Search

This Perspective aims at providing a road map to computational heterogeneous photocatalysis highlighting the knowledge needed to boost the design of efficient photocatalysts. A plausible computational framework is suggested focusing on static and dynamic properties of the relevant excited states as well of the involved chemistry for the reactions of interest. This road map calls for explicitly exploring the nature of the charge carriers, the excited-state potential energy surface, and its time evolution. Excited-state descriptors are introduced to locate and characterize the electrons and holes generated upon excitation. Nonadiabatic molecular dynamics simulations are proposed as a convenient tool to describe the time evolution of the photogenerated species and their propagation through the crystalline structure of photoactive material, ultimately providing information about the charge carrier lifetime. Finally, it is claimed that a detailed understanding of the mechanisms of heterogeneously photocatalyzed reactions demands the analysis of the excited-state potential energy surface.

In this study, the effect of nitrogen substitution in wolframite-type ScTaO4 was investigated using density- functional theory calculations. First, structural and mechanical properties, as well as the dynamical stability of ScTaO4 were examined deeply for the ambient-pressure structure. Subsequently, we studied how lattice vibrations are affected by hydrostatic pressure and determined the elastic moduli of ScTaO4. The results of our study show that the monoclinic structure of ScTaO4 is rigid and non-compressible. In addition, band- structure calculations show that ScTaO4 has a wide direct band-gap of 4.04 eV, which in turn leads to a possible tuning of electronic properties. We have found that this task can be conducted by partially substi- tuting oxygen atoms in the unit cell with nitrogen atoms. Both band-structure calculations and charge- density analyses revealed a narrowing in the band gap caused by the presence of nitrogen atoms, which act as a shallow acceptor state, resulting in weak repulsive interactions and structural distortions in both Sc and Ta coordination polyhedra; reducing the crystal symmetry from monoclinic to triclinic.

The challenges imposed by climate change require the continued improvement and identification of materials for the development of green technologies. Point defect engineering is a promising technology for producing green hydrogen by taking advantage of catalytic hydrogen evolution reactions. In this work, we investigate the role of anionic and cationic vacancy point defects, as well as the nature of the active sites, in the catalytic activation of Cu2WS4 single layers. The stability of the pristine and defective structures of Cu2WS4 has been thoroughly investigated using density-functional theory calculations. A deep analysis of the formation enthalpy indicates that the Cu vacancy is the chemically most favorable vacancy. However, the calculated adsorption energy indicates that the presence of such vacancies slightly enhances the hydrogen evolution reaction. In contrast, the formation of an S vacancy considerably magnifies the same reaction in Cu2WS4 single layers.

ilustraciones, fotografías En este trabajo se reporta la síntesis del material denominado ferrocobaltita lantánida La2FeCoO6 mediante el método modificado de Pechini. Se realizó el estudio experimental de las propiedades estructurales, morfológicas y magnéticas y se implementó la teoría funcional de la densidad mediante el código VASP para analizar las bandas de energı́a, densidad de estados y las propiedades termofísicas que caracterizan a este material. El refinamiento de Rietveld de los patrones experimentales de difracción de rayos X reveló la cristalización de este material en una estructura monoclínica similar a la perovskita, grupo espacial P21 /n (#14). Por otra parte, las imágenes de microscopía electrónica de barrido y microscopía de fuerza atómica revelaron que la morfología de la superficie es esencialmente policristalina, con tamaños de grano medios entre 177 y 188 nm. Se aplicó la espectroscopia dispersiva de rayos X mostrando en el material La, Fe, Co y O en las proporciones estequiométricas esperadas. Las curvas de histéresis magnética revelan el carácter ferromagnético de este material para diferentes temperaturas evaluadas. Además, las curvas de respuesta I-V muestran un comportamiento similar al de un semiconductor con una figura de exponente de mérito de 1.53 del tipo varistor. El comportamiento de semiconductor ferromagnético sugiere que la ferrocobaltita lantánida La2FeCoO6 tiene una potencial aplicación en dispositivos espintrónicos. Se efectuaron cálculos ab inicio de las propiedades electrónicas y termodinámicas para la ferrocobaltita lantánida La2FeCoO6 . Para efectuar los cálculos se usó GGA+U para incluir la energía de intercambio y correlación con la corrección del potencial de Hubbard indicado para los átomos de Fe y Co. Por medio de estos cálculos se estableció que la La2FeCoO6 se comporta como un semiconductor, con un gap de energı́a de E g = 2,35 eV . Se observaron fuertes hibridaciones entre los orbitales de O-2p en la banda de valencia con los estados Fe 2+ -3d y Co 4+ -3d permiten explicar la naturaleza ferromagnética del material a través del mecanismo de superintercambio entre estados de alto espı́n de Fe 2+ con estados de bajo espı́n de Co 4+ mediado por los orbitales O 2− . La dependencia del calor especı́fico con respecto a la temperatura y la presión, el coeficiente de expansión térmica, la temperatura de Debye y el parámetro de Grüneisen, se calcularon a partir de la ecuación de estado utilizando el modelo de cuasi-armónico de Debye. Se observa que los resultados obtenidos para este material a nivel teórico son compatibles con los resultados experimentales reportados. (Texto tomado de la fuente) In this research is reported the synthesis for the lanthanide ferrocobaltite La2FeCoO6 using the modified Pechini route. The experimental study was carried out of structural, morphological and magnetic properties and density functional theory was used for studying energy bands, the density of states and termophysical properties of this perovskite. The Rietveld refinement of the experimental patterns of X-ray diffraction reveald the crystallization of this material in a monoclinic structure belonging to the space group P21/n (#14). Besides, the images of scanning electron microscope (SEM) and atomic force microscope (AFM) reveled that the morphologic of the sample is esscentially polycrystaline distribution wich is formed by grains of random sizes and shapes between 177 and 188 nm. Dispersive X-ray spectroscopy was applied showing in the material La, Fe, Co and O in the expected stoichiometric proportions. The magnetic hysteresis curves reveal the ferromagnetic character of this material for different evaluated temperatures. In addition, the I-V response curves show behavior similar to a semiconductor with an exponent of merit figure of 1.53 of the varistor type. The ferromagnetic semiconductor behavior suggests that lanthanide ferrocobaltite La2FeCoO6 has a potential application in spintronic devices. Ab initio calculations of the electronic and thermodynamic properties for the ferrocobaltite of the perovskite-type La2FeCoO6 are reported. To carry out the calculations, GGA+U was used to include the energy of exchange and correlation with the correction of the Hubbard potential indicated for the Fe and Co atoms. Through these calculations it was established that La2FeCoO6 behaves like a semiconductor, with an energy gap of E g = 2,35 eV . Strong hybridizations were observed between the O-2p orbitals in the valence band with the Fe 2+ -3d and Co 4+ − 3d states, allowing to explain the ferromagnetic nature of the material through the super-exchange mechanism between high-spin states of F e 2+ with low-spin states of Co 4+ mediated by O 2− orbitals. The dependence of specific heat with respect to temperature and pressure, as well as the coefficient of thermal expansion, the Debye temperature, and the Grüneisen parameter were calculated from the equation of state, using the quasi-harmonic Debye model. Theoretical results obtained are comparable with the experimental values obtained in the literature. Maestría Magíster en Ciencias - Física Física de Nuevos Materiales.

Las propiedades de átomos y moléculas confinados son distintas a las del sistema libre correspondiente, lo que ha motivado su estudio en los últimos años para su aplicación en numerosos campos. Esta Tesis Doctoral busca estudiar el confinamiento cuántico de átomos y moléculas y analizar su estabilidad una vez abandonan el confinamiento. Para ello, se han considerado sistemas de uno y dos electrones y distintos potenciales que modelen la cavidad de confinamiento. Para resolver la ecuación de Schrodinger independiente del tiempo se han desarrollado e implementado distintas metodologías adecuadas a cada caso, con lo que se han obtenido los valores de energía y las funciones de onda de los sistemas en estudio para distintas condiciones de confinamiento. A su vez, esto ha permitido estudiar su estabilidad una vez desaparece el confinamiento frente a procesos de autoionización o excitación a través del cálculo de las probabilidades de transición entre los estados confinados y los estados del sistema libre. Como sistema de un electrón se ha estudiado el átomo de hidrogeno confinado en distintas situaciones. Se han considerado como potenciales de confinamiento una barrera esférica infinita, un pozo gaussiano, una barrera finita cuadrada y un pozo finito cuadrado. Mediante el análisis del espectro para cada potencial se ha observado que existe una gran dependencia entre la energía y el modelo de confinamiento considerado, al igual que sucede con la probabilidad de ionización del átomo tras abandonar la cavidad confinante. Según las condiciones de confinamiento, se ha obtenido que esta probabilidad puede alcanzar valores por encima de 0.5. Por otro lado, como sistemas de dos electrones se han estudiado la molécula de hidrogeno y el átomo de helio, para los que se han considerado como modelos de confinamiento una combinación de dos potenciales de Woods-Saxon y una combinación de pozos y barreras, respectivamente. Mediante el estudio de su espectro, se ha analizado como afecta a la estabilidad del sistema su posición en el interior de la cavidad, para lo cual se han tenido en cuenta distintos sistemas de referencia en los que realizar el análisis. Como resultado de ello, se ha obtenido que efectivamente existe una gran dependencia entre la posición del sistema y su estabilidad. Así, se ha visto que el valor más ligado de energía no siempre se obtiene para el sistema situado en el centro de la cavidad de confinamiento, por lo que esta no sería la posición más estable del mismo. The properties of confined atoms and molecules are different from those of the free system, which has motivated their study in recent years for their application in several fields. The purpose of this Ph.D. Thesis is the study of the quantum confinement of atoms and molecules and the analysis of their stability once they leave the confinement. Consequently, one- and two-electron systems and different potentials to model the confining cavity have been considered. To solve the timeindependent Schrodinger equation, different methodologies have been developed and implemented for each case, therefore energy values and wave functions of the systems under study have been obtained for different confinement conditions. Furthermore, using these eigenvalues and eigenfunctions, it has been possible to study the stability of the systems once confinement is removed with respect to selfionisation or excitation processes by calculating the transition probabilities from confined to free states. As a one-electron system, the confined hydrogen atom has been studied in different situations. An infinite spherical barrier, a Gaussian well, a finite squarebarrier and a finite square-well have been considered as confining potentials. By analysing the energy spectrum for each potential, it has been observed that there is a strong dependence between the energy and the confinement model considered, as in the case of the ionisation probability of the atom after leaving the confining cavity. Depending on the confinement conditions, it has been found that this probability can reach values above 0.5. On the other hand, the hydrogen molecule and the helium atom have been studied as two-electron systems, for which a combination of two Woods-Saxon potentials and a combination of wells and barriers, respectively, have been considered as confinement models. By studying their spectra, we have analysed how their position inside the cavity affects the stability of the system, therefore different reference systems could be considered to perform the analysis. As a result, it was found that there is a strong dependence between the position of the system and its stability. Thus, it has been observed that the lowest energy value is not always obtained for the system located at the centre of the confining cavity, which suggests that this would not be the most stable position of the system.

The electronic structure of nitrobenzene (C6H5NO2) has been studied by means of the complete active space self-consistent field (CASSCF) and multi-state second-order perturbation (MS-CASPT2) methods. To this end, an active space of 20 electrons distributed in 17 orbitals has been selected to construct the reference wave function. In this work, we have calculated the vertical excitation energies and the energy barrier for the dissociation of the molecule on the ground state into phenyl and nitrogen dioxide. After applying the corresponding vibrational corrections to the electronic energies, it is demonstrated that the MS-CASPT2//CASSCF values obtained in this work yield an excellent agreement between calculated and experimental data. In addition, other active spaces of lower size have been applied to the system in order to check the active space dependence in the results. UNIVERSIDAD DE MALAGA

The ability to control the chemical conformation of a system via external stimuli is a promising route for developing molecular switches. For eventual deployment as viable sub-nanoscale components that are compatible with current electronic device technology, conformational switching should be controllable by a local electric field (i.e. E-field gateable) and accompanied by a rapid and significant change in conductivity. In organic chemical systems the degree of π-conjugation is linked to the degree of electronic delocalisation, and thus largely determines the conductivity. Here, by means of accurate first principles calculations, we study the prototypical biphenyl based molecular system in which the dihedral angle between the two rings determines the degree of conjugation. In order to make this an E-field gateable system we create a net dipole by asymmetrically functionalising one ring with: (i) electron withdrawing (F, Br and CN), (ii) electron donating (NH2), and (iii) mixed (NH2/NO2) substituents. In this way, the application of an E-field interacts with the dipolar system to influence the dihedral angle, thus controlling the conjugation. For all considered substituents we consider a range of E-fields, and in each case extract conformational energy profiles. Using this data we obtain the minimum E-field required to induce a barrierless switching event for each system. We further extract the estimated switching speeds, the conformational probabilities at finite temperatures, and the effect of applied E-field on electronic structure. Consideration of these data allow us to assess which factors are most important in the design of efficient gateable electrical molecular switches.

The new class of oxide heterojunctions with mixed dimensionalities holds great promise for energy and environmental applications. However, the existing fabrication strategies typically involve low-yield and multistep processes leading to the formation of powders that necessitate binding agents when used for electrochemical applications; thereby, the durability and performance of the resultant electrode may be adversely impacted. To address these challenges, the present work first reports a high-temperature counter-current gas flow technique for rapid fabrication (5–10 min) of centimetre-size, self-adhesive, free-standing 3D patches made of ZnO-based woven nanowires. Furthermore, the high applicability of the method was shown by layer-by-layer assembly of the ZnO and layers of 0D heteroatoms including Bi2O3, CdO, SnO2, and carbon forming stratified oxide heterojunction (SOH) nanostructures with midgap states within their electronic bandgap region. This work is innovative in that the ZnO and the fabricated SOHs are synthesised through a sustainable and large-scale method based on microrecycling of waste materials. The engineering of the electronic band positions can modify the functionality of the SOH patches by optimising the potentials required for catalytic reactions. As a representative, the SOH nanostructures were tested for anodic electrocatalytic water oxidation to H2O2. The results showed that the ZnO–CdO patch has the lowest overpotential of 150 mV and outstanding stability at 2.33 V vs. RHE. Furthermore, the results of first-principles density functional theory (DFT) calculations (i) confirmed realignments of the band position due to the formation of midgap states, and (ii) revealed that significant improvements in the electrocatalytic H2O2 performance can be achieved with overpotentials as low as 0.19 and 0.31 V for ZnO–CdO and ZnO–Bi2O3, respectively. This work offers an ultrafast fabrication strategy to synthesise binder-free SOH nanostructures with an engineered electronic structure that can be an alternative to state-of-the-art noble metal electrocatalysts such as Pt.

Over recent decades, much effort has been made to lengthen spin relaxation/decoherence times of single-molecule magnets and molecular qubits by following different chemical design rules such as maximizing the total spin value, controlling symmetry, enhancing the ligand field or inhibiting key vibrational modes. Simultaneously, electronic structure calculations have been employed to provide an understanding of the processes involved in the spin dynamics of molecular systems and served to refine or introduce new design rules. This review focuses on contemporary theoretical approaches focused on the calculation of spin relaxation/decoherence times, highlighting their main features and scope. Fundamental aspects of experimental techniques for the determination of key Single Molecule Magnet/Spin Qubit properties are also reviewed.

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2022, Tutors: Bruno Juliá-Díaz, Alexandre Dauphin, We study the electronic structure of graphene and the corresponding phases. We first examine the non-interacting part of the Hamiltonian and show the appearance of Dirac cones. Later we study the full Hamiltonian through Mean-Field theory and show a process of symmetry breaking that leads to the formation of a Charge Density Wave (CDW) phase. We show how Automatic Differentiation, a tool from Machine Learning, can be used as an alternative to the Self-Consistent Field method

Por último, se han analizado los desplazamientos en frecuencias con el potencial de electrodo en dos moléculas cargadas como son los iones cianuro y 4-cianobenzoato utilizando el modelo de clústeres metálicos lineales donde se pone de manifiesto la existencia de una estructura electrónica dual del híbrido metal-molécula, sintonizable por el potencial aplicado, que presentan características y naturalezas diferentes, además de una respuesta diferente al potencial aplicado. La interpretación de los espectros SERS (Surface-Enhanced Raman Spectroscopy) en base a la estructura electrónica del sistema en estudio, resulta complejo ya que depende de cada sistema metal-molécula en particular. La única información que se dispone de forma experimental, son los cambios en intensidades relativas, respecto al Raman de la disolución, y los desplazamientos en frecuencias debido al proceso de adsorción o bien al potencial de electrodo aplicado en las experiencias electroquímicas. Por otro lado, para obtener información de la estructura electrónica del sistema en estudio desde el punto de vista teórico, es necesario desarrollar modelos teóricos de la interfase metal-adsorbato además, de seleccionar el nivel de cálculo. La comparación entre los datos experimentales y los teóricos permiten conocer la naturaleza del fenómeno SERS en un determinado sistema. En esta Tesis Doctoral se han analizado los cambios en intensidades relativas y los desplazamientos en frecuencia en los espectros SERS, a diferentes potenciales de electrodo, de las moléculas neutras de piridazina y 4-cianopiridina en base a un mecanismo de transferencia de carga y haciendo uso de un modelo sencillo de complejo superficial metal-molécula. También, se ha realizado la comparación entre dos modelos teóricos que intentan similar el potencial de electrodo aplicado en las experiencias SERS electroquímicas y que son un modelo de clústeres lineales de plata con diferentes densidades de carga y un modelo de campo eléctrico. Esta comparación se ha realizado en el análisis de los espectros SERS de una molécula emblemática en este campo como es la piridina.

Heterometallic lanthanide [LnLn′] coordination complexes that are accessible thermodynamically are very scarce because the metals of this series have very similar chemical behaviour. Trinuclear systems of this category have not been reported. A coordination chemistry scaffold has been shown to produce molecules of type [LnLn′Ln] of high purity, i.e. exhibiting high metal distribution ability, based on their differences in ionic radius. Through a detailed analysis of density functional theory (DFT) based calculations, we discern the energy contributions that lead to the unparalleled chemical selectivity of this molecular system. Some of the previously reported examples are compared here with the newly prepared member of this exotic list, [Er2Pr(LA)2(LB)2(py)(H2O)2](NO3) (1) (H2LA and H2LB are two β-diketone ligands). A magnetic analysis extracted from magnetization and calorimetry determinations identifies the necessary attributes for it to act as an addressable, conditional multiqubit spin-based quantum gate. Complementary ab initio calculations confirm the feasibility of these complexes as composite quantum gates, since they present well-isolated ground states with highly anisotropic and distinct g-tensors. The electronic structure of 1 has also been analyzed by EPR. Pulsed experiments have allowed the establishment of the quantum coherence of the transitions within the relevant spin states, as well as the feasibility of a coherent control of these states via nutation experiments., The authors thank the European Union's Horizon 2020 research and innovation programmes QUANTERA project SUMO and FET-OPEN grant 862893 FATMOLS (GA and OR) the Spanish AEI for grants PGC2018-098630-B-I00 (GA and DA), PID2020-118329RB-I00 (OR), PID2020-112806RB-I00 (DGR and CB) and the Severo Ochoa Excellence Accreditation 2020–2023 CEX2019-000925-S (DGR and CB), the ICIQ foundation (DGR and CB) and the Generalitat de Catalunya for a Beatriu de Pinós fellowship (2019-BP-00015, DR), an FI grant (also financed by AGAUR and the European Social Fund, DGR) an FI-SDUR Grant (2020-FISDU-00492, DM), the CERCA program and the ICREA Academia 2018 Prize (GA), as well as the National EPR Facility at the University of Manchester for support with the EPR measurements. This work used resources of the ALBA synchrotron through access to beamline 13-XALOC.

[eng] The structure and stability of molecules, the formation of supramolecular aggregates in the solid state or in solution and, therefore, many chemical and biological processes in which they participate depend heavily on noncovalent interactions (NCIs). To fully exploit these weak interactions in order to reach specific chemical, biological and technological goals, a thorough understanding of their properties at the molecular level is crucial. In this doctoral thesis, different types of NCIs have been studied in detail in the gas phase using a combination of structural and computational methods. On one hand, searches in the Cambridge Structural Database (CSD) have revealed unnoticed intramolecular and intermolecular short contacts that dictate the structure of several families of molecules in their crystalline phases. Moreover, the analysis of the experimental structures has helped to identify the geometrical preferences that maximize the interaction strength. On the other hand, accurate density functional theory (DFT) calculations and other computational techniques have provided reliable information needed to unveil the energetics and physical nature of these interactions. The noncovalent interactions studied in this work are (1) Li···Li and X···X through-ring interactions in Li2X2 rings, (2) lone-pair-carbonyl interactions in acyl halides, (3) electrostatically disfavoured Br···C=O contacts, (4) sigma-hole interactions between lead(II) and sulphur or oxygen, (5) [N···I···N]+ halonium bonding, and (6) azido···azido contacts in metal complexes with different interaction topologies. The results indicate that, although the atoms or moieties involved are different, the origin of all these attractive interactions lies in the subtle interplay of the various contributions (Pauli exchange-repulsion, electrostatics, charge transfer, polarization, and dispersion forces) acting upon the formation of noncovalently-bonded systems, differing only in the relative weights of these forces in the overall interaction strength., [cat] L'estructura i l'estabilitat de les molècules, la formació d'agregats supramoleculars en estat sòlid i en solucions i, per tant, de molts processos químics i biològics en què participen depenen en gran manera de les interaccions no covalents (NCI). Per aprofitar al màxim aquestes interaccions febles en l'assoliment d'objectius químics, biològics i tecnològics específics, és crucial conèixer a fons les seves propietats a nivell molecular. En aquesta tesi doctoral s'han estudiat amb detall diferents tipus de NCI en fase gasosa mitjançant una combinació de mètodes estructurals i computacionals. D'una banda, les cerques a la Base de Dades Estructurals de Cambridge (CSD) han revelat contactes curts intramoleculars i intermoleculars desapercebuts que dicten l'estructura de diverses famílies de molècules en les seves fases cristal·lines. A més, l'anàlisi de les estructures experimentals ha ajudat a identificar les preferències geomètriques que maximitzen la força d'interacció. D'altra banda, els càlculs precisos basats en la teoria del funcional de la densitat (DFT) i altres tècniques computacionals han proporcionat informació fiable necessària per revelar l’energia i la naturalesa física d'aquestes interaccions. Les interaccions no covalents estudiades en aquest treball són (1) interaccions a través de l’anell Li···Li i X···X en anells de Li2X2, (2) interaccions carbonil-parell solitari en halurs d'acil, (3) contactes Br···C=O electroestàticament desafavorits, (4) interaccions “sigma-hole” entre plom(II) i sofre o oxigen, (5) l’enllaç d’haloni [N···I···N]+, i (6) contactes azido···azido en complexos metàl·lics amb diferents topologies d'interacció. Els resultats indiquen que, tot i que els àtoms o fragments implicats són diferents, l'origen de totes aquestes interaccions atractives rau en la interacció subtil de les diverses contribucions (intercanvi-repulsió de Pauli, electroestàtica, transferència de càrrega, polarització i forces de dispersió) que actuen sobre la formació de sistemes enllaçats no covalentment, els quals difereixen només en els pesos relatius d'aquestes forces per a la força global d'interacció.

Dentro de la nanotecnología, la espintrónica y la electrónica molecular han acaparado la atención de la comunidad científica en los últimos tiempos, y han sido acogidas en el área de la nanotecnología. Cambios drásticos en el comportamiento físico de los dispositivos se explican por los fenómenos de ´ındole cuántico que comienzan a manifestarse por lo reducido del tamaño de los materiales. Esto definitivamente llama a la búsqueda de alternativas nuevas y mejores, que ofrezcan propiedades que aseguren un crecimiento sostenido de estas tecnologías a través de la innovación. La espintrónica es la rama de la nanotecnología que explota el espín electrónico por sobre la carga electrónica. En este sentido muchas moléculas, pol´ımeros y nanoestructuras que poseen propiedades electrónicas y magnéticas interesantes se posicionan de gran forma ante posibles aplicaciones tales como: almacenamiento de informaci´on de alta densidad, c´omputo cu´antico, sistema de memoria no volátil, sensores magnéticos, válvulas de espín, filtros de espín, transistores moleculares, etc. La necesidad de diseñar, estudiar y preparar nuevos dispositivos, eficientes y de viable preparación en condiciones accesibles, requiere el trabajo de investigación en el tipo de materiales en los que se han verificado condiciones para desarrollarlos. Dentro de las más interesantes se encuentran: nanotubos de carbono, nanocintas de carbono, fulerenos y el grafeno. Todos ellos han mostrado propiedades muy interesantes hasta el momento, demostrando un potencial uso en campos como: energ´ıa, electrónica y magnetismo; y varios de ellos incorporados ya en productos tecnológicos. En esta tesis se desarrollaron cálculos de simulación computacional basados en la Teoría del Funcional de la Densidad para analizar dos sistemas carbonosos: el grafeno y las nanocintas de carbono. Estas nanoestructuras carbonosas fueron modificadas qu´ımicamente mediante dopado con elementos metálicos como hierro y níquel; y elementos no metálicos como boro, oxígeno, nitrógeno y monóxido de carbono. Se ensayaron diferentes porcentajes de dopado, mediante uso de diferentes superceldas, lo que permitió entender el efecto de dilución del dopante en los mismos. En el caso de las nanocintas, se ensayaron diferentes tipos de dopado, en región central y borde, generando efectos magnéticos interesantes. Finalmente se analizaron las consecuencias vi estructurales y mecánicas del dopado en borde en nanocintas de carbono, en las que se evidenció un mecanismo electrónico complejo que explica la variación en la dureza de las nanocintas en función del cambio en la estructura electrónica producido por el dopado en borde. Como resultado de este trabajo se ha evidenciado el efecto de dopantes metálicos y no metálicos en grafeno y nanocintas de carbono.

This theoretical work presents the electronic and magnetic properties of Cr doped SrFe12O19 using first-principles calculations based on density functional theory. An improvement in the magnetic properties of SrFe12O19 is predicted when doped with the Cr3+ ion. It was found that the magnetic Cr3+ ion preferentially replaces Fe3+ ion at 2a, 4f1, and 12k sites. The calculation demonstrates that if Cr ion occupies the 4f1 site, a total magnetization of 52.920 emu/g is obtained. On the other hand, SrFe11.5Cr0.5O19 behaves as a semiconductor when Cr occupies the 2a and 12k sites while behaves like half-metal when Cr occupies the 4f1 site. The higher magnetization, identified for the SrFe11.5Cr0.5O19 compound, provides unique qualities in the applications of permanent magnets and/or related devices like spintronics and magnetic semiconductors.

International audience; The single-molecule magnet (SMM) properties of a series of ferrocenium complexes, [Fe(h5-C5R5)2]+ (R=Me, Bn), are reported. In the presence of an applied dc field, the slow dynamics of the magnetization in [Fe(h5-C5Me5)2]BArF are revealed. Multireference quantum mechanical calculationsshow a large energy difference between the ground and first excited states, excluding the commonly invoked, thermally activated (Orbach-like) mechanism of relaxation. In contrast, a detailed analysis of the relaxation time highlights that both direct and Raman processes are responsible forthe SMM properties. Similarly, the bulky ferrocenium complexes, [Fe(h5-C5Bn5)2]BF4 and [Fe(h5-C5Bn5)2]PF6, also exhibit magnetization slow dynamics, however an additional relaxation process is clearly detected for these analogous systems.

The ground state (GS) of Chichibabin's polycyclic hydrocarbons (CPHs) can be singlet (open- or closed-shell (OSS or CS)) or triplet (T), depending on the elongation of the π-system and the exocyclic substituents. CPHs with either a small singlet-triplet energy gap (ΔEST) or even a triplet GS have potential applications in optoelectronics. To analyze the effect of the size and exocy-clic substituents on the nature of the GS of CPHs, we have selected a number of them with different substituents in the exocyclic carbon atoms and different ring chain lengths. OPBE/cc-pVTZ level of theory was used for the optimization of the systems. The aromaticity of the resulting electronic structures was evaluated with HOMA, NICS, FLU, PDI, Iring, and MCI aromaticity indices. Our results show that the shortest π-systems (one or two rings) have a singlet GS. However, systems with three to five rings favor OSS GSs. Electron-withdrawing groups (EWGs) and aromatic substituents in the exocyclic carbons tend to stabilize the OSS and T states, while electron-donating groups slightly destabilize them. For CS, OSS, and T states, aromaticity measures indicate a gain of aromaticity of the six-membered rings of the CPHs with the increase in their size and when CPHs incorporate EWGs or aromatic substituents. In general, the CPHs analyzed present small singlet-triplet energy gaps, and in particular, the ones contain-ing EWGs or aromatic substituents present the smallest singlet-triplet energy gaps A.P. and M.S. thank the ́ Ministerio de Economi ́ a y Competitividad (MINECO) of Spain for projects CTQ2014-59832-JIN and CTQ2017-85341- P, Generalitat de Catalunya for project 2017SGR39, Xarxa de Referencia en Qui ̀ mica Teo ́ rica i Computacional, the ICREA ̀ Academia prize 2014 awarded to M.S., and European Fund for Regional Development (FEDER) grant UNGI10-4E-801

Understanding the interactions among magnetic nanostructures is one of the key factors to predict and control the advanced functionalities of 3D integrated magnetic nanostructures. In this work, the focus is on different interconnected Ni nanowires forming an intricate, but controlled, and ordered magnetic system: Ni 3D Nanowire Networks (3DNNs). These self-ordered systems present striking anisotropic magnetic responses, depending on the interconnections’ position between nanowires. To understand their collective magnetic behavior, the magnetization reversal processes are studied within different Ni 3D Nanowire Networks compared to the 1D nanowire 1DNW array counterparts. The systems are characterized at different angles using first magnetization curves, hysteresis loops, and First Order Reversal Curves techniques, which provided information about the key features that enable macroscopic tuning of the magnetic properties of the 3D nanostructures. In addition, micromagnetic simulations endorse the experiments, providing accurate modeling of their magnetic behavior. The results reveal a plethora of magnetic interactions, neither evident nor intuitive, which are the main role players controlling the collective response of the system. The results pave the way for the design and realization of 3D novel metamaterials and devices based on the nucleation and propagation of ferromagnetic domain walls both in 3D self-ordered systems and future nano-lithographed devices., M.M.G. and O.C.C. acknowledge the financial support from the project PID2020-118430GB-100 (MICINN). J.B.P., and F.P. acknowledge the financial support from PID2019-106165GB-C21 (MICINN) and M. López-Haro and J.J. Calvino from the University of Cádiz for the acquisition of the electron tomography series. The authors also acknowledge the service from the X-SEM Laboratory at IMM, and funding from MINECO under project CSIC13-4E-1794 with support from the EU (FEDER, FSE). The authors acknowledge the support for simulation hardware from J.L. Mesa at INTA. D.N. acknowledges the financial support from the project PID2019-108075RB-C31 and the grant RYC-2017-22820 funded by MICINN/10.13039/501100011033 and by “ESF Investing in your future”.

[eng] The principal aim of this thesis is the understanding of the electronic structure of organic diradical and polyradical molecules. Unveiling the properties that give them the electronic, magnetic, and optical properties to be applied as main components in optoelectronic devices. Specifically, the objectives that have been achieved are i) the characterization of organic compounds with diradical and polyradical character, and their electronic, magnetic, and spectroscopic properties; ii) the detailed description of the singlet fission mechanism, as well as the proposal of a new system that, theoretically, is able of carrying out efficiently the singlet fission process; and iii) the use of quantum mechanical methods (specifically RAS-SF) and computational tools to get a proper description of the electronic structure of the ground state of systems in which non-dynamic correlation plays an important role. In the first place, organic compounds whose optical and magnetic properties make them interesting in the field of optoelectronic materials were studied. The relationship between molecular structure and the radical character was found by the study of linear and cyclic acenes and small triangular fragments derivatives of graphene. While the diradical and tetraradical character increase together with the size of the linear and cyclic compounds, the triangular structures (TGNF, the acronym for Triangulene Graphene Nano Fragments) are open-shell systems with high-spin ground state multiplicity. Furthermore, a manner to tune the spin multiplicity in the ground state of TGNFs is proposed through heteroatom doping or hydrogenation, which offers a way to design larger graphene nanofragments with well-defined spin-multiplicity. Towards larger systems, the increment of the size is associated with the increase of the polyradical character. This thesis presents the rationalization of the electronic structure of organic macrocycles with high polyradical character. Concretely, from triradicals to decaradicaloids (up to 10 radical centers). The properties triggered by the open-shell character of the ground state are as diverse as surprising. For instance, AWA systems (annulenes-within-annulenes) have been characterized in collaboration with experimental groups for the first time. The global aromaticity exhibit by these macrocycles responds to the radical interaction in each of the annulenes and is governed by both aromaticity rules, Hückel's, and Baird's simultaneously. On the other hand, the singlet fission process (SF) was expanded from the classical model, which involves five electronic states, to a model that includes double excitations (D states), a seven-state model. Using a simple model, it is estimated that the D states can play an active role in SF, as well as the necessary conditions to maximize their participation as an initial or intermediate state in the process. The feasibility of spiro systems carrying out SF is exposed., [spa] El principal objetivo de esta tesis es poner al descubierto las propiedades químicas que permiten que procesos fotofísicos complejos tomen lugar en moléculas orgánicas, y que les permite tener aplicaciones en materiales optoelectrónicos. Puntualmente, los objetivos que se han cumplido son i) la caracterización de di y poliradicales orgánicos y sus propiedades electrónicas, magnéticas y espectroscópicas; ii) la descripción detallada del mecanismo de fisión de singuletes, así como proponer un sistema que teóricamente es capaz de realizarlo eficientemente; y iii) la utilización de métodos de la mecánica cuántica (específicamente RAS-SF) y herramientas computacionales que permiten la descripción apropiada de la estructura electrónica del estado fundamental de sistemas en los que la correlación no-dinámica representa un papel importante. En primer lugar, se estudiaron compuestos orgánicos cuyas propiedades ópticas y magnéticas los hacen interesantes en el campo de materiales optoelectrónicos. Se encontró la dependencia del carácter radical en la topología de nanoestructuras de grafeno (acenos) lineales y cíclicos, y pequeñas laminas de estructura triangular. Mientras el carácter diradical y tetraradical crece con el tamaño de los compuestos lineales y cíclicos, las estructuras triangulares (TGNF, siglas en inglés de Triangulene Graphene Nano Fragments) son compuestos de capa abierta. Además, se propone una forma de modular la multiplicidad de espín en el estado basal de los TGNF mediante el dopaje con heteroátomos. El incremento del tamaño del sistema permite obtener molecular con alto carácter poliradical. En esta tesis se presenta la racionalización de la estructura electrónica de macrociclos orgánicos con carácter triradical hasta decaradicaloide (10 centros radicalarios). Las propiedades que se derivan de estas características son tan variadas como sorprendentes, por ejemplo, se han caracterizado sistemas AWA (anulenos-dentro de-anulenos) cuya aromaticidad global responde a la suma de la aromaticidad en cada uno de los anulenos y esta regida por las reglas de aromaticidad de Hückel y Baird simultaneamente. El proceso de fisión de singuletes (SF) fue expandido del modelo clásico, que involucra cinco estados electrónicos, a un modelo que incluye excitaciones dobles (estados D), modelo de siete estados. Usando un modelo sencillo se estima que los estados D pueden jugar un papel activo en SF, así como las condiciones necesarias para maximizar su participación como estado inicial o intermediario en el proceso. Se expone la factibilidad de que los sistemas spiro lleven a cabo SF.

Layered group V transition-metal trichalcogenides are paradigmatic low-dimensional materials providing an ever increasing series of unusual properties. They are all based on the same basic building units, one-dimensional MX3 (M = Nb, Ta; X = S, Se) trigonal-prismatic chains that condense into layers, but their electronic structures exhibit significant differences leading to a broad spectrum of transport properties, ranging from metals with one, two, or three charge density wave instabilities to semimetals with potential topological properties or semiconductors. The different physical and chemical properties are shown to be related with subtle structural differences within the layers that result in half-, third-, or quarter-filled quasi-one-dimensional Nb dz2-type bands, providing a clear-cut illustration of the intimate link between structural and electronic features within a family of solids. An interesting yet not sufficiently explored feature of these solids is the polymorphism. Based on both experimental and new theoretical results, we examine this aspect for NbS3 and show that at least seven different polymorphs with a stability compatible with the presently known phases of this compound are possible. We discuss a simple rationale for the physical properties of the presently known polymorphs as well as predictions for those that have still not been characterized or prepared. It is argued that some of the presently unknown polymorphs may have been prepared in an uncontrolled way as mixtures of different phases which could not be structurally characterized. The rich landscape of structures and properties found for this van der Waals material is suggested to represent an ideal platform for the preparation of flakes with fine-tuned properties for applications in new electronic and optoelectronic devices., This work was supported by the Spanish MICIU through grants PGC2018-096955-B-C44 and PGC2018-093863-B-C22, the Severo Ochoa FUNFUTURE (CEX2019-000917-S) Excellence Centre distinction, the Maria de Maeztu Units of Excellence Program (MDM-2017-0767), and Generalitat de Catalunya (grants nos. 2017SGR1506 and 2017SGR1289). S.C. was supported by FONDECYT, Chile (project 11171063).

En cierta forma, la mecánica cuántica da solución a todos los problemas de la química, lo único que hay que hacer es resolver las ecuaciones de Schrödinger para las moléculas de interés. Desafortunadamente, el costo computacional de resolver estas ecuaciones crece exponencialmente con el número de electrones y para más de ~100 electrones resulta imposible resolverlas con precisión química (~2 kcal/mol). Las ecuaciones de Kohn-Sham (KS) de la teoría del funcional de la densidad (density functional theory, DFT) permiten reformular las ecuaciones de Schrödinger usando la densidad de probabilidad electrónica como la variable central sin necesidad de calcular las funciones de onda de Schrödinger. El costo de resolver las ecuaciones de Kohn-Sham solo crece como N3, donde N es el número de electrones, lo que ha llevado a la inmensa popularidad de la DFT en química. A pesar de esta popularidad, incluso las aproximaciones más sofisticadas de las KS-DFT llevan a errores que limitan el uso de métodos basados exclusivamente en la densidad electrónica. En este artículo se discute cómo pueden desarrollarse nuevos métodos que escalen linealmente con N usando densidades de fragmentos como las variables principales en lugar de densidades totales, así como la forma en que estos métodos proveen una respuesta atractiva a la pregunta del subtítulo: ¿cuál es la forma de los átomos en las moléculas?. In some sense, quantum mechanics solves all the problems in chemistry: The only thing one has to do is solve the Schrödinger equation for the molecules of interest. Unfortunately, the computational cost of solving this equation grows exponentially with the number of electrons and for more than ~100 electrons, it is impossible to solve it with chemical accuracy (~ 2 kcal/mol). The Kohn-Sham (KS) equations of density functional theory (DFT) allow us to reformulate the Schrödinger equation using the electronic probability density as the central variable without having to calculate the Schrödinger wave functions. The cost of solving the Kohn-Sham equations grows only as N3, where N is the number of electrons, which has led to the immense popularity of DFT in chemistry. Despite this popularity, even the most sophisticated approximations in KS-DFT result in errors that limit the use of methods based exclusively on the electronic density. By using fragment densities (as opposed to total densities) as the main variables, we discuss here how new methods can be developed that scale linearly with N while providing an appealing answer to the subtitle of the article: What is the shape of atoms in molecules?.

Most of the archetypal aromatic compounds present high symmetry and have degenerate highest-occupied molecular orbitals. These orbitals can be fully occupied resulting in a closed-shell structure or can be same-spin half-filled. This closed-shell or same-spin half-filled electronic structure provides an extra stabilisation and it is the origin of several rules of aromaticity such as the Hückel 4N+2 rule, the lowest-lying triplet excited state 4N Baird rule, the 4N rule in Möbius-type conformation, the 4N+2 Wade-Mingos rule in closo boranes or the 2(N+1)2 Hirsch rule in spherical aromatic species. Combinations of some of these rules of aromaticity can be found in some particular species. Examples of these combinations will be discussed and the validity of some of these rules will be assessed. Moreover, it is possible to establish connections with some of these rules of (anti)aromaticity and, therefore, they can be partially generalised, which represents a step forward to a future unified theory of aromaticity Financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) (Project No. CTQ2017-85341-P) and the DIUE of the Generalitat de Catalunya (Project No. 2017SGR39 and ICREA Academia 2014 to M.S.) is acknowledged. The EU under the FEDER grant UNGI10-4E-801 has also funded this research

Nonlinear optical properties (NLOPs) play a major role in photonics, electro-optics and optoelectronics, and other fields of modern optics. The design of new NLO molecules and materials has benefited from the development of computational tools to analyze the relationship between the electronic structure of molecules and their optical response. In this paper, we present a new means to analyze the response property through the partition of NLOPs in terms of orbital contributions (PNOC). This tool can be used to obtain a real-space representation of the NLOPs, providing a powerful visualization aid to connect the magnitude of the optical property with some parts of the molecule. Unlike other methods to analyze NLOPs, the PNOC decomposes the optical property into orbitals of the unperturbed system, furnishing this method with the ability to assess the performance of single- and multi-determinant electronic structure methods. PNOC can be also used to design small basis sets for an accurate description of large systems, saving a substantial amount of computer time for the calculation of optical properties This research has been funded by Spanish MINECO/FEDER Projects CTQ2014-52525-P and EUIN2017-88605, the Catalan DIUE 2014SGR931 and IKERBASQUE

In this work, using DFT + U formalism, we investigate the effect of order–disorder in the A-site occupation by La3+ and Sr2+ on the stability of the ferromagnetic order in La2/3Sr1/3MnO3 with R 3 - c symmetry. To date, a detailed theoretical discussion of such phenomenon, using a combination of different representations of the electronic structure, is still missing in the Literature. We employed structural models consisting of 120 atom supercells constructed according to the precise stoichiometry of the compound. Two configurations, describing randomized and ordered occupation of the La3+/Sr2+ ions, were evaluated. We demonstrate that the ferromagnetic arrangement of La2/3Sr1/3MnO3 with randomly distributed La3+ and Sr2+ ions is more stable. In such configuration we find that the Mn3+ and Mn4+ ions are not distinguished, favoring the double-exchange mechanism, enhanced by the higher degree of covalence in the Mn O bonds near the Fermi level between the spin-up Mn-eg orbitals and the O-p orbitals.

[ES] Dado su tamaño subnanométrico, los clusters metálicos están regidos por el confinamiento cuántico, lo que les hace más "moleculares" y menos "metálicos". En consecuencia, manifiestan propiedades que difieren con respecto a las de partículas más grandes del mismo elemento, y que a menudo son ventajosas para la catálisis de reacciones específicas. Además, su menor tamaño los hace más económicos, con una mayor superficie expuesta. Todo ello hace que los clusters sean opciones muy interesantes en catálisis, y su estudio, síntesis y aplicación ha crecido continuamente desde su descubrimiento en los años 90. Esta tesis se ha centrado principalmente en el cobre, del que se presenta, en primer lugar, un estudio fundamental sobre la disociación de oxígeno por clusters de diferentes tamaños. Después, se explora computacionalmente la catálisis de las oxidaciones de CO y propeno, confirmando que los clusters de Cu5 (o inferior) son prometedores para reacciones de oxidación. Las dos reacciones utilizadas son buenos ejemplos de la aplicación potencial en industria, sea para reducir emisiones de CO o para producir epóxido de propeno, que es un intermedio importante en la producción de plásticos y adhesivos, entre otros. Además, también se estudió la influencia de dos soportes en los clusters de cobre y su capacidad de oxidación: N-grafeno como un sistema más inerte y ceria como uno que puede participar activamente en reacciones de oxidación. Finalmente, se incluyen otros dos estudios más específicos, sobre la capacidad de los clusters de Pt3 y Pd3 para catalizar reaciones de acoplamiento C-C como la reacción de Heck, importante para la síntesis de productos de la química fina, y sobre la reacción CO + NO en clusters de Pt, motivado por su uso potencial como catalizadores para la conversión de esas especies en los menos perjudiciales CO2 y N2 en motores de combustión interna., [CA] Atès que són de grandària subnanomètrica, els clusters metàl·lics estan regits pel confinament quàntic, el qual els fa més "moleculars" i menys "metàl·lics". En conseqüència, manifesten propietats que són diferents a les de partícules més grans del mateix element, i que sovint són avantatjoses per a la catàlisi de reaccions específiques. A més a més, la seua menor grandària fa que siguen més econòmics, amb una major superfície exposada. Així, els clusters són una opció molt interesant en catàlisi, i el seu estudi, síntesi i aplicació ha cres-cut contínuament des del seu descobriment als anys 90. Aquesta tesi s'ha centrat principalment en el coure, del qual es presenta, en primer lloc, un estudi fonamental sobre la dissociació de l'oxígen per clusters de diferents grandàries. Després, s'explora computacionalment la catàlisi de les oxidacions de CO i de propè, confirmant que els clusters de Cu5 (o inferior) són prometedors per a reaccions d'oxidació. Les dues reaccions utilitzades són bons exemples de l'aplicació potencial en indústria, siga per reduir emissions de CO o per produir epòxid de propè, que és un intermedi important en la producció de plàstics i adhesius, entre altres. A més, també es va estudiar la influència de dos suports en els clusters de coure i la seua capacitat d'oxidació: N-grafè com a un sistema més inert i cèria com a un que pot participar activament en reaccions d'oxidació. Finalment, s'inclouen altres dos estudis més específics, sobre la capacitat dels clusters de Pt3 y Pd3 per catalitzar reaccions d'acoblament C-C com la reacció de Heck, important per a la síntesi de productes de la química fina, i sobre la reacció CO + NO als clusters de Pt, motivat pel seu ús potencial com a catalitzadors per a la conversió d'eixes espècies en els menys perjudicials CO2 i N2 als motors de combustió interna., [EN] Due to their subnanometric size, metal clusters belong to the regime affected by quantum confinement, which makes them more "molecular" and less "metallic". As a result, they exhibit properties that differ with respect to those of larger particles of the same element, and which are often advantageous in the catalysis of specific reactions. Besides, their smaller size makes them more economic and with a higher surface exposed. All of this renders metal clusters very interesting options for catalysis, and their study, synthesis and application has steadily increased since their discovery in the 90s. In this work we have largely focused on copper, of which a fundamental study on the oxygen dissociation by clusters of different sizes is first presented. Then, the catalysis of the CO and propene oxidation reactions is theoretically explored, confirming that Cu5 (or smaller) clusters are promising systems for oxidation reactions. The two reactions used are good examples of the potential application in industry, either to reduce CO emissions or to produce propene epoxide, an important intermediate in the production of plastics and adhesives, among others. In addition, the influence of two supports in the copper clusters and their oxidation capability is explored: on N-graphene as a more inert system and on ceria as one that can actively participate in oxidation reactions. Finally, two other more specific studies are included, regarding the capability of Pt3 and Pd3 clusters to undergo C-C coupling reactions such as the Heck reaction, important for the synthesis of many products of fine chemistry, and regarding the CO + NO reaction on Pt clusters, motivated by their potential use as catalysts for the conversion of those species in less harmful CO2 and N2 in internal combustion engines., En primer lugar me gustaría agradecer al Ministerio de Economía y Competitividad de España (MINECO) por la financiación de esta tesis mediante el programa Severo Ochoa (SVP-2013-068146), incluyendo los costes adicionales de mi estancia de investigación (EEBB-I-17-12057).

Perovskite-like materials which include magnetic elements have relevance due to the technological perspectives in the spintronics industry. In this work, the magnetic, structural and electronic properties of the Ba₂CoMoO₆ double perovskite are investigated. Calculations are carried out through the Full-Potential Linear Augmented Plane Wave method within the framework of the Density Functional Theory with exchange and correlation effects in the Generalized Gradient and Local Density approximations, including spin polarization. From the minimization of energy as a function of volume using Murnaghan’s state equation the equilibrium lattice parameter and cohesive properties of this compound were obtained. The study of the electronic structure was based in the analysis of the electronic density of states, and the band structure, showing that this compound evidences a conductive character for a spin channel and insulation for the other, and presents an integer value for the effective magnetic moment (3.0 μB), which allows it to be classified as a half-metallic material. The effects of pressure and temperature on thermophysical properties such as specific heat, Debye temperature, coefficient of thermal expansion and the Grüneisen parameter were calculated and analyzed from the state equation of the system. Obtained results reveal that, in the low-temperature regime, the specific heat at constant volume and pressure presents an analogous behavior to each other, with a tendency to the limit of Dulong-Petit typical of the structures of cubic perovskite-type, showing a value of 246.3 J/mol.K at constant volume and slightly higher values at constant pressure. The dependence of the thermal expansion coefficient, the temperature of Debye and the Grüneisen parameter with the increase in temperature are discussed in relation to other perovskite-like materials., Los materiales de tipo perovskita que incluyen elementos magnéticos tienen relevancia debido a las perspectivas tecnológicas en la industria de la espintrónica. En este trabajo se efectúa un estudio exhaustivo de las propiedades magnéticas, estructurales y electrónicas de la perovskita doble Ba₂CoMoO₆. Los cálculos se realizan a través del método de ondas planas aumentadas y linealizadas dentro del marco de la teoría del funcional de la densidad con efectos de intercambio y correlación en las aproximaciones del gradiente generalizado y de densidad local, incluyendo polarización de espín. A partir de la minimización de la energía en función del volumen, utilizando la ecuación de estado de Murnaghan, se obtuvieron el parámetro de red de equilibrio y las propiedades cohesivas de este compuesto. El estudio de la estructura electrónica se basó en el análisis de la densidad electrónica de estados y la estructura de bandas, mostrando que este compuesto evidencia un carácter conductor para un canal de espín y aislante para el otro, presentando un valor entero para el momento magnético efectivo (3.0 μB), que permite clasificarlo como un material espín-metálico. Los efectos de la presión y la temperatura sobre las propiedades termofísicas, como el calor específico, la temperatura de Debye, el coeficiente de expansión térmica y el parámetro Grüneisen, se calcularon y analizaron a partir de la ecuación de estado del sistema. Los resultados obtenidos revelan que, en el régimen de baja temperatura, el calor específico a volumen y presión constantes presenta un comportamiento análogo entre sí, con una tendencia al límite de Dulong-Petit típico de las estructuras de tipo perovskita cúbica, mostrando un valor de 246.3 J/mol.K a volumen constante y valores ligeramente más altos a presión constante. La dependencia del coeficiente de expansión térmica, la temperatura de Debye y el parámetro Grüneisen con el aumento de temperatura se discute en relación con otros materiales de tipo perovskita., Instituto de Física La Plata, Grupo de Estudio de Materiales y Dispositivos Electrónicos

Anions have often been considered to act essentially as electron donors or acceptors in molecular conductors. However there is now growing evidence that they play an essential role in directing the structural and hence electronic properties of many of these systems. After reviewing the basic interactions and different ground states occurring in molecular conductors we consider in detail how anions influence the structure of donor stacks and often guide them toward different types of transitions. Consideration of the Bechgaard and Fabre salts illustrates how anions play a crucial role in directing these salts through complex phase diagrams where different conducting and localized states are in competition. We also emphasize the important role of hydrogen bonding and conformational flexibility of donors related to BEDT-TTF and we discuss how anions have frequently a strong control of the electronic landscape of these materials. Charge ordering, metal to metal and metal to insulator transitions occurring in these salts are considered., Work in Spain was supported by MINECO (Spain) through Grants FIS2015-64886-C5-4-P and CTQ2015-64579-C3-3-P, and Generalitat de Catalunya (2017SGR1506, 2017SGR1289 and XRQTC). E. C. acknowledges support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant SEV-2015-0496. This review is based on recent collaborative works and pertinent discussions with N. Avarvari, P. Batail, C. Bourbonnais, C. Coulon, M. de Souza, M. Dressel, M. Fourmigue´, P. Foury-Leylekian, V. Ilakovac, D. Je´rome, M. Lang, E. Molins, Y. Nogami, C. Pasquier, H. Sawa and S. Tomic´.

In this doctoral research thesis we perform the calculation of the absorption coefficient and the relative refractive index change for the linear and its non-linear correction, due to the intense laser effect, for a double quantum well with asymmetry in the barriers. The asymmetric double quantum well is formed by cubic-phase Gallium Nitride (c-GaN) in the wells regions and by Gallium Aluminum Nitride, also in its cubic-phase, in the barriers with an aluminum concentration c-AlxGa1−xN. This heterostructure, due to the difference in its lattice constants, is strained, so in this work we present a comparative study of the optical properties of interest in order to determine the importance of the strain in the system, this was done when implementing the Van der Walle’s Model-Solid Theory. The electronic structure, once considered the effect of the strain on the height of the barriers, is done by means of the diagonalization method, using the solution for a rectangular well of infinite barriers of width L as the base of the expansion, all the computation was performed under the approximation of effective mass and parabolic band. The optical properties are calculated with well-established expressions in the literature in the frame of the formalism of the compact density-matrix. En esta investigación doctoral realizamos el cálculo del coeficiente de absorción y del cambio relativo del índice de refracción lineal y su corrección no lineal, debida al efecto de láser intenso, para un pozo cuántico doble con asimetría en las barreras. El pozo cuántico doble asimétrico está formado por Nitruro de Galio en fase cúbica (c-GaN) en la región de los pozos y por Nitruro de Galio Aluminio, también en su fase cubica, en las barreras con una concentración de aluminio c-AlxGa1−xN. Esta heteroestructura, por la diferencia en sus constantes de red, se encuentra tensionada, así que en este trabajo presentamos un estudio comparativo de las propiedades ópticas de interés con el fin de determinar la importancia de la tensión en el sistema, lo cual se hizo al implementar la Teoría del Sólido Modelo de Van der Walle. La estructura electrónica, una vez considerado el efecto de la tensión en la altura de las barreras, se hace mediante el método de diagonalización usando como base de la expansión la solución al pozo rectangular de barreras infinitas de ancho L, bajo la aproximación de masa efectiva y banda parabólica. Las propiedades ópticas se calculan con expresiones bien establecidas en la literatura que se obtienen mediante el formalismo de la matriz densidad compacta.

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2019, Tutor: Jordi Cirera Fernández, Metal-organic cages (MOCs) containing four Fe(II) metal centres ([Fe4]) represent a novel class of molecules that have attracted a lot of interest due to, among others, its efficiency in encapsulating greenhouse gases such as SF6. On the top of that, the [Fe4] molecules exhibit spin-crossover (SCO) behaviour, which can be tuned as a function of the guest molecule encapsulated in the system. Due to their size, conventional electronic structure calculations are not suited to study the origin of such interaction and its effect on the SCO behaviour and different computational methodologies need to be developed in order to study the nature of the host-guest interaction. First, by performing DFT calculations, the Fe-N bond will be modelled using a Morse potential. This data will be adjusted simultaneously against an energy scan along the totally symmetric normal mode in order to generate an ab initio force field for the molecule. Also, a series of electronic structure calculations will be carried out at CASSCF/NEVPT2 level to extract the relevant Angular Overlap Model parameters for a very simple molecule, [Fe(NH3)6]2+, representative of the Fe(II) environment in [Fe4] MOCs; necessary to construct the force field. In order to see if the results of [Fe(NH3)6]2+ can be extrapolated to more complex systems like the [Fe4] cages, the ammonia results will be compared with systems of increasing complexity yet closely related to the actual Fe environment in the [Fe4] system: [Fe(py)6]2+ and [Fe(C6H6N2)3]2+. Once validated, then the complete force field for the [Fe4] system will be assembled and tested. Finally, LFMM simulations will be run in the NVE, NVT or NPT/NσT ensemble to study the interaction between the [Fe4] system and the SF6 molecule in a large enough system so the crystal packing effects can be properly modelled

The electronic structure of AgCuO2, and more specifically the possible charge delocalization and its implications for the transport properties, has been the object of debate. Here the problem is faced by means of first-principles density functional theory calculations of the electron and phonon band structures as well as molecular dynamics simulations for different temperatures. It is found that both Cu and Ag exhibit noninteger oxidation states, in agreement with previous spectroscopic studies. The robust CuO2 chains impose a relatively short contact distance to the silver atoms, which are forced to partially use their dz2 orbitals to build a band. This band is partially emptied through overlap with a band of the CuO2 chain, which should be empty if copper were in a Cu3+ oxidation state. In that way, although structural correlations could roughly be consistent with an Ag+Cu3+O2 formulation, the appropriate oxidation states for the silver and copper atoms become Ag(1+δ)+ and Cu(3−δ)+, and as a consequence, the stoichiometric material should be metallic. The study of the electronic structure suggests that Ag atoms form relatively stable chains that can easily slide despite the linear coordination with oxygen atoms of the CuO2 chains. Phonon dispersion calculations and molecular dynamics simulations confirm the stability of the structure although pointing out that sliding of the silver chains is an easy motion that does not lead to substantial modifications of the electronic structure around the Fermi level and, thus, should not alter the good conductivity of the system. However, this sliding of the silver atoms from the equilibrium position explains the observed large thermal factors., This work was supported by MINECO (Spain) through Grant Nos. FIS2015-64886-C5-4-P, RTI2018-097753-B-I00, CTQ2015-64579-C3-3-P, and MAT2015-65192-R, Generalitat de Catalunya (2017SGR1506, 2017SGR1289, and XRQTC), and FONDECYT (Chile) through Project No. 11107163. E.C., N.C.-P., and A.G. acknowledge support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant No. SEV-2015-0496, and P.A. is supported from the Maria de Maeztu Units of Excellence Program under Grant No. MDM-2017-0767.

We report anomalies observed in photoelectron spectroscopy measurements performed on α-(BEDT-TTF)2I3 crystals. In particular, above its metal-insulator transition temperature (T = 135 K), we observe the lack of a sharp Fermi edge in contradiction with the metallic transport properties exhibited by this quasi-bidimensional organic material. We interpret these unusual results as a signature of a one-dimensional electronic behavior confirmed by DFT band structure calculations. Using photoelectron spectroscopy we probe a Luttinger liquid with a large correlation parameter (α > 1) that we interpret to be caused by the chain-like electronic structure of α-(BEDT-TTF)2I3 surface doped by iodine defects. These new surface effects are inaccessible by bulk sensitive measurements of electronic transport techniques., We acknowledge A. Santander-Syro, M. Gabay and P. Auban- Senzier for fruitful discussions. Work in Spain was supported by the Spanish Ministerio de Econom a y Competitividad (Grants FIS2015-64886-C5-4-P and CTQ2015-64579-C3-3-P) and Generalitat de Catalunya (2017SGR1506, 2017SGR1289 and XRQTC). E.C. acknowledges support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant SEV-2015-0496. P.A. acknowledges support from the Maria de Maeztu Units of Excellence Program under Grant MDM-2017-0767.

Resumen de la comnunicación La molécula de Tchischibabin1 representa un ejemplo de dirradical orgánico tipo kekulé. La química de dirradicales orgánicos ha tenido un renacer en los últimos años debido a las interesantes aplicaciones en electrónica orgánica. Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

We present detailed studies of the high-field magnetoresistance of the layered organic metal κ-(BETS)2Mn- [N(CN)2]3 under a pressure slightly above the insulator-metal transition. The experimental data are analyzed in terms of the Fermi surface properties and compared with the results of first-principles band structure calculations. The calculated size and shape of the in-plane Fermi surface are in very good agreement with those derived from Shubnikov-de Haas oscillations as well as the classical angle-dependent magnetoresistance oscillations. A comparison of the experimentally obtained effective cyclotron masses with the calculated band masses reveals electron correlations significantly dependent on the electron momentum. The momentum- or band-dependent mobility is also reflected in the behavior of the classical magnetoresistance anisotropy in a magnetic field parallel to layers. Other characteristics of the conducting system related to interlayer charge transfer and scattering mechanisms are discussed based on the experimental data. Besides the known high-field effects associated with the Fermi surface geometry, new pronounced features have been found in the angle-dependent magnetoresistance, which might be caused by coupling of the metallic charge transport to a magnetic instability in proximity to the metal-insulator phase boundary., We are grateful to N.D. Kushch for providing the high-quality crystals for our studies and to P.D. Grigoriev for numerous useful discussions. The work was supported by the German Research Foundation (DFG) via the Grant No. KA 1652/4-1. The high-field measurements were done under support of the LNCMI-CNRS, member of the European Magnetic Field Laboratory (EMFL). V.N.Z. acknowledges the support from RFBR Grant No. 18-02-00280. Work in Spain was supported by the Spanish Ministerio de Economa y Competitividad (Grants No. FIS2015-64886-C5-4-P and No. CTQ2015-64579-C3-3-P) and Generalitat de Catalunya (2017SGR1506, 2017SGR1289, and XRQTC). E.C. acknowledges support from the Severo Ochoa Centers of Excellence Program under Grant No. SEV-2015-0496. P.A. acknowledges support from the Maria de Maeztu Units of Excellence Program under Grant No. MDM-2017-0767.

Comunicación a congreso en forma de póster The importance of antiaromatic molecules in Chemistry and more recently in material science is rising. In the particular case of applications in electronics, the antiaromatic skeleton offers the favourable situation of high energy (i.e., strong destabilization) occupied molecular orbitals allowing to ease oxidation, doping or electron transfer to form conductive and photo-active substrates. Furthermore these systems show intriguing characteristics, such as paratropic ring current and π –bond localization, opposite the characteristics expected from aromaticity[1]. In this contribution, we will show new recently synthesized polycyclic compounds based on fused antiaromatic bis-phenylene which have resulted to be stable enough to be spectroscopically characterisable[2],[3]. We study two different isomer such as shown in Figure 1, either with linear arrangement of the rings or with angular disposition. The research is based on the study of neutral and oxidized species of the two molecules. Oxidation was carried out by electrochemistry and the generated species were analyzed by UV-Vis-NIR absorption spectroscopy. On the other hand, the redox forms were formed by chemical oxidation on which the Raman spectra were also obtained. All results are interpreted and guided theoretically through the use of DFT quantum chemical calculations. We conclude that oxidation generates the radical cation and the dication species which have completely different electronic and molecular structures which are in consonance with the varying degree of antiaromatic character. Stable cations and dications of antiaromatic molecules are scarce and therefore unexplored. We contribute to this knowledge by studying these in oligomers of biphenylene. Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech.

Se estudia la estructura electrónica de las perovskitas BiFeO3 e Y CrO3. Se utilizó una disposición antiferromagnética para los átomos de ambos materiales. Para el BiFeO3 se utilizaron los arreglos antiferromagnéticos tipo A y G. Para el Y CrO3 se utilizaron los arreglos antiferromagnéticos tipo A, C y G. El trabajo se desarrolló en el marco de la teoría del funcional de densidad, para lo cual se utilizó el paquete de simulación Quantum Espresso y pseudopotenciales ultrasuaves junto a la aproximación de densidad local considerando además al parámetro de Hubbard con un valor de 2,43 eV y 1,13 eV para los átomos de hierro y cromo respectivamente. Para el BiFeO3 se obtuvo un gap de energía de 1,4 eV para el arreglo tipo A y un gap de 1,8 eV para el arreglo tipo G. Para el Y CrO3 se obtuvo un gap de energía de 1,30 eV para el arreglo tipo A, para el arreglo tipo C se obtuvo un gap de 1,32 eV y para el arreglo tipo G se obtuvo un gap de 1,6 eV. Se determinó que el arreglo antiferromagnético tipo G es el más estable para ambos materiales. Tesis