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Applied cutting-edge electronic structure and phonon simulations provide a reliable knowledge about the stability of perovskite structures and their electronic properties, which are crucial for des...

The electronic structure of a natural pyrite sample from Navajun, Spain was studied using both core-level x-ray and valence band ultraviolet photoelectron spectroscopies (XPS and UPS, respectively)...

Biomaterials play a crucial role in tissue engineering as a functional replacement, regenerative medicines, supportive scaffold for guided tissue growth, and drug delivery devices. The term biomaterial refers to metals, ceramics, and polymers account for the vast majority. In the case of polymers, hydrogels have emerged as active materials for an immense variety of applications. Epoxy-based hydrogels possess a unique network structure that enables very high levels of hydrophilicity and biocompatibility. Hydrogel such as Medipacs Epoxy Polymers (MEPs) models were constructed to understand water’s behavior at the water/hydrogel interface and hydrogel network. We computed the Gibbs dividing surface (GDS) to define the MEP/water interface, and all the physicochemical properties were computed based on GDS. We calculated the radial distribution function (RDF), the 2D surface roughness of the immersed MEPs. RDF analysis confirmed that the first hydration shell is at a distance of 1.86 A, and most of the water molecules are near the hydroxyl group of the MEPs network. Hydrogen bonds (H-bonds) analysis was performed, and the observation suggested that the disruption of the H-bonds between MEP chains leads to an increase in the polymer matrix’s void spaces. These void spaces are filled with diffused water molecules, leading to swelling of the MEP hydrogel. The swelling parameter was estimated from the fitted curve of the yz-lattice of the simulation cell. The MEP/water interface simulation results provide insightful information regarding the design strategy of epoxy-based hydrogel and other hydrogels vital for biomedical applications.

Novel mono- and bimetallic nanoparticles (MNPs and BNPs) decorated surface of perovskite-type KTaO3 photocatalysts were successfully synthesized by hydrothermal reaction of KTaO3 followed by photodeposition of MNPs/BNPs. The effect of noble metal type (MNPs = Au, Ag, Pt, Pd, Rh, Ru or BNPs = Au/Pt, Ag/Pd, Rh/Ru), amount of metal precursor (0.5, 1.0, 1.5 or 2.0 wt%) as well as photoreduction method (simultaneous (both) or subsequent (seq) deposition of two metals) on the physicochemical and photocatalytic properties of MNPs- and BNPs-KTaO3 have been investigated. All as-prepared photocatalysts were subsequently characterized by UV–Vis diffuse reflectance spectroscopy (DRS), Brunauer-Emmett-Teller (BET) specific surface area and pore size distribution measurement, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (PXRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) emission spectroscopy. The crystal structure was performed using visualization for electronic and structural analysis (VESTA). The photocatalytic activity under Vis light irradiation was estimated in phenol degradation in aqueous phase and toluene removal in gas phase, while under UV–Vis light irradiation was measured amount of H2 generation from formic acid solution. The absorption properties of O2 and H2O molecules on KTaO3(1 0 0) surface supported by Au or Au/Pt NPs was also investigated using density-functional theory (DFT). The experimental results show that, both MNPs-KTaO3 and BNPs-KTaO3 exhibit greatly enhanced pollutant decomposition efficiency under Vis light irradiation and highly improved H2 production under UV–Vis light irradiation compared with pristine KTaO3. MNPs deposition on KTaO3 surface effects by disperse metal particle size ranging from 11 nm (Ru NPs) to 112 nm (Au NPs). Simultaneous addition of Au/Pt precursors results in formation of agglomerated larger metal nanoparticles (50–100 nm) on KTaO3 surface than subsequent deposition of Au/Pt with composition of concentrated smaller metal nanoparticles (>50 nm) on KTaO3 surface. The 0.5 Au/1.5 Pt-KTaO3_both and 2.0 Rh-KTaO3 reveal the highest Vis-induced activity among prepared samples in aqueous phase (14.75% of phenol decomposition after 90 min of irradiation) and gas phase (41.98% of toluene removal after 60 min of irradiation), respectively. The theoretical calculations confirmed that adsorption energy of O2 and H2O molecules was increased after loading of Au or Au/Pt NPs on KTaO3(1 0 0) surface. Control tests with scavengers show that O2 − radical is significantly involved in phenol oxidation under Vis light irradiation, which proposed mechanism is based on direct electron transfer from MNPs/BNPs to conduction band of KTaO3. The highest amount of H2 evaluation is obtained also by 0.5 Au/1.5 Pt-KTaO3_both after 240 min of UV–Vis light irradiation (76.53 µmol/min), which is eleven times higher than for pristine KTaO3 (6.69 µmol/min). Moreover, the most photocatalytic samples for each model reaction present good repeatability and stability after subsequent three cycles. Summarized, MNPs- and BNPs-KTaO3 are promising material in advanced applications of photocatalysis.

Novel rare earth-doped K2Ta2O6 (RE-K2Ta2O6) photocatalysts were successfully synthesized by one-step hydrothermal method. The effect of dopant type (RE = Y, Yb, Ho, Pr, Er) and amount of rare earth precursor (2, 4, 8 and 10 mol%) on the physicochemical and photocatalytic properties of RE-K2Ta2O6 have been investigated. All as-prepared materials were subsequently characterized by UV–vis diffuse reflectance spectroscopy (DRS), Brunauer-Emmett-Teller (BET) specific surface area measurement, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, mass magnetic susceptometry and photoluminescence (PL) emission spectroscopy. The photocatalytic activity under UV–vis light irradiation was estimated in phenol degradation in aqueous phase, toluene removal in gas phase and H2 generation from formic acid solution. The experimental results show that, novel RE-K2Ta2O6 exhibits greatly improved degradation efficiency under UV–vis light irradiation compared with pristine K2Ta2O6. The Er-K2Ta2O6 and Pr-K2Ta2O6, obtained by introducing of 2 mol% of RE ions during synthesis, reveal the highest photocatalytic activity among prepared samples in aqueous phase (33% of phenol decomposition after 90 min of irradiation) and gas phase (45% of toluene removal after 60 min of irradiation), respectively. Moreover, both photocatalysts present good stability after subsequent three cycles. The active species trapping test shows that OH and O2 − radicals are significantly involved in phenol oxidation under UV–vis light irradiation. The amount of H2 evolution increases with increasing addition of Er dopant into K2Ta2O6 lattice. The highest H2 production is obtained for 10 mol% Er-K2Ta2O6 after 240 min of UV–vis light irradiation (15.40 μmol/min). Enhanced photoactivity performance can be attributed to incorporation of RE ions at K+ lattice site in RE-K2Ta2O6, probably leading to formation of new RE 4f states below the conduction band of K2Ta2O6 structure. To investigate the localization of RE ions in K2Ta2O6 structure, the band structure and partial density of the states (PDOS) have been investigated. Computer simulations were performed using plane-wave based Vienna ab-initio simulation package (VASP) with the generalized gradient approximation (GGA) by Perdew-Burke-Ernzerhof (PBE). Moreover, inclusion of RE ions in K2Ta2O6 causes predominance pyrochlore phase formation over perovskite in regular cubic structure. Summarized, RE-doped K2Ta2O6 is promising material in photocatalytic degradation of organic pollutants and H2 generation processes. Our work may provide valuable information for rare earth doping semiconductor with improved photocatalytic performance.

Applications of self-assembled monolayers (SAMs) on surfaces are prevalent in modern technologies and drives the need for a better understanding of the surface domain architecture of SAMs. To explore structural interaction at the interface between gold surfaces and a hydroxyl-terminated alkanethiol, 11-hydroxy-1-undecanethiol, (C11TH) we have employed a combined computational and experimental approach. Density functional theory (DFT) calculations were carried out on the thiol–gold interface using both the Perdew–Burke–Ernzerhof (PBE) and van der Waals (optB86b) density functionals. Our ab initio molecular dynamics (AIMD) simulations revealed that the interface consists of four different distinguished phases, each with different C11TH orientations. Experiments involved deposition of C11TH SAMs onto gold, with the resultant surfaces examined with X-ray photoelectron spectroscopy (XPS) and ellipsometry. Weighted average projected density of states (PDOS) of the different phases were photoionization cross section corrected and these were confirmed by experimental XPS data. Computed molecular parameters including tilt angles and the thickness of SAMs also agreed with the XPS and ellipsometry results. Hydrogen bonding arising from the terminal hydroxyl groups is the primary factor governing the stability of the four phases. Experimental results from XPS and ellipsometry along with DFT simulation results provide insights into the formation of the different orientations of SAM on Au(111) which will guide future efforts in the self-assembled SAMs architecture for other thiols or metal substrates.

One of the challenges in the field of heterogeneous photocatalysis is the development of new type of photoactive materials activated by low-power and low-cost irradiation sources, and to understand the mechanism of photocatalytic reaction in the presence of those materials. Photocatalysis based on Er-TiO2 has become an attractive process to remove contaminants from aquatic environments. Unfortunately, there is no available literature presenting the systematic experimental and theoretical study of the effects of erbium ions on the photocatalytic activity of TiO2 and visible light excitation mechanism of this system. Accordingly, to obtain solar-responsive photocatalysts, nanostructured Er-TiO2 was prepared via hydrothermal method. The physicochemical characterization of samples indicate that modified photocatalysts contain both doped erbium and the surface adsorbed erbium species (e.g., Er2O3, Er(OH)x). Predicted electronic structures of Er-doped TiO2 indicate that the formation of new Er-4f states within the band gap region of TiO2 might be crucial for observed photocatalytic effects towards Vis response. However, the up-conversion process is not responsible for Vis activity and efficient O2 − generation. Moreover, Er modification of TiO2 results in an inhibition of the growth of titania crystallites, and thus an increase in the specific surface area, which are also beneficial for an efficient degradation of pollutants.

Bismuth tungstate (Bi2WO6) can work as a photocatalyst but suffers from rapid recombination of photogenerated charge carriers. Herein, density functional theory (DFT) simulations revealed that the formation of a thermodynamically stable AgInS2(112)/Bi2WO6(010) heterojunction could promote charge separation and enhance the photoactivity of Bi2WO6. To confirm these theoretical predictions, a new type of photocatalysts in the form of Bi2WO6 flower-like microspheres decorated with different amounts of AgInS2 quantum dots (QDs) was obtained using a three-step procedure. The optimized system, obtained by embedding 1 wt% AgInS2 QDs on a Bi2WO6 matrix, possessed an enhanced photocatalytic activity for both phenol degradation and water splitting under visible light irradiation (λ > 420 nm), as well as good reusability and stability during prolonged storage. Finally, DFT calculations of the adsorption energies of reagents (O2, H2O, and H2 molecules) on Bi2WO6 and AgInS2/Bi2WO6 surfaces showed that the surface of the AgInS2(112)/Bi2WO6(010) interface was more active, allowing this system to strongly interact with surrounding species such as H2, O2, and H2O and thereby inducing photocatalytic oxidation of OH– to •OH, reduction of O2 to O2•– or reduction of H+ to H2.

Using density-functional theory, we have computed the structural and electronic properties of paramagnetic vivianite crystal Fe3(PO4)2·8H2O and its (010)-(1 × 1) and (100)-(1 × 1) surfaces. The properties of bulk vivianite are studied with a set of functionals: HSE06, PBE, AM05, PBEsol, and PBE with on-site Coulomb repulsions corrections (PBE+U). The appropriate U parameter is estimated by considering the HSE06 results, and it is used to study the vivianite surfaces. The computed surface energy predicts the (010) surface to be the most stable. The less stable (100) surface is observed to have important reconstructions with the spontaneous formation of a water molecule at the surface and two hydroxide hydrate anions per unit cell. Using thermodynamical considerations within DFT, we have calculated the phase diagram of the (010) surface in equilibrium with hydrogen gas. The results suggest that under ultralow hydrogen pressure, the (010) surface with two hydrogen vacancies is stable. The electronic structur...

Considerable interest is focussed on high-k dielectrics as replacements for the gate oxide in future MOSFETs. Atomic Layer Deposition (ALD) is the method of choice to produce conformal thin films for the gate dielectric, but a deeper understanding of this method is needed for process optimisation. For ALD of alumina, we use first principles density functional (DFT) calculations to describe the surface intermediates and pathways of precursor adsorption/decomposition at the atomic scale, yielding quantitative reaction energetics. This reveals the intrinsic limits on ALD growth rate as a function of OH coverage.

The present work focuses on the superconductivity phenomenon in La-doped SrTiO3 crystal, and the Jahn-Teller (JT) polarons in BaTiO3 crystal. In the case of SrTiO3 crystal this study shows the occurrence of free electrons in the conduction band of the crystal and explains a number of the experimental results obtained for the superconducting La-doped SrTiO3 existing under 0.3K. Free electrons are found to be situated within the Ti-O planes similar as in the case of the high temperature superconducting (HTS) cuprates. Results obtained for the JT polarons in BaTiO3 crystal explain a number of experimental observations and coincide with the other results obtained independently by the polaron theory. A phenomenological model explaining a bipolaron formation in titanates is suggested and analysed in the present work.

We have studied computationally the CdF 2 crystal doped with Al, Ga, In, Sc and Y impurities, using the density-functional theory and the ultrasoft pseudopotential method. In particular, we focus on the bistability behavior of the dopants. The formation energies, ionization levels and spatial configurations are calculated. For bistable donor configurations (DX centers), the overlap populations and energy barriers are also estimated. The results obtained for the DX centers associated with Ga and In atoms in the negative charge state are in a good agreement with previous works. We predict the existence of bistability in the case of the Sc-doped CdF 2 . In all the investigated cases for the DX centers we find an antibonding state between the impurity and the nearest Cd atoms.

Self-trapped hole polarons in technologically important perovskite-type ceramic of BaTiO3 have been modeled by means of the quantum chemical method modified for crystal calculations. The computations are carried out in the self-consistent field (SCF) manner using the embedded molecular cluster model. The spatial configuration of a hole polaron, displacement of defect-surrounding atoms, and wave functions of the polaron ground and excited states are obtained and analyzed. The probability of spontaneous hole self-trapping is estimated in the perfect lattice of the BaTiO3 crystal by calculating the value of the hole self-trapping energy as a difference of the atomic relaxation energy and the hole localization energy. This value is found to be negative, −1.49 eV, which demonstrates the preference of the self-trapped polaron state. The calculated polaron absorption energy, 0.5 eV, is discussed in light of the available experimental data. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 79: 358–366, 2000

First principles density functional theory and generalised gradient approximation (GGA) have been exploited to investigate Fe-doped ilmenite-type MgSiO 3 mineral. Strong electron correlation effects not included in a density-functional formalism are described by a Hubbard-type on-site Coulomb repulsion (the DFTU approach). Microstructure of equilibrium geometries, electronic band structures as well as magnetic properties are computed and discussed in detail. Hartree-Fock methodology is used as an extra tool to study optical properties of the same system. For equilibrium state of the doped mineral we find zigzag-type atomic rearrangements around the Fe impurity. The inclusion of correlation effects leads to an improved description of the electronic properties. In particular, it is discovered that Fe incorporation produces local energy levels within the band-gap of the material. Using ?SCF method optical absorption energies are found to be equal to 2.2 and 2.6 eV leading to light absorption at longer wavelengths compared to the undoped MgSiO 3. Our results provide evidence on the occurrence of local magnetic moment in the region surrounding iron dopant. According to the outcomes, the Fe?Mg reaction can be described as substitutionally labile with Fe 2 complex being found in the high-spin state at low pressure MgSiO 3-ilmenite conditions. © 2012 Elsevier B.V. All rights reserved.

We investigate, experimentally and theoretically, the initial stage of the formation of Ca/Si and Si/F structures that occurs during the adsorption of CaF${}_{2}$ molecules onto a bare Si(100) surface heated to 1000 K in a low-coverage regime (0.3 monolayer). A low-temperature (5 K) scanning tunneling microscope (STM) is used to observe the topographies and the electronic properties of the exposed silicon surfaces. Our atomic-scale study reveals that several chemical reactions arise during CaF${}_{2}$ deposition, such as dissociation of the CaF${}_{2}$ molecules and etching of the surface silicon dimers. The experimental and calculated STM topographies are compared using the density functional theory, and this comparison enables us to identify two types of reacted structures on the Si(100) surface. The first type of observed complex surface structure consists of large islands formed with a semiperiodic sequence of 3 \ifmmode\times\else\texttimes\fi{} 2 unit cells. The second one is made of isolated Ca adatoms adsorbed at specific sites on the Si(100)-2 \ifmmode\times\else\texttimes\fi{} 1 surface.

We have performed density functional calculations with on-site Coulomb repulsion corrections of systems that may be involved in the Verwey transition in magnetite (Fe(3)O(4)). We find that the lowest energy solution for the minority spin wavefunction in the cubic cell involves orbitally ordered Fe-d and O-p states, which breaks cubic symmetry. This leads to partial charge ordering that triggers a Jahn-Teller distortion and band-gap opening. Our results show this to be the essential mechanism of the Verwey transition. Applying ionic relaxation within a larger tetragonal cell, three patterns of charge ordering are compared and a Pmca pattern matching x-ray data is found to be the most stable.

Hole self-trapping in technologically important BaTiO 3 and SrTiO 3 crystals is studied using a quantum-chemical method developed for periodic systems, e.g., crystals. The computations are carried out in the self-consistent-field manner using the embedded molecular cluster model. The possibility of spontaneous hole self-trapping is estimated by employing an analytical procedure based on the energetical favourability principle. Our results support the self-trapping of radiation-induced hole in BaTiO 3 crystals while such an outcome is not predicted for SrTiO 3 crystals. Computational results are discussed in the light of available experimental data on light-induced absorption spectra measurements in BaTiO 3 crystals.

From an interplay between simultaneously recorded noncontact atomic force microscopy and scanning tunneling microscopy images and simulations based on density functional theory, we reveal the location of single hydrogen species in the surface and subsurface layers of rutile ${\mathrm{TiO}}_{2}$. Subsurface hydrogen atoms (${\mathrm{H}}_{\mathrm{sub}}$) are found to reside in a stable interstitial site as subsurface OH groups detectable in scanning tunneling microscopy as a characteristic electronic state but imperceptible to atomic force microscopy. The combined atomic force microscopy, scanning tunneling microscopy, and density functional theory study demonstrates a general scheme to reveal near surface defects and interstitials in poorly conducting materials.

In this work we investigate some of the key factors in simultaneously recorded scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) images of the TiO(2)(110) surface, particularly the role of tip size and orientation in the obtained contrast pattern, and the importance of tip-surface relaxations and surface impurities in measured currents. We show that, while using multi-channel scanning modes provides an increase in physical data from a given measurement and greatly aids in interpretation, it also demands much greater rigor in simulations to provide a complete comparison.

The low-temperature (12 K) adsorption of single Au atoms on Si(100) is studied by scanning tunneling microscopy (STM). Comparison between experimental and calculated STM topographies as well as density-functional-theory calculations of the adsorption energies enable us to identify two adsorption configurations of Au atoms between Si-dimer rows (BDRs) and on top of Si-dimer rows (TDRs). In both adsorption configurations, the Au atoms are covalently bound to two Si atoms through a partial electron transfer from Si to Au. STM manipulation confirms that the TDR adsorption configuration is metastable, whereas the BDR one is the most stable configuration.

The electronic structure, chemical bonding, geometry, and effects produced by Sr-doping in CaCO3 have been studied on the basis of density-functional theory using the VASP simulation package and molecular-orbital theory utilizing the CLUSTERD computer code. Two calcium carbonate structures which occur naturally in anhydrous crystalline forms, calcite and aragonite, were considered in the present investigation. The obtained diagrams of density of states show similar patterns for both materials. The spatial structures are computed and analyzed in comparison to the available experimental data. The electronic properties and atomic displacements because of the trace element Sr-incorporation are discussed in a comparative manner for the two crystalline structures. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2008

In this article, we study the origin of the corrugating and anticorrugating states through the electronic properties of the Si(1 0 0) surface via a low-temperature (9 K) scanning tunneling microscope (STM). Our study is based on the analysis of the STM topographies corrugation variations when related to the shift of the local density of states (LDOS) maximum in the [Formula: see text] direction. Our experimental results are correlated with numerical simulations using the density-functional theory with hybrid Heyd-Scuseria-Ernzerhof (HSE06) functional to simulate the STM topographies, the projected density of states variations at different depths in the silicon surface as well as the three dimensional partial charge density distributions in real-space. This work reveals that the Si(1 0 0) surface exhibits two anticorrugating states at +0.8 and +2.8 V that are associated with a phase shift of the LDOS maximum in the unoccupied states STM topographies. By comparing the calculated data with our experimental results, we have been able to identify the link between the variations of the STM topographies corrugation and the shift of the LDOS maximum observed experimentally. Each surface voltage at which the STM topographies corrugation drops is defined as anticorrugating states. In addition, we have evidenced a sharp jump in the tunnel current when the second LDOS maximum shift is probed, whose origin is discussed and associated with the presence of Van Hove singularities.

Starting from the theoretical prediction of the $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ structure using density-functional theory in the generalized gradient approximation, we have studied the (1 1 1), (0 0 1), (1 1 0), and (1 5 0) surfaces. The surface energies and their corresponding structures are computed and compared with predictions for (0 0 0 1) $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ and available experimental results for $\ensuremath{\gamma}$-alumina surfaces. (1 1 1) and (0 0 1) surfaces are predicted to be equally stable, but to show quite different structure and reactivity. Whereas a low coverage of highly reactive trigonal Al occurs on (1 1 1), (0 0 1) exhibits a more dense plane of both five-coordinate and tetrahedral Al. Microfaceting of a (1 1 0) surface into (1 1 1)-like planes is also observed. The implications for the structure of ultrathin dielectric films and for the surfaces of disordered transition aluminas are discussed.

Using the density functional theory (DFT) within the local density approximation (LDA) and a method based on the Hartree-Fock (HF) approximation, we study the structural, electronic and optical properties of F-centers (two electrons in an oxygen-vacancy) in the tetragonal lattice of the perovskite-type BaTiO3 crystal. In this structure the F-center has two non-equivalent positions due to the two different O atoms; namely, the F-center situated in the Ti-O-Ti chains along the [001] polarization axes (Va0) and the F-center situated in the Ti-O-Ti chains within the x-y plane (Vb0). Both cases are considered in our study. The obtained results point out that the crystal structure containing Va0 center is energetically more stable than that of the Vb0) center. The two electrons of the F-center are found to be localized on the two adjacent Ti ions and their ground state is composed mainly of Ti 3dz2 atomic orbital. Besides, we have computed the allowed optical transitions for the F-center using the so-called ΔSCF method. As a result the obtained absorption energies are found to be equal to 1.47 eV for the Va0 center as well as 3.07 and 4.27 eV for the Vb0 center.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The controlled growth of alumina films by atomic layer deposition (ALD) is of great interest to the electronics industry, as high-k dielectrics are being sought for the next-generation of MOSFETS. We present a theoretical study of the alumina polymorph γ-Al2O3 based on density functional theory (DFT). The predicted bulk structure for γ-Al2O3 has the Al vacancy sites widely separated, which is in agreement with other theoretical predictions. We estimated the energy of several γ-Al2O3 surfaces namely: (111), (110) and (110). The atomic and electronic structure of the most stable (111) surface is discussed and compared with α-Al2O3 (0001). The adsorption and dissociation of the H2O onto the (111) surface is considered and H-diffusion is simulated. Finally, a model for the γ-Al2O3 (111)/aluminium-hydroxide interface is proposed and considered as an intermediate stage in the γ-alumina film growth mechanism.

We have used a quantum-chemical method developed for crystal calculations to investigate self-trapped hole polarons in technologically important BaTiO 3 perovskite-type crystal. The tetragonal structure of this material is considered in the present work, The computations are carried out in a self-consistent-field manner using the embedded molecular cluster model. The spatial configuration of a hole polaron and displacements of defect-surrounding atoms are obtained and analyzed. The probability of spontaneous hole self-trapping in a perfect crystal lattice is estimated by calculating the hole self-trapping energy as a difference of the atomic relaxation energy and the hole localization energy. This value is found to be negative, -0.87 eV, which demonstrates the preference of the self-trapped polaron state compared to a free hole state in the valence band. The computed polaron absorption energy, 0.4 eV is found to be in an excellent agreement with the available experimental data and independent estimations of the polaron theory.

The BaTiO3 and SrTiO3 crystals have many technological applications due to their unusual magnetic and dielectric properties. Using the INDO method modified for crystals which had shown reliability in a number of applications, we study self-trapped and impurity-trapped polarons in cubic lattices of both materials and La-doping of the SrTiO3 crystal. The obtained results are discussed in terms of the available experimental data.

Using first-principles density functional theory calculations within the generalized gradient approximation (GGA) as well as the GGA+U approach, we study Al-doped ?-Fe 2O 3 crystals. Structural, electronic, magnetic and optical properties due to impurity incorporation have been investigated and discussed in detail. Atomic displacements and Bader charges on atoms have been computed, showing that Al dopant converts the chemical bonding in its neighbourhood into a more ionic one. This work enhances our knowledge about how a crystalline lattice reacts in the presence of an Al impurity. It was found that Al incorporation produces some local changes in the band structure of the material without the creation of local energy levels within the band gap. The results provide evidence for changes in the magnetic moments in the vicinity of a defect, which means that ?-Fe 2O 3 doped with aluminum might not act as an antiferromagnetic substance.