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A comparative analysis of the electronic structure obtained in the DFT/LDA and LDA + DMFT approaches of the possible isostructural analogues of iron superconductors InCo2As2 and KInCo4As4 with the electronic structure of the parent high-temperature superconductor system BaFe2As2 is carried out. It is established that in spite of the rather large value of the electron-electron correlations (local Coulomb interaction on the Co-$$3d$$ shell $$U = 4.0$$ eV, the Hund exchange interaction $$J = 0.85$$ eV), in the considered systems a relatively small quasiparticle mass renormalization 1.2–1.35 at the Fermi level is observed. The correlation effects lead to the remarkable shift and compression of the spectrum below –0.8 eV. The band structure of InCo2As2 near the Fermi level is qualitatively similar to the previously studied BaCo2As2, and differs significantly from the band structure of BaFe2As2. In the KInCo4As4 system, the bands near the Fermi level resemble the band structure of BaFe2As2, and the Fermi surfaces have a similar topology. This indirectly points to the possibility of superconductivity in KInCo4As4. Also according to the results of LDA + DMFT calculations it is seen that with a rather small hole or electron doping in the KInCo4As4 system will experience topological Lifshitz transitions. We believe that the synthesis of the InCo2As2 and KInCo4As4 compounds considered in this paper is important for the study of superconductivity in this class of materials.

The photocatalytic properties of Bi2-xTi2O7-1.5x (x = 0, 0.5) pyrochlores are examined via ab initio calculations and experiments. A coprecipitation method is applied for the synthesis of nanopowder pyrochlores. The pyrochlore phase formation starts at 500 °C (Bi2Ti2O7) and 550 °C (Bi1.5Ti2O6.25). Nanopowders are found to be a metastable character of pyrochlore phases. The presence of bismuth and oxygen vacancies enhances the thermal stability of the Bi1.5Ti2O6.25 phase in comparison with the Bi2Ti2O7 phase. The estimated crystallite size is 30-40 nm with noticeable agglomerates of about 100-300 nm according to scanning electron microscopy (SEM) and with the formation of particles (510-580 nm) in the aqueous medium. The isoelectric points of the nanopowders seem to be shifted to the strongly acidic region, resulting in the formation of negative surface particle charges of -33 mV (Bi2Ti2O7) and -27 mV (Bi1.5Ti2O6.25) at pH 5.88 in distilled water. The specific surface area is 11.5 m2/g (Bi2Ti2O7) and 12.00 m2/g (Bi1.5Ti2O6.25). The use of the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional allows achieving an excellent agreement between theoretical and experimental structural parameters. The screened Coulomb hybrid HSE03 functional is the most appropriate for describing the optoelectronic properties. Bismuth titanate pyrochlores are wide-gap semiconductors with strong abilities to be active photocatalysts under visible irradiation. The optical Eg values for direct/indirect transition according to the experiment, 3.19/2.94 eV (x = 0) and 3.24/3.03 eV (x = 0.5), and the DFT/HSE03 calculations, 2.92/2.87 (x = 0) and 3.42/- eV (x = 0.5), are in the visible light region and are close. The calculated low effective masses of the charge carriers and suitable band edge positions confirm the ability of the pyrochlores to act as photocatalysts. The photocatalytic activity has been evaluated through the decomposition of rhodamine B under visible irradiation. Bi2Ti2O7 shows the highest activity in comparison with Bi1.5Ti2O6.25, which is in good agreement with theoretically predicted and experimentally revealed characteristics.

Structural and electronic properties, Fermi surface topology, and chemical bonding features in layered 1111-oxyarsenides LaRhAsO and LaIrAsO are studied and compared using ab initio simulations. The nonmagnetic metal LaIrAsO is found to be weakly sensitive to both electronic and hole doping. However, the Rh-containing phase is predicted to exhibit weak band magnetism and can turn into a nonmagnetic state by hole doping. Therefore, the LaRhAsO oxyarsenide can be considered as a possible “electronic” analogue of the LaFeAsO compound, the basic phase of layered FeAs superconductors.

Combined ab initio and experimental study of Cr doping into bismuth titanate pyrochlore was carried out for the first time. Accurate first-principles density functional theory calculations were performed considering a Hubbard U correction (DFT+U) to account for on-site Coulomb interactions of the Cr 3d states. The possibility to synthesize a novel pyrochlore-type compound with a high dopant content Bi1.5Cr0.5Ti2O7 (Bi site doping) in a fine powder state was shown via coprecipitation method, while the single-phase Bi2Ti1.5Cr0.5O7 (Ti site doping) could not be obtained. Detailed descriptions of thermostability, structural, optoelectronic, and magnetic properties of Cr-doped pyrochlores in the fine (particles size 100-300 nm) and "bulk" (1-50 μm) powder states are presented based on the well-matched results of theoretical and experimental investigations. According to the Rietveld refinement of the X-ray diffraction data, Bi1.5Cr0.5Ti2O7 compound is an A-site deficient pyrochlore (Bi1.38Cr0.30)(Ti1.84Cr0.16)O6.44 with chromium distribution between both cationic sites. Metastability of fine powder Cr-containing pyrochlore phase was revealed during long-thermal annealing, while the bulk powder sample was stable up to its melting point 1230 °C. According to the study of electronic structure and optical properties, Cr-doped pyrochlores are wide-band semiconductors with light absorption in the range of 300-500 nm and perspective as photocatalytic active materials under visible light irradiation. Paramagnetic behavior with effective magnetic moment 3.92 μB (Bi1.6Cr0.1Ti2O7-δ) and 3.01 μB (Bi1.5Cr0.5Ti2O7) was experimentally observed. All chromium in magnetically diluted pyrochlore Bi1.6Cr0.1Ti2O7-δ exists in the form of Cr3+ monomers, whereas in the more concentrated magnetic Bi1.5Cr0.5Ti2O7 composition Cr3+-O-Cr3+ dimers may also be present, with a fraction equal to 0.39. This investigation constitutes the first approach to the electronic, structural, optical, and magnetic properties of d-elements doped bismuth titanate pyrochlores from experimental and theoretical viewpoints, emphasizing the power of DFT+U to provide insights and to complement the experimental characterization of these new compounds.

The energies of formation of vacancies in the carbon and silicon sublattices, the independent elastic constants, the all-round compression, shear and Young’s moduli, and the anisotropy coefficients are determined for the complete and nonstoichiometric cubic phases of 3C-SixCy (x, y = 1.0–0.75) by ab initio methods of the band theory. In the formalism of the density functional perturbation theory (DFPT), the phonon dispersion dependences are obtained for these phases (the comparison with the experiment is given for the complete phase). It is shown that the mechanical characteristics of the phases become strongly anisotropic upon the transition from 3C-SiC0.875 to 3C-SiC0.75. It is established from the analysis of the phonon dispersion curves that the 3C-SiC0.875 and 3C-SiC0.75 phases, in contrast to the complete 3C-SiC phase, are dynamically unstable at T = 0 K.

Prepared by the precursor method, lithium-doped cadmium oxide samples were synthesized using mixed formates of Cd 1- x Li x (HCOO) 2 ·2H 2 O (0 ≤ x ≤ 0.075) as a precursor. NMR spectra were obtained on the lithium nuclei incorporated in cadmium oxide; and it is demonstrated that there is only one type of impurity centers to be found. The first-principle method using a projector augmented wave (PAW) approach is applied to carry out electronic band structure calculations and to analyze optical absorption spectra for the cases of lithium atoms occupying interstitial sites and of lithium atoms substituting for cadmium or oxygen atoms. Calculations are carried out to determine energy-efficient preferences for sites of impurity centers, which indicate as the most probable the presence of lithium atoms at the interstitial sites, with vacancies observed in the cadmium sublattice. The presence of lithium atoms at the interstitial sites appears to be explaining the conductivity increase and the red-shift of the absorption edge observed in experiments.

Combined ab initio and experimental study of Mg-, Ca-, Sr-, Ba-doped bismuth titanate pyrochlores (BTO) has been carried out. It is revealed that the substitution of bismuth atoms by alkaline earth elements is a preferable process rather than the substitution of titanium atoms according to the structural and energy parameters from the first-principles calculations. The pyrochlore-type compounds Bi1.5Mg(Ca)xTi2O7-δ have been obtained at x = 0.25. Bismuth titanate pyrochlores doped with Mg and Ca are wide-band semiconductor materials with the maximum of light absorption at wavelengths

The ab initio projector augmented wave (PAW) method is used to calculate the electronic structure of Li-doped cadmium oxide with NaCl structure. The preference energy for Li atoms in interstitial sites and the energy of impurity oxidation are calculated. Interstitial positions for Li atoms are shown to be stable under thermodynamic equilibrium, but Li atoms can substitute Cd atoms in presence of vacancies in the oxygen sublattice. We consider the following complexes: one Li atom in the interstitial site and the other Li atom in Cd position; one Li atom in Cd position and one oxygen vacancy; a pair of oxygen vacancies; and show that these complexes are formed to have the shortest possible distance between their components. The band gap substantially decreases when Li atoms occupy interstitial sites to explain considerable increase of experimental conductivity.

Ab initio calculations were used to study the properties of a series of hexagonal (Fe2N-like) subcarbides M2C, where M = Tc, Ru, Rh, Pd, Re, Os, Ir, and Pt, and to calculate their equilibrium structural parameters, electronic properties, phase stability, elastic constants, compression modulus, shear modulus, Young’s modulus, compressibility, Pugh’s indicator, Poisson ratio, elastic anisotropy indices, and also hardness, Debye temperature, sound velocity, and low-temperature heat capacity. It is found based on these results that all the subcarbides are mechanically stable; however, their formation energies Eform are positive with respect to a mixture of d-metal and graphite. In addition, the calculation of the phonon spectra of these subcarbides shows the existence of negative modes, which indicates their dynamical instability. Thus, a successful synthesis of these subcarbides at normal conditions is highly improbable.

New Bi1.5Mg0.9-xLixNb1.5O7–δ (x = 0.25; 0.40) and Bi1.4RE0.1Mg0.5Li0.4Nb1.5O7–δ (RE – Eu, Ho, Yb) compounds with the pyrochlore structure were synthesized. The displacements of the A-site atoms (96g) and O' ones (32e) as well as the Li and RE atoms distribution in the A-sites were determined. The dopant distribution was proven by ab initio calculations. The most preferable (Bi1.5Li0.5)(Nb1.5Mg0.5)O7 model was predicted with a direct band gap of 3.18 eV corresponding to the experimental Eg for Bi1.5Mg0.5Li0.4Nb1.5O7–δ. The thermal stability of the compounds in air up to 1100–1220 °C and the reducing atmosphere up to 400 °C was determined. The charge disbalance in the A2O' sublattice and the oxygen vacancies predetermine the dielectric behavior of the ceramics up to 200 °C, the mixed conductivity at high temperatures (T > 200 °C), and the proton transport up to 400 °C. © 2021 Elsevier Ltd. This work is supported by the German-Russian Interdisciplinary Science Center (G-RISC) funded by the German Federal Foreign Office via the German Academic Exchange Service (DAAD) (the research project No M-2020a-4_d), and by RFBR according to the research project No 19-03-00642 A. The study was performed using the equipment of the Center for Shared Use of Scientific Equipment ?Khimiya? of the Institute of Chemistry FRC Komi Science Center, Ural Branch of the Russian Academy of Science. We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at PETRA III using beamline P02.1.

Using the ab initio projector augmented wave (PAW) method, calculations are performed for the electronic energy-band structure of titanium dioxide having the structure of anatase doped with boron, nitrogen, and carbon. Thermodynamic characteristics are determined for the formation of impurity centers, such as the preference energy for the interstitial position, the energy of impurity oxidation, and the energy of oxygen vacancy formation. It is shown that under the conditions of thermodynamic equilibrium the interstitial position of boron atoms is stable, whereas carbon atoms, depending on the oxygen pressure, can occupy both interstitial positions and substitutional positions of oxygen atoms, and nitrogen atoms replace oxygen atoms. It is shown that the presence of oxygen vacancies promotes the thermodynamic stability of carbon and nitrogen atoms. The obtained densities of electronic states correspond to ESR spectroscopy data, which indicates the presence of spin-polarized electrons in the states of the oxygen vacancy.

Iron and carbon doped nanostructured titanium dioxide with different morphology of aggregates was synthesized using the developed precursor technique. Glycolate of the general composition Ti1-xFex(OCH2CH2O)2-x/2 (0 ≤ x ≤ 0.1) was used as a precursor. The synthesized samples of the compositions Ti1-xFexO2, Ti1-xFexO(2-x/2)-yCy and Ti1-xFexO(2-x/2)-yCy:nC were characterized by X-ray diffraction, scanning electron microscopy, ultraviolet and visible absorption spectroscopy, as well as by vibration and X-ray photoelectron spectroscopy methods. In addition, they were tested as photocatalysts in the hydroquinone oxidation reaction under ultraviolet and visible irradiation. It is established that the insertion of iron into the structure of carbon-doped anatase (TiO2-yCy) suppresses its photocatalytic activity in the visible range of the spectrum, but leads to no change under ultraviolet irradiation. Globular samples of Ti1-xFexO(2-x/2)-yCy containing no more than 2.5 at% Fe show the maximum photocatalytic activity. To clarify the reasons for the observed complex dependence of photocatalytic activity on the concentration of the dopant first-principles, calculations of the electronic band structure and optical absorption of anatase doped with iron and carbon are performed and discussed.

Recently, new FeAs-based high-temperature superconductors CaAFe4As4 (A = K, Rb, and Cs) with a layered tetragonal crystal structure were synthesized (T C ∼ 30 K). In this letter, we report for the first time the band structures, Fermi surface topology, total and partial densities of electronic states, and interatomic interactions for CaAFe4As4 as estimated by means of the first-principle FLAPW-GGA calculations. The obtained data for CaAFe4As4 phases are analyzed in comparison with each other and with other related FeAs systems. The interatomic bonding picture can be represented as a highly anisotropic mixture of metallic, covalent, and ionic contributions, which are realized inside Fe4As4-layered blocks and between these blocks and Ca, A atomic sheets. The Fermi surfaces of these systems have a multisheet character and are compiled of a large number of cylinders at the edges and in the central part of the Brillouin zone. It is established that the high-temperature superconductivity in CaAFe4As4 compounds as in other related systems correlates well with such structure parameters as bond angles and anion height.

The electronic structure, Fermi surface, Sommerfeld and Pauli paramagnetic susceptibility coefficients, cohesive energies, phase and point defect formation energies, elastic constants, bulk, shear, and Young moduli, Poisson ratios, and Vickers microhardness of niobium boronitride Nb2BN are determined by the ab initio FLAPW-GGA full-potential method. The obtained values are discussed in comparison with similar data for Mo2BC and other related binary carbides, nitrides, and borides of transition metals, and with available experimental data.

For the first time the comparison of the theoretical and experimental data have shown that UV–vis absorption in the region of 600 nm in colloidal solution of NbS 3 can be described by the d-d electronic transitions to the antibonding molecular orbital. It is proved that the process leads to excitation of metal-metal bond.

Using ab initio calculations, the data have been obtained on the structural, electronic, and optical properties of bismuth titanates with the pyrochlore type structure and compounds with the substitution of scandium or indium atoms for bismuth and titanium atoms. The results of the theoretical calculations agree with the experimentally obtained structural and optical characteristics of the synthesized compounds doped with scandium or indium. It has been shown that the substitution of scandium or indium atoms for bismuth atoms in the pyrochlore structure is energetically favorable. The energies corresponding to the direct and indirect electronic transitions in scandium- and indium-doped bismuth titanates have been determined based on the optical spectroscopy data obtained for the studied samples. These energies are in agreement with the theoretically calculated values.

A sample of lutetium carboboride LuB2C was synthesized from a mixture of lutetium hydride, boron and carbon by annealing in argon. The temperature dependence of the heat capacity Cp(T) (2-300 K) and lattice parameters a(T), b(T), and c(T) (5-300 K) of the carboboride was experimentally determined. The experimental values of the heat capacity were fitted with the approximation Cp(T) = aT + ΣCD + CE + CTLS(T). Here the first term is the electronic contribution, the second is the sum of the Debye components, the third is the Einstein contribution, and the fourth is the contribution to the heat capacity due to the vibrations of the two-level systems which are formed in the Lu-subsystem due to the asymmetry of the B-C atomic arrangement around the Lu3+-ions and, as a consequence, the possible transition of the lutetium atoms between spatially close, but energetically non-equivalent positions. A strong anisotropy of the thermal expansion of the carboboride was revealed. Along the c axis the coefficient of thermal expansion monotonically increases; in the basal plane, the expansion is practically not observed. The temperature dependence of the unit cell volume Vu(T) has been analyzed in the Debye-Einstein approximation taking into account the electronic contribution and effect of two-level systems. The values of the Gruneisen parameters corresponding to different modes of the phonon spectrum of the carboboride have been determined. The frequencies of the lattice vibrations, determined in a Raman scattering experiment, are in satisfactory agreement with the parameters obtained from Cp(T) using the Debye-Einstein approximation. Using ab initio band theory methods and an exchange-correlation functional in the PBE form in the VASP package, it was established that the total energies of these two crystal structures differ by no more than 0.01 eV f.u.-1. Calculations of the thermodynamic properties of LuB2C yielded similar results for orthorhombic and tetragonal phases of the carboboride.

Density functional theory was used to study pressure-induced phase transitions of zircon to monazite in doped yttrium orthophosphate, Y1–хLaхPO4, for х = 0, 0.0625, 0.125. The pressures of the phase transition, the elastic moduli and the universal elastic anisotropy index were calculated. It was shown that with increasing lanthanum concentration in Y1–xLaxPO4, the transition pressure increases. According to the Birch–Murnaghan equation of state, this effect is associated with a decrease in the critical volume. The increased stability of the doped zircon phase compared to YPO4 is attributed to the more significant increase in the anisotropy and distortions of REO8 polyhedra and RE–O–P chains found for the optimized structures at critical volumes.

Point defect formation within graphene and ultra-thin graphite films is considered by means of molecular dynamics simulations in the framework of the density functional tight-binding approach. The barrier energy for vacancy formation is estimated and two types of defect formation are revealed.

Using density functional theory the electronic structure, chemical bond parameters, phase formation energies, and intrinsic defects in metal and non-metal sublattices of chalcogenides MX2 (M = Nb, Mo, W; X = Se, Te) are determined. For compounds with X = Te a monotonic decrease in charges on metal and non-metal atoms occurs with increasing atomic number, however, for compounds with X = Se this order is violated. With increase in the metal atomic number for both selenides and tellurides, the formation energies increases, i.e. the stability of these phases decreases. The formation energies of vacancies in both sublattices of these systems have a non-monotonic character. For MX2 (M = Mo, W; X = Se, Te) systems the formation of vacancies in the chalcogen sublattice results in the semiconductor–metal transition, and vacancies in the metal sublattice decrease the band gap.

The disordered perovskite-like phase LaCu0.5Ti0.5O3±δ (LCTO) with Pnma space group is synthesized by solid-state reaction route. Coulometric and thermogravimetric measurements in 300–1223 K temperature and 10−16 – 0.21 atm oxygen partial pressure ranges reveal narrow equilibrium oxygen content interval for the studied compound. Moreover, LCTO is shown to possess both over- and non-stoichiometry depending on external conditions. The complete reduction of LaCu0.5Ti0.5O3±δ results in a formation of La2O3, La2Ti2O7–x oxides and metallic copper. Defect chemistry modeling and DFT calculations evidence oxygen vacancies formed in LaCu0.5Ti0.5O3±δ at reduction can be considered mobile species. A decomposition mechanism for studied cuprate is proposed based on experimental results.

The ab initio projector augmented wave method of the density functional theory is applied for studying energy characteristics of oxygen vacancies and migration pathways in Pr1−xYxBaCo2O6−δ. It is found that the negative formation energy leads to initial appearance of oxygen vacancies in PrO planes thus providing the pathway for oxygen migration at small δ's. The increase of oxygen vacancy population in PrO planes and replacement of Pr by Y both favor a weakening of Co O bonds, which facilitates the increase in the amount of oxygen vacancies in CoO2 layers. Moreover, the yttrium dopants enlarge the bottleneck for oxygen jumps along ⟨101⟩ directions over oxygen vacancies in PrO and CoO2 layers. As a result, the respective ‘zigzag’ oxygen migration pathway becomes predominant with increasing oxygen non-stoichiometry and yttrium doping. Thus, the layered perovskite-like cobaltites give an example of a peculiar diffusion process where minority defects provide the transport channel while majority defects maintain favorable geometry of the migration pathway.

Based on the results of ab initio band structure calculations it was found that in a series of ternary and quarternary compounds (some antiperovskites, 1111-oxyarsenides and non-centrosymmetric superconductors) containing the atoms of platinum-group metals the latter reveal the anionic behavior. They are characterized with negative effective atomic charges, and the character of electronic density distribution in these crystals confirms the electronic transfer from cations towards platinum-groups metal atoms, as well as towards the atoms of non-metals. The possible reasons why platinum-groups metal atoms can rival with non-metallic atoms for being anions are briefly discussed.

Features of the electronic structure of layered ZrCuSiAs-like LaZnAsO1–δ phases at δ = 0.11 and 0.44 are briefly discussed based on the results of ab initio calculations. It is shown that the effect of oxygen vacancies on the electronic structure of non-stoichiometric phases is equivalent to the effect of an electron dopant and the charge compensation occurs inside [La–O] structural blocks due to variation of the occupation of lanthanum electronic states.

Quasi-one-dimensional (1D) Ti1 − xLnxO2 − x/2 anatase solid solutions were prepared by heating Ti1 − xLnx(OCH2CH2O)2 − x/2 precursors, where Ln = Nd, Eu, Tb, Er (x = 0.025), or Sm (0.005 ≤ x ≤ 0.025), in air. These solid solution were found to have a photocatalytic activity in hydroquinone oxidation in aqueous solution under exposure to UV radiation. UV-Vis absorption spectra were recorded for Ti1 − xLnxO2 − x/2 (Nd, Sm, Eu, Tb, or Er). The electronic structure and optical absorption spectra were calculated for anatase doped with neodymium, samarium, or terbium.

Very recently, on the example of hole- and spin-doped BaZn2As2, quite an unexpected area of potential applications of 122-like phases was proposed as a promising platform for searching the new diluted magnetic semiconductors (DMSs) (2013; K. Zhao, et al, Nature Commun. 4:1442). Herein, by means of the first-principles calculations, we have examined in detail the basic structural, elastic, electronic properties and the peculiarities of the inter-atomic bonding in α and β polymorphs of 122-like BaZn2As2 – a parent phase of the new DMSs. Our characterization of these materials covers the optimized structural parameters, the main elastic parameters (elastic constants, bulk, shear, and Young’s moduli, Poisson’s ratio, anisotropy indexes, and Pugh’s criterion), as well as electronic bands and densities of electronic states.

By means of DFT-based ab initio calculations, we examine two polymorphs of the newly synthesized 1111-like MgFeSeO as possible new superconducting systems. We have found that the polymorph with blocks [MgO], where Mg atoms are placed in the centers of O4 tetrahedra, is dynamically unstable unlike the ZrCu-SiAs-type polymorph with oxygen atoms placed in the centers of Mg4 tetrahedra. The characterization of this material covers the structural, elastic properties, electronic band structure, density of electronic states, and Fermi surface. Our calculations suggest that a high critical temperature for MgFeSeO may be achieved as a result of electron or hole doping through ion substitutions or through creation of lattice vacancies.

The 6,7Li NMR spectra and the 7Li spin–lattice relaxation rate were measured on polycrystalline samples of Li2ZrO3, synthesized at 1050 K and 1300 K. The 7Li NMR lines were attributed to corresponding structural positions of lithium Li1 and Li2 by comparing the EFG components with those obtained in the first-principles calculations of the charge density in Li2ZrO3. For both samples the line width of the central 7Li transition and the spin–lattice relaxation time decrease abruptly at the temperature increasing above ~500 K, whereas the EFG parameters are averaged (〈νQ〉=42 (5) kHz) owing to thermally activated diffusion of lithium ions.

The absorption spectra of the precursor-derived solid solutions Zn1 − x M x O (M = Fe, Co, Cu) with a tubular morphology of aggregates have been investigated in the ultraviolet and visible regions. The maximum metal concentration x in the Zn1 − x M x O solid solutions is 0.075 for iron, 0.2 for cobalt, and 0.1 for copper. It has been found that the optical absorption and the band gap of the Zn1 − x M x O compounds depend on the type of dopant. The obtained experimental data have been interpreted using the results of the performed ab initio calculations of the electronic band structure and optical absorption.

We report the LDA + DMFT (method combining local density approximation with dynamical mean-field theory) results for spectral properties of the superconductor NdFeAsO0.9F0.1 in the paramagnetic phase. The calculated momentum-resolved spectral functions are in good agreement with angle-resolved photoemission spectra (ARPES). The obtained effective quasiparticle mass enhancement (m*/m = 1.4) is smaller than the one in isostructural parent compound LaFeAsO which critical temperature under the same fluorine doping (LaFeAsO0.9F0.1) is two times lower. Our results demonstrate that in quaternary FeAs-based superconductors of the same class, changes of the crystal structure caused by substitution of one rare-earth atom, implicitly result in reduction of the electronic correlation strength. © 2013 Pleiades Publishing, Inc.

Very recently, low-temperature superconductivity was discovered for the intermetallic compound HfCuGe2 (2013; Cava RJ, et al, EPL 101:67001.), which was declared as "a non-magnetic analog of the 1111 iron pnictides". Herein, by means of the first-principles calculations, we have examined in detail the structural, elastic, and electronic properties of HfCuGe2, as well as of the isostructural and isoelectronic phases ZrCuGe2, HfCuSi2, and ZrCuSi2, which are analyzed in comparison with a set of 1111-like phases. The obtained close similarity of the electronic factors, namely, the topologies of the near-Fermi bands, the Fermi surfaces, as well as the DOS values at the Fermi level for superconducting HfCuGe2 and other examined 112 phases allowed us to assume that low-temperature superconductivity may be expected also for ZrCuGe2, HfCuSi2, and ZrCuSi2. © 2013 Elsevier Ltd. All rights reserved.

A comparative analysis of the structural, electronic, mechanical, and magnetic properties and relative stability has been carried out in terms of ab initio calculations for four possible polymorphic modifications of rhenium dinitride, whose nonmetallic lattices contain both individual nitrogen atoms and dimers N2. It has been found that the recently synthesized hexagonal polymorph ReN2 (structural type 2H-MoS2) is a weak d0 magnet in which the magnetic state is formed due to spin splitting of N 2p states.

A comparative analysis of 6,7Li NMR spectra is performed for the samples of monoclinic lithium titanate obtained at different synthesis temperatures. In the 7Li NMR spectra three lines are found, which differ in quadrupole splitting frequencies vQ and according to ab initio EFG calculations are assigned to three crystallographic sites of lithium: Li1 (vQ ∼ 27 kHz); Li2 (vQ ∼ 59 kHz); Li3 (vQ ∼ 6 kHz). The dynamics of lithium ions is studied in a wide temperature range from 300 K to 900 K. It is found that the narrowing of 7Li NMR spectra as a result of thermally activated diffusion of lithium ions in the low-temperature Li2TiO3 sample is observed at a higher temperature in comparison with a sample of high-temperature lithium titanate. Based on the analysis of 6Li NMR spectra it is assumed that there is mixed occupancy of lithium and titanium sites in the corresponding layers of the crystal structure of low-temperature lithium titanate, which hinders lithium ion transfer over regular crystallographic sites.

Quasi-one-dimensional solid solutions of the composition Ti1 − x Fe x O2 − x/2 (0.005 ≤ x ≤ 0.050) with the anatase-type structure and extended aggregates have been prepared by the precursor method. The absorption spectra of the solid solutions have been investigated in the ultraviolet and visible regions, and the photocatalytic activity in the oxidation reaction of hydroquinone in water has been estimated. It has been found that the synthesized solid solutions serve as photocatalysts only under ultraviolet irradiation, and their photoactivity increases with an increase in the dopant concentration. The first-principles calculations of the electronic band structure and optical absorption in iron-doped anatase and rutile have been performed using the pseudopotential method LSDA + U (with the VASP software package). The on-site exchange-correlation parameters have been calibrated in the calculations of the electronic band structure of hematite α-Fe2O3 and ilmenite FeTiO3. It has been shown that, despite the appearance of impurity states within the band gap of anatase and rutile, doping with iron does not cause substantial absorption in the visible region, which correlates with the increase in photocatalytic activity only under ultraviolet irradiation. The most probable cause of the experimentally observed absorption in the visible region is the presence of finely dispersed hematite impurities in the obtained samples.

By means of the first-principles calculations, the elastic properties of the newest platinum-based superconductors CaPt3P and SrPt3P are evaluated and analyzed in comparison with their nearest pnictogen-containing chemical analogues: superconducting SrPtAs and SrPt2As2. Our studies reveal that CaPt3P and SrPt3P are mechanically stable materials, which adopt relatively weak elastic anisotropy and will behave in a ductile manner. As compared with SrPtAs and SrPt2As2, the examined phases demonstrate increased lattice stiffness and will exhibit higher hardness. Nevertheless, the SCs CaPt3P and SrPt3P should be classified as soft materials.

The first-principles calculations were performed to predict the elastic and electronic properties of the superconducting ThCr2Si2-type phases CaPd2As2 and SrPd2As2 in comparison with the non-superconducting CeMg2Si2-type phase BaPd2As2. Besides, the same properties were compared for CeMg2Si2- and ThCr2Si2-type polymorphs of BaPd2As2. We found that all these phases are mechanically stable and belong to soft materials with low hardness. The near-Fermi region is formed by the valence states of the blocks [Pd2As2] with decisive contributions of Pd 4d states. The values of N(E F) increase in the sequence: CaPd2As2 < SrPd2As2 < BaPd2As2, i.e. in the reverse sequence relative to the transition temperatures T C. Thus, the change in T C cannot be explained by the electronic factor, i.e. by the simple correlation T C∼N(E F). Most likely the decrease in T C in the sequence CaPd2As2 → SrPd2As2 and the absence of a superconducting transition in BaPd2As2 are related to the structural factors and the peculiarities of the electron–phonon coupling mechanism.

Within electron density functional theory with the use of the Vienna ab-initio simulation package (VASP), the effect of the sp substitutional impurities of fluorine (n-type dopant), nitrogen, and carbon (p-type dopants) on the electronic and magnetic properties of tungsten trioxide WO3 is studied. It is established that these impurities induce the transformation of tungsten trioxide (nonmagnetic semiconductor) into nonmagnetic metal (WO3:F), magnetic semimetal (WO3:N), or magnetic metal (WO3:C) states.

By means of the first principles calculations, the influence of arsenic deficiency on the electronic and magnetic properties of the layered phase LaFeAsO was examined. We found that As vacancies affect mainly the near-Fermi Fe-derived bands leading to their splitting and changing their dispersion and the level of filling. The Fermi surface for LaFeAs1−xO adopts a complicated multi-sheet morphology, with a number of linked 2D-like sheets and a set of new sheets (which are absent for ideal LaFeAsO) at the lateral sides of the Brillouin zone. Besides, for the most stable As-deficient phase an antiferromagnetic stripe-like spin configuration was found, for which an inhomogeneous distribution of magnetic moments of Fe atoms arises near and away from vacancy owing to differences in local magnetic moments of non-equivalent Fe atoms.

Based on ab initio band structure calculations, we have systematically examined the monoclinic tungsten trioxide doped with all 3d metals (M=Sc,Ti,…,Ni,Cu) focusing on the effects of these impurities on the electronic and magnetic properties of e-WO3. We found that all of the 3d impurity atoms can be divided into two main groups. The first group includes the atoms of the beginning of the 3d row, and for these systems (for example, e-WO3:Ti) the highest magnetic moments and the peculiarities of the near-Fermi states are due to the atoms of the oxygen lattice of trioxides. On the contrary, the impurity atoms of the end of the 3d row play a dominant role in the formation of magnetic and electronic properties. These results are fair for calculations within both GGA and GGA+U approximations. Finally, there are so-called “border” systems (for example, e-WO3:Cr), where the Coulomb interactions have a critical influence on the electronic and magnetic parameters.

Very recently (2011, Advanced Materials , 23, 4248), an elegant exfoliation approach was proposed to prepare a new family of two-dimensional (2D)-like transition metal carbides and nitrides from layered MAX phases. This discovery has provided the next stage in the design of new graphene-like inorganic materials with intriguing functionalities and applications. This overview is intended as an introduction to this newest family of graphene-like quasi-2D nanocarbides and nanonitrides of d metals, which are known today also as MXenes. Here the authors discuss the available results achieved for MXenes (2011-2012), when a group of various MXenes (2D-Ta 4 C 3 , Ti 3 (C 0.5 N 0.5 ) 2 , TiNbC and some others) have been successfully synthesised, a set of their properties (conductivity, mechanical behaviour etc.) were measured and systematical theoretical studies of structural, electronic properties, chemical bonding and the stability of MXenes were initiated. Besides, possible applications of MXenes as promising materials for Li ion batteries, sensors or 2D electronics etc. are noted.

This work reports on the elastic, electronic properties and inter-atomic bonding of the newly discovered superconductor Th2NiC2 as obtained within ab initio calculations. We found that Th2NiC2 is mechanically stable and it will behave as a ductile material exhibiting enhanced elastic anisotropy in shear and a rather low hardness. The inter-atomic bonding in Th2NiC2 is described as an anisotropic mixture of covalent and ionic contributions, where inside [Th2C2] blocks covalent Th–C bonds emerge, and these blocks are bonded with the Ni atomic sheets through directional Ni–C bonds. Additionally, some inter-atomic ionic Th–C interactions occur owing to charge transfer Th → C. Our data reveal that for Th2NiC2 the Fermi level is located in a deep DOS minimum and the experimentally observed increase in TC in the sequence Th2NiC2 → Th1.8Sc0.2NiC2 may be explained by the growth of N(EF). We also speculate that an increase in N(EF) (and, probably, in TC) for Th2NiC2-based materials may be also achieved by an alternative way: by electron doping – for example, by partial substitution of V for Th or Cu for Ni, as well as by partial substitution of N for C with the formation of Th–Ni carbonitrides like Th2NiC2−xNx.

Very recently (2012, Phys. Rev Lett., 109, 035502) a new hexagonal (s.g. P63/mmc, ♯194) ternary phase Nb2GeC, which belongs to so-called 211-like MAX phases, was discovered. In order to get a systematic insight into the structural, elastic, and electronic properties of Nb2GeC, we used two complementary DFT-based first-principles approaches (as implemented in the VASP and Wien2k packages) to calculate the optimized structural parameters, band structure, densities of state, Fermi surface, and a set of elastic parameters: elastic constants (Cij), bulk modulus (B), compressibility (β), shear modulus (G), Young’s modulus (Y), and elastic anisotropy indexes, which were discussed in comparison with available data. Besides, the inter-atomic bonding picture for Nb2GeC was discussed using electron density maps and Bader’s charge analysis.

Employing first-principles band structure calculations, we have examined the electronic, optical properties and the peculiarities of the chemical bonding for six newly synthesized layered quaternary 1111-like chalcogenide fluorides SrAgSF, SrAgSeF, SrAgTeF, BaAgSF, BaAgSeF, and SrCuTeF, which are discussed in comparison with some isostructural 1111-like chalcogenide oxides. We found that all of the studied phases AMCh F ( A =Sr, Ba; M =Cu, Ag; and Ch =S, Se, Te) are semiconductors for which the fitted “experimental” gaps lie in the interval from 2.23 eV (for SrAgSeF) to 3.07 eV (for SrCuTeF). The near-Fermi states of AMCh F are formed exclusively by the valence orbitals of the atoms from the blocks ( MCh ); thus, these phases belong to the layered materials with “natural multiple quantum wells”. The bonding in these new AMCh F phases is described as a high-anisotropic mixture of ionic and covalent contributions, where ionic M–Ch bonds together with covalent M–Ch and Ch–Ch bonds take place inside blocks ( MCh ), while inside blocks ( A F) and between the adjacent blocks ( MCh )/( A F) mainly ionic bonds emerge.

We have examined theoretically the electronic band structure and Fermi surface (FS) of the tetragonal low-temperature superconductor Bi2Pd. Our main results are that (i) the Pd 4d and Bi 6p states determine the main peculiarities of the multiple-sheet FS topology, thus for this material a complicated superconducting gap structure with different energy gaps on different FS sheets should be assumed; (ii) the effect of spin-orbit coupling is of minor importance for the distributions of the near-Fermi electronic states; and (iii) this phase adopts a 3D-like type owing to directional bonds between the adjacent atomic sheets.

Structural, electronic properties and relative stability of quasi-two-dimensional (2D) free-standing planar nano-block (NBs) structures Tin+1Al0.5Cn and Tin+1Cn (n = 1 and 2), which can be prepared using the recently developed procedure of exfoliation of corresponding NBs from MAX phases, were examined within first principles calculations in comparison with parent MAX phases Ti3AlC2 and Ti2AlC. We found that in general Tin+1Cn and Tin+1Al0.5Cn NBs retain the atomic geometries of the corresponding blocks of the MAX phases, but some structural distortions for the NBs occur owing to the lowering of the coordination number for atoms in the external Ti sheets of the nano-block structures. Our analysis based on their cohesive and formation energies reveals that the stability of the nano-block structures increases with index n (or, in other words, with a growth of the number of Ti–C bonds), the Al-containing NBs becoming more stable than the “pure” Ti–C NBs. Our data show that the magnetization of the simulated planar nano-block structures can be expected; so, for the Ti3C2 nano-block the most stable will be the spin configuration, where within each external Ti sheet the spins are coupled ferromagnetically together with antiferromagnetic ordering between opposite external titanium sheets of this nano-block.

The tetragonal (s.g. I 4/ nmm ; #129) silicide arsenide ZrCuSiAs is well known as a structural type of the broad family of so-called 1111-like quaternary phases which includes now more than 150 representatives. These materials demonstrate a rich variety of outstanding physical properties (from p-type transparent semiconductors to high-temperature Fe-based superconductors) and attracted a great interest as promising candidates for a broad range of applications. At the same time, the data about the electronic and elastic properties of the ZrCuSiAs phase itself, as well as of related silicide arsenides and germanide arsenides are still very limited. Here for a series of six isostructural and isoelectronic 1111-like phases which includes both synthesized (ZrCuSiAs, HfCuSiAs, ZrCuGeAs, and HfCuGeAs) and hypothetical (TiCuSiAs and TiCuGeAs) materials, systematical studies of their structural, elastic, electronic properties and stability trends are performed by means of first-principles calculations.

A full-potential FLAPW-GGA method is used to study for the first time the electronic structure of new layered HgCuSeO oxyselenide with a tetragonal structure. The band structure, density of electron states, Fermi surface, and effective atomic charges are obtained and analyzed; the coefficients of low-temperature heat capacity and paramagnetic Pauli susceptibility are estimated. It is shown that the new layered HgCuSeO phase can be characterized as non-magnetic ionic metal.

By means of first-principles calculations, we have systematically studied the structural, elastic, electronic, and thermodynamic properties of two related layered 122-type intermetallics: tetragonal LaNi 2 P 2 and LaNi 2 Ge 2 . The optimized lattice constants, cell volumes, electronic bands, Fermi surface topology, and densities of states were obtained and analyzed. We found that the bonding picture in the examined phases is of a complex anisotropic character and can be described as a mixture of metallic, ionic, and covalent contributions. We predicted that LaNi 2 P 2 , and LaNi 2 Ge 2 are mechanically stable and relatively soft materials with high compressibility and rather small hardness. Both phases will exhibit comparable elastic anisotropy and behave in a brittle manner. Sound velocities, Debye temperatures, and heat capacity C p ( T ) were evaluated for LaNi 2 P 2 and LaNi 2 Ge 2 in the low-temperature limit, and are in reasonable agreement with available experiments.