First-Principles Linear Combination of Atomic Orbitals Calculations of KSiF Crystal: Structural, Electronic, Elastic, Vibrational and Dielectric Properties.

The results of first-principles calculations of the structural, electronic, elastic, vibrational, dielectric and optical properties, as well as the Raman and infrared (IR) spectra, of potassium hexafluorosilicate (KSiF; KSF) crystal are discussed. KSF doped with manganese atoms (KSF:Mn) is known for its ability to function as a phosphor in white LED applications due to the efficient red emission from Mn⁴⁺ activator ions. The simulations were performed using the CRYSTAL23 computer code within the linear combination of atomic orbitals (LCAO) approximation of the density functional theory (DFT).

Effect of Rh Doping on Optical Absorption and Oxygen Evolution Reaction Activity on BaTiO (001) Surfaces.

In the present work, we investigate the potential of modified barium titanate (BaTiO), an inexpensive perovskite oxide derived from earth-abundant precursors, for developing efficient water oxidation electrocatalysts using first-principles calculations. Based on our calculations, Rh doping is a way of making BaTiO absorb more light and have less overpotential needed for water to oxidize. It has been shown that a TiO-terminated BaTiO (001) surface is more promising from the point of view of its use as a catalyst.

Modeling of the Lattice Dynamics in Strontium Titanate Films of Various Thicknesses: Raman Scattering Studies.

While the bulk strontium titanate (STO) crystal characteristics are relatively well known, ultrathin perovskites' nanostructure, chemical composition, and crystallinity are quite complex and challenging to understand in detail. In our study, the DFT methods were used for modelling the Raman spectra of the STO bulk (space group I4/mcm) and 5-21-layer thin films (layer group p4/mbm) in tetragonal phase with different thicknesses ranging from ~0.8 to 3.

Water Splitting on Multifaceted SrTiO Nanocrystals: Calculations of Raman Vibrational Spectrum.

Various photocatalysts are being currently studied with the aim of increasing the photocatalytic efficiency of water splitting for production of hydrogen as a fuel and oxygen as a medical gas. A noticeable increase of hydrogen production was found recently experimentally on the anisotropic faces (facets) of strontium titanate (SrTiO, STO) nanoparticles. In order to identify optimal sites for water splitting, the first principles calculations of the Raman vibrational spectrum of the bulk and stepped (facet) surface of a thin STO film with adsorbed water derivatives were performed.

Oxygen Vacancy Formation and Migration within the Antiphase Boundaries in Lanthanum Scandate-Based Oxides: Computational Study.

The atomic structure of antiphase boundaries in Sr-doped lanthanum scandate (LaSrScO) perovskite, promising as the proton conductor, was modelled by means of DFT method. Two structural types of interfaces formed by structural octahedral coupling were constructed: edge- and face-shared. The energetic stability of these two interfaces was investigated.

The electronic properties of SrTiO with oxygen vacancies or substitutions.

The electronic properties, including bandgap and conductivity, are critical for nearly all applications of multifunctional perovskite oxide ferroelectrics. Here we analysed possibility to induce semiconductor behaviour in these materials, which are basically insulators, by replacement of several percent of oxygen atoms with nitrogen, hydrogen, or vacancies. We explored this approach for one of the best studied members of the large family of ABO perovskite ferroelectrics - strontium titanate (SrTiO).

Evidence for the formation of two types of oxygen interstitials in neutron-irradiated α-AlO single crystals.

Due to unique optical/mechanical properties and significant resistance to harsh radiation environments, corundum (α-AlO) is considered as a promising candidate material for windows and diagnostics in forthcoming fusion reactors. However, its properties are affected by radiation-induced (predominantly, by fast neutrons) structural defects. In this paper, we analyze thermal stability and recombination kinetics of primary Frenkel defects in anion sublattice - the F-type electronic centers and complementary oxygen interstitials in fast-neutron-irradiated corundum single crystals.

BaCoO monoclinic structure and chemical bonding analysis: hybrid DFT calculations.

Cobalt based perovskites have great potential for numerous applications. Contrary to a generally assumed hexagonal space group (SG P6/mmc) model as the ground state of BaCoO (BCO), our hybrid DFT calculations with B1WC density functional and the symmetry group-subgroup derived crystal structure model support the ground state of BCO to be indeed monoclinic, in agreement with recent experimental predictions [Chin et al., Phys.

First principles calculations of the vibrational properties of single and dimer F-type centers in corundum crystals.

The present paper investigates the F-type centers in α-AlO through their electronic and vibrational properties from first principle calculations using a periodic supercell approach, a hybrid functional, and all-electron Gaussian basis sets as implemented in the CRYSTAL17 code. Single F-type and dimer F-type centers related to oxygen vacancies in various charge states were considered. The defect-induced vibrational modes were identified and found to appear mainly in the low (up to 300 cm) and high (above 700 cm) frequency regions, depending on the defect charge.

Atomic, electronic and magnetic structure of an oxygen interstitial in neutron-irradiated AlO single crystals.

A single radiation-induced superoxide ion [Formula: see text] has been observed for the first time in metal oxides. This structural defect has been revealed in fast-neutron-irradiated (6.9×10 n/cm) corundum (α-AlO) single crystals using the EPR method.

Hybrid density functional theoretical study of NASICON-type NaTi(PO) (x = 1-4).

Sodium Super Ionic Conductor (NASICON) structured phosphate framework compounds represent a very attractive class of materials for their use as Na-ion battery electrodes. A series of NASICON-structured NaTi(PO) compounds corresponding to varying degrees of sodiation (x = 1-4) have been investigated using high-level hybrid density functional theory calculations using the Linear Combination of Atomic Orbitals and Gaussian-type basis set formalism together with hybrid B1WC and HSE06 exchange-correlation functionals. Using primitive cells of NaTi(PO) compounds with different stoichiometry, sodium sublattice structure and titanium oxidation states are constructed and analyzed using group theoretical symmetry considerations.

Distinctive features of diffusion-controlled radiation defect recombination in stoichiometric magnesium aluminate spinel single crystals and transparent polycrystalline ceramics.

MgAlO spinel is important optical material for harsh radiation environment and other important applications. The kinetics of thermal annealing of the basic electron (F, F) and hole (V) centers in stoichiometric MgAlO spinel irradiated by fast neutrons and protons is analyzed in terms of diffusion-controlled bimolecular reactions. Properties of MgAlO single crystals and optical polycrystalline ceramics are compared.

First-principles comparative study of perfect and defective CsPbX (X = Br, I) crystals.

First principles Density Functional Theory (DFT) hybrid functional PBESOL0 calculations of the atomic and electronic structure of perfect CsPbI3, CsPbBr3 and CsPbCl3 crystals, as well as defective CsPbI3 and CsPbBr3 crystals are performed and discussed. For the perfect structure, decomposition energy into binary compounds (CsX and PbX2) is calculated, and a stability trend of the form CsPbBr3 > CsPbI3 > CsPbCl3 is found. In addition, calculations of the temperature-dependent heat capacity are performed and shown to be in good agreement with experimental data.

Interface-induced enhancement of piezoelectricity in the (SrTiO)/(BaTiO) superlattice for energy harvesting applications.

We present the results of a detailed first principles study of the piezoelectric properties of the (SrTiO)/(BaTiO) heterostructure using the 3D STO/BTO superlattice model. The atomic basis set, hybrid functionals and slabs with different numbers of STO and BTO layers were used. The interplay between ferroelectric (FE) and antiferrodistortive (AFD) displacements is carefully analyzed.

First-principles calculations of iodine-related point defects in CsPbI.

We present here first principles hybrid functional calculations of the atomic and electronic structure of several iodine-related point defects in CsPbI, a material relevant for photovoltaic applications. We show that the presence of neutral interstitial I atoms or electron holes leads to the formation of di-halide dumbbells of I (analogous to the well-known situation in alkali halides). Their formation and one-electron energies in the band gap are determined.

Thermodynamic stability of non-stoichiometric SrFeO: a hybrid DFT study.

SrFeO3-δ is a mixed ionic-electronic conductor with a complex magnetic structure that reveals a colossal magnetoresistance effect. This material and its solid solutions are attractive for various spintronic, catalytic and electrochemical applications, including cathodes for solid oxide fuel cells and permeation membranes. Its properties strongly depend on oxygen non-stoichiometry.

Electromechanical Properties of BaSrTiO Perovskite Solid Solutions from First-Principles Calculations.

An enhancement of the piezoelectric properties of lead-free materials, which allow conversion of mechanical energy into electricity, is a task of great importance and interest. Results of first-principles calculations of piezoelectric/electromechanical properties of the BaSrTiO (BSTO) ferroelectric solid solution with a perovskite structure are presented and discussed. Calculations are performed within the linear combination of atomic orbitals (LCAO) approximation and periodic-boundary conditions, using the advanced hybrid functionals of density functional theory (DFT).

First-principles calculations of oxygen interstitials in corundum: a site symmetry approach.

Using site symmetry analysis, four possible positions of interstitial oxygen atoms in the α-AlO hexagonal structure have been identified and studied. First principles hybrid functional calculations of the relevant atomic and electronic structures for interstitial O atom insertion in these positions reveal differences in energies of ∼1.5 eV.

Use of site symmetry in supercell models of defective crystals: polarons in CeO.

In supercell calculations of defective crystals, it is common to place a point defect or vacancy in the atomic position with the highest possible point symmetry. Then, the initial atomic structure is often arbitrary distorted before its optimization, which searches for the total energy minimum. In this paper, we suggest an alternative approach to the application of supercell models and show that it is necessary to preliminarily analyze the site symmetry of the split Wyckoff positions of the perfect crystal supercell atoms (which will be substituted or removed in defective crystals) and then perform supercell calculations with point defects for different possible site symmetries, to find the energetically most favorable defect configuration, which does not necessarily correspond to the highest site symmetry.

Thermodynamic stability of stoichiometric LaFeO and BiFeO: a hybrid DFT study.

BiFeO perovskite attracts great attention due to its multiferroic properties and potential use as a parent material for BiSrFeO and BiSrFeCoO solid solutions in intermediate temperature cathodes of oxide fuel cells. Another iron-based LaFeO perovskite is the end member for well-known solid solutions (LaSrFeCoO) used for oxide fuel cells and other electrochemical devices. In this study an ab initio hybrid functional approach was used for the study of the thermodynamic stability of both LaFeO and BiFeO with respect to decompositions to binary oxides and to elements, as a function of temperature and oxygen pressure.

Ab Initio Study of BiFeO3: Thermodynamic Stability Conditions.

BiFeO3 is investigated intensively, mainly as a multiferroic material. In this paper, the state-of-the-art ab initio hybrid functional approach with atomic basis sets was employed for a study of the stability range of BiFeO3 with respect to its decomposition into binary oxides and elementary metals, as a function of temperature and oxygen partial pressure. The calculated atomic and electronic structure of BiFeO3 was compared with previous LDA+U calculations using plane-wave basis sets.

Static and dynamic screening effects in the electrostatic self-assembly of nano-particles.

In the description of charge screening in the electrostatic self-assembly of nanoparticles (molecules) embedded into a polar solvent, the static screening effects (a contribution associated with the rapid spatial redistribution of small and highly mobile ions of a solvent) are traditionally treated phenomenologically, using the Yukawa short-range potential for describing the interaction between these particles. However, this model has a limited range of applicability being valid only for infinitely diluted systems and high salt concentrations. During a slow self-assembling process with nanoparticle formation, very dense structural elements (aggregates) are formed, in which the distances between the nanoparticles could become comparable to the Debye radius in the Yukawa potential.

Theory of non-equilibrium critical phenomena in three-dimensional condensed systems of charged mobile nanoparticles.

A study of 3d electrostatic self-assembly (SA) in systems of charged nanoparticles (NPs) is one of the most difficult theoretical problems. In particular, the limiting case of negligible or very low polar media (e.g.

Combined theoretical and experimental analysis of processes determining cathode performance in solid oxide fuel cells.

Solid oxide fuel cells (SOFC) are under intensive investigation since the 1980's as these devices open the way for ecologically clean direct conversion of the chemical energy into electricity, avoiding the efficiency limitation by Carnot's cycle for thermochemical conversion. However, the practical development of SOFC faces a number of unresolved fundamental problems, in particular concerning the kinetics of the electrode reactions, especially oxygen reduction reaction. We review recent experimental and theoretical achievements in the current understanding of the cathode performance by exploring and comparing mostly three materials: (La,Sr)MnO3 (LSM), (La,Sr)(Co,Fe)O3 (LSCF) and (Ba,Sr)(Co,Fe)O3 (BSCF).

Formation and migration of oxygen vacancies in La(1-x)Sr(x)Co(1-y)Fe(y)O(3-δ) perovskites: insight from ab initio calculations and comparison with Ba(1-x)Sr(x)Co(1-y)Fe(y)O(3-δ).

The formation and migration of oxygen vacancies in the series of (La,Sr)(Co,Fe)O(3-δ) perovskites, which can be used as mixed conducting SOFC cathode materials and oxygen permeation membranes, are explored in detail by means of first principles density functional calculations. Structure distortions, charge redistributions and transition state energies during the oxygen ion migration are obtained and analyzed. Both the overall chemical composition and vacancy formation energy are found to have only a small impact on the migration barrier; it is rather the local cation configuration which affects the barrier.