Lanthanum doping of the first-order Ruddlesden-Popper (RP) phases Sr2-x La x FeO4 (0 ≤ x ≤ 1) is investigated by density functional theory (DFT) calculations. Excess volume and excess energy of this solid solution series are calculated. The evolution of the electronic structure with increasing La content is discussed based on the calculated Fe magnetic moment. It is found that for the same average Fe oxidation state, the electronic structure in Sr2-x La x FeO4 displays a more delocalized character than in corresponding Sr2FeO4‑δ composition. This is attributed to the fact that oxygen vacancies VO •• which are formed under Fe-O bond breaking (and which carry a doubly positive charge) represent a stronger local distortion than singly charged substitutional LaSr • dopants.

Detailed first-principles calculations of structural, electronic, elastic, thermodynamic and vibrational properties of two spinel crystals, MgAl2O4 (MAO) and ZnAl2O4 (ZAO): neat and doped with Ti3+ ions, at ambient and elevated hydrostatic pressure, are reported. Special attention is given to the location of Ti3+ 3d level in the hosts' band gap. Various exchange-correlation functionals are employed for that purpose; the best agreement with the experimental data is obtained for the M06 functional, which places the Ti3+ state at 4.39 eV above the valence band top in MgAl2O4 and at 4.08 eV in ZnAl2O4. Crystal field splitting of the Ti3+ 3d states is calculated for different pressures; dependence of the crystal field strength 10Dq on pressure and Ti3+-O2-distance is analyzed. Our calculations of the Debye temperature (based on the knowledge of elastic constants) result in close agreement with the corresponding experimental data. Doping with Ti3+ ions leads to a slight decrease of the elastic parameters and lowering the Debye temperature by 20-40 K, because the Ti3+-O2-chemical bonds become longer and softer when compared with the Al3+-O2-ones in undoped materials. As a result, slight red shift of the most prominent features in the vibrational spectra is expected; this is confirmed by the performed calculations. Obtained results give a deeper insight into the properties of doped optical materials, highlight the effect of added impurity ions on their physical parameters and may serve as useful guides for smart materials engineering with wide opportunities of fine tuneability of their most important characteristics for potential applications.

Sr2FeO4-s is a first-order Ruddlesden-Popper perovsike-type material, prospective as a positrode (O-electrode) material for solid oxide and protonic ceramic electrochemical cells. By means of DFT + Ueff, oxygen vacancies (V center dot center dot O ) and interstitials (O"i ) have been modelled in a broad oxygen stoichiometry range of s = 0.125 to 0.5. For each s value, all symmetry inequivalent configurations were identified, yielding the V center dot center dot O formation energy in the range of 2.0-2.4 eV, almost independent of s (unlike for simple perovskites). We assign the individual Fe charge states in Sr2FeO4-s ("oxidation state map") based on magnetic moment and average Fe-O distance. Interestingly, V center dot center dot O and Fe3+ energetically prefer not to be nearest neighbors. Regarding oxygen interstitial species, a Sr-O2-2-Fe defect is the most stable form. Oxygen interstitial transport mechanism occurs predominantly within the SrO layer with the intermediate formation of a single oxide O2-i and an average barrier of 0.5 eV. Oxygen vacancy migration is preferred along a zig-zag path within the FeO2 layer with an effective barrier of 0.6 eV. The comprehensive analysis of Sr2FeO4-s-serving as prototype also for higher Ruddlesden-Popper phases-including oxidation state map and oxygen defect migration, paves the way for coming investigations of the protonated materials.

BaFeO3-delta is a prototypical "triple-conducting" perovskite combining electronic, proton and oxygen vacancy conductivities. Here, the interaction energies of protons at different sites with Ga3+, Sc3+, In3+, and Y3+ dopants on the Fe site in BaFeO3 are calculated using density functional theory (DFT). The effect of the dopants on the respective proton transfer barriers is also investigated. While for the smaller Ga3+ and Sc3+ dopants a slight trapping of protons in the first and second shell around the dopant is found, in the case of the strongly oversized In3+ and Y3+ the first shell exhibits a repulsive behaviour for protons (despite attractive electrostatic interaction). The calculated proton transfer barriers for different configurations depend sensitively on the local geometry. They follow the previously derived correlations with O-H bond lengths and O & sdot;& sdot;& sdot;O distances in BaFeO3-delta, corroborating that these quantities are physically meaningful descriptors for proton transfer in perovskites. Overall, a very complex energy landscape is obtained, and the consequences for long-range proton transport are discussed only qualitatively. The combination of a proton-repulsive first shell and the tendency for increased proton barriers suggests that for BaFeO3-delta, instead of the very oversized Y3+ smaller dopants should be considered.

Composites of two perovskites are one possibility to combine protonic and p-type electronic conductivity as required for oxygen electrodes in protonic ceramic electrochemical cells. The BaCeO3-BaFeO3 system can be acceptor-doped to increase proton uptake and transport. However, preceding experiments [C. Berger et al., J. Mater. Chem. A 10(2022)2474; C. Nader et al., Solid State Ionics 406 (2024) 116474] indicated that the dopants are inhomogeneously distributed between the two phases, which is decisive for hydration ability and proton conductivity of such composites. Here, we use extended density functional theory calculations (DFT+U, Hubbard approach) for a comprehensive characterization of the BaCeO3-BaFeO3 system including acceptors. Supercells of various compositions are calculated to derive chemical reaction energies, for example for the transfer of defects between the phases. Two key aspects related to the hydration ability of such materials are: (i) The development of the electronic structure with increasing Fe content in a (hypothetical) single-phase BaCe1-xFexO3 perovskite. (ii) The distribution of acceptors (Ga3+, Sc3+, In3+, Y3+) and oxygen vacancies (V center dot center dot O ) between Ce- and Fe-rich phases. The segregation driving forces of acceptor dopant and V center dot center dot O are calculated individually. V center dot center dot O have the largest driving force towards the Fe-rich phase; ion radii and acid/base properties of the different acceptor dopants play a secondary role. The co-segregation of acceptors and V center dot center dot O into the ferrate phase unfortunately decreases the hydration ability of the Ce-rich proton conductor phase. Analogous trends are expected for related proton- and hole-conductor perovskite composites, which partially counteracts the intended mixed conductivity.

Cesium hexafluorosilicate (Cs2SiF6, CSF) and potassium hexafluorosilicate (K2SiF6, KSF) compounds are suitable hosts for luminescent impurities. In this work, the results of first-principle calculations of the basic properties of both these compounds are discussed and compared with the available experimental and theoretical data. The simulations were performed using the CRYSTAL23 computer code within the linear combination of atomic orbitals (LCAO) method of the density functional theory (DFT) and the advanced hybrid DFT-HF exchange-correlation B1WC functional. A comparative study of the structural, electronic, and elastic properties of the two materials is presented, along with a study of the dependence of properties on external pressure in the range of 0-20 GPa. In particular, the electronic properties with an emphasis on the effective atomic charges (by means of Mulliken analysis) and the chemical bonding properties (by means of crystal orbital overlap population (COOP) analysis) were addressed, with regards to the pressure effects. The structure of the valence bands at 0 and 20 GPa was compared. The vibrational properties of CSF and KSF were calculated, including the simulation of the one-phonon IR and Raman spectra. The calculated Raman spectra exhibit excellent agreement with the experimental ones. The pressure dependences of sound speeds and the Debye temperature are evaluated.

The linear combination of atomic orbitals (LCAO) method is advantageous for calculating important bulk and surface properties of crystals and defects in/on them. Compared to plane wave calculations and contrary to common assumptions, hybrid density functional theory (DFT) functionals are actually less costly and easier to implement in LCAO codes. However, choosing the proper basis set (BS) for the LCAO calculations representing Guassian-type functions is crucial, as the results depend heavily on its quality. In this study, we introduce a new basis set (BS) visual representation, which helps us (1) analyze the collective behavior of individual atoms’ shell exponents (s, p, and d), (2) better compare different BSs, (3) identify atom-type invariant relationships, and (4) suggest a robust method for building a local all-electron BS (denoted as BS1) from scratch for each atom type. To compare our BS1 with the others existing in the literature, we calculate the basic bulk properties of SrTiO3 (STO) in cubic and tetragonal phases using several hybrid DFT functionals (B3LYP, PBE0, and HSE06). After adjusting the exact Hartree–Fock (HF) exchange of PBEx, HSEx, and the state-of-the-art meta-GGA hybrid r2SCANx functionals, we find the r2SCAN15 and HSE27 for BS1, with the amount of exact HF exchange of 0.15 and 0.27, respectively, perform equally well for reproducing several most relevant STO properties. The proposed robust BS construction scheme has the advantage that all parameters of the obtained BS can be reoptimized for each new material, thus increasing the quality of DFT calculation predictions.

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 (K2SiF6; KSF) crystal are discussed. KSF doped with manganese atoms (KSF:Mn4+) 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). For the study of KSF, we have applied and compared several DFT functionals (with emphasis on hybrid functionals) in combination with Gaussian-type basis sets. In order to determine the optimal combination for computation, two types of basis sets and four different functionals (three advanced hybrid—B3LYP, B1WC, and PBE0—and one LDA functional) were used, and the obtained results were compared with available experimental data. For the selected basis set and functional, the above-mentioned properties of KSF were calculated. In particular, the B1WC functional provides us with a band gap of 9.73 eV. The dependencies of structural, electronic and elastic parameters, as well as the Debye temperature, on external pressure (0–20 GPa) were also evaluated and compared with previous calculations. A comprehensive analysis of vibrational properties was performed for the first time, and the influence of isotopic substitution on the vibrational frequencies was analyzed. IR and Raman spectra were simulated, and the calculated Raman spectrum is in excellent agreement with the experimental one.

In the present work, we investigate the potential of modified barium titanate (BaTiO3), 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 BaTiO3 absorb more light and have less overpotential needed for water to oxidize. It has been shown that a TiO2-terminated BaTiO3 (001) surface is more promising from the point of view of its use as a catalyst. Rh doping expands the spectrum of absorbed light to the entire visible range. The aqueous environment significantly affects the ability of Rh-doped BaTiO3 to absorb solar radiation. After Ti→Rh replacement, the doping ion can take over part of the electron density from neighboring oxygen ions. As a result, during the water oxidation reaction, rhodium ions can be in an intermediate oxidation state between 3+ and 4+. This affects the adsorption energy of reaction intermediates on the catalyst’s surface, reducing the overpotential value.

O–O and O–H distances co-determine the proton migration barrier in triple conducting BaFeO 3− δ .

We present theoretical justification for distorted Ruddlesden-Popper (RP) phases of the first-order by using hybrid density functional theory (DFT) calculations and group-theoretical analysis. We, thus, demonstrate the existence of the Jahn-Teller effect around an Fe[Formula: see text] ion in Sr[Formula: see text]FeO[Formula: see text]. On the calculation side, we have established a combination of Wu-Cohen (WC) exchange and Perdew-Wang (PW) correlation in a three-parameter functional WC3PW, giving the most accurate description of Sr[Formula: see text]FeO[Formula: see text] from the comparison of three hybrid DFT functionals. Self-consistently obtained Hartree-Fock exact exchange of 0.16 demonstrates consistent results with the experimental literature data. Importantly, we explain conditions for co-existing proper and pseudo-Jahn-Teller effects from the crystalline orbitals, symmetry-mode analysis and irreps products. Moreover, phonon frequency calculations support and confirm the results of symmetry-mode analysis. In particular, the symmetry-mode analysis identifies a dominating irreducible representation of the Jahn-Teller mode (X2+) and corresponding space group (SG) of ground state structure (SG Cmce model). Therefore, the usually suggested high-symmetry tetragonal crystal structure (SG I4/mmm model) is higher in energy by 121 meV/f.u. (equivalent to the Jahn-Teller stabilization energy) compared with the distorted low-symmetry structure (SG Cmce model). We also present diffraction patterns for the two crystal symmetries to discuss the differences. Therefore, our results shed light on the existence of low-symmetry RP phases and make possible direct comparisons with future experiments.

In this work, the electronic structure and properties of NASICON-structured A4V2(PO4)3, where A = Li, Na, K were studied using hybrid density functional theory calculations. The symmetries were analyzed using a group theoretical approach, and the band structures were examined by the atom and orbital projected density of states analyses. Li4V2(PO4)3 and Na4V2(PO4)3 adopted monoclinic structures with the C2 space group and averaged vanadium oxidation states of V+2.5 in the ground state, whereas K4V2(PO4)3 adopted a monoclinic structure with the C2 space group and mixed vanadium oxidation states V+2/V+3 in the ground state. The mixed oxidation state is the least stable state in Na4V2(PO4)3 and Li4V2(PO4)3. Symmetry increases in Li4V2(PO4)3 and Na4V2(PO4)3 led to the appearance of a metallic state that was independent of the vanadium oxidation states (except for the averaged oxidation state R32 Na4V2(PO4)3). On the other hand, K4V2(PO4)3 retained a small band gap in all studied configurations. These results might provide valuable guidance for crystallography and electronic structure investigations for this important class of materials.

The atomic structure of antiphase boundaries in Sr-doped lanthanum scandate (La1−xSrxScO3−δ) 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 mechanisms of oxygen vacancy formation and migration in both types of interfaces were modelled. It was shown that both interfaces are structurally stable and facilitate oxygen ionic migration. Oxygen vacancy formation energy in interfaces is lower than that in the regular structure, which favours the oxygen vacancy segregation within such interfaces. The calculated energy profile suggests that both types of interfaces are advantageous for oxygen ion migration in the material.

Sr-doped lanthanum scandate La1−xSrxScO3−δ (LSS) is a promising perovskite-type material for electrochemical applications such as proton conductors. Oxygen vacancy is a common defect in ABO3-type perovskites. It controls ion transport as well as proton uptake. The energetic, structural, and electronic properties of oxygen vacancy in LSS are studied deploying the DFT method with meta-GGA functional. The vacancy formation energies in LSS were calculated for various Sr concentrations. Unlike other perovskites, in this material, the electrons are trapped at the oxygen vacancy site (the F-type centres, common in ionic oxides like MgO and Al2O3) rather than localised on the nearest to the vacancy B-cations. The process of oxygen vacancy formation is considered relative to Sr concentration x and oxygen nonstoichiometry factor δ. Three primary regimes are discussed: (I) localized at the vacancy electrons, x/δ < 2, (II) electron charge balanced system, x/δ = 2, and (III) delocalized electron holes, x/δ > 2. For x/δ ≥ 2 oxygen vacancy formation energy reaches the saturation level of ~3.5 eV, which is potentially beneficial for the proton uptake.

We use the hybrid B1WC density functional calculations and the supercell model to analyze properties of BaCoO3 (BCO) in the corner-sharing CoO6 octahedra geometry. We compare the atomic and electronic structures for ideal and defective BCO containing a single oxygen vacancy (Vo). We find that the ideal BCO is better described by the triclinic space group (SG) P1 supercell than the cubic SG Pm3m model. The site symmetry approach allows us to derive several Vo supercell models on the basis of SGs P4mm, Pmm2 or Amm2 from the aristotype cubic SG Pm3m. The low symmetry Pmm2 model suggests the 0.4 eV formation energy of Vo in BCO. Furthermore, sta-bility of this result is confirmed by the calculation of Vo in the supercell without symmetry (SG P1). The ideal and defective BCO are characterized by the intermediate spin (IS) states of Co4+ and Co4+& Co3+ ions, respectively. Additional complexity is introduced by the smooth Co magnetic moment dependence on the Co-Vo distance.

Phosphate frameworks with NASICON structure are among the most studied and applied Li- and Na-ion battery electrode and electrolyte materials. In this work, the NASICON-structured Na3-xV2-xTix(PO4)3 with x = 0.0, 0.25, 0.5, 0.75 and 1.0 are successfully prepared by conventional solid-state synthesis and characterized in detail as potential aqueous Na-ion battery positive electrodes with improved charge capacity and cycling stability. Structural analysis using powder X-ray diffractometry indicates that titanium substitutes vanadium at arbitrary concentration without significant distortion of the NASICON structure. The results show that titanium content in this system directly correlates with its aqueous stability when cycled in simple 1 M Na2SO4 aqueous electrolyte within the vanadium redox potential range. Electrochemical kinetics and charge capacity measurements show Na2VTi(PO4)3 as well as Na2.25V1.25Ti0.75(PO4)3 to be stable positive electrodes in simple aqueous electrolyte solutions. Hybrid density functional theory analysis of V-O chemical bonding suggests that it is stabilized by the presence of titanium in the NASICON structure. In this work, we show that the observed capacity loss in full symmetric cells is caused by the capacity imbalance between positive and negative electrodes which progresses during cycling but not the aqueous materials stability per se. This imbalance is caused by several parasitic reactions, the most important being the oxygen reduction reaction catalyzed by Ti(III) species. Careful mitigation and management of this reaction could, in principle, allow for the preparation of truly capacity balanced cells (i.e. without a need of any electrode overcapacity), and superior cycling stability.

NAtrium SuperIonic CONductor (NASICON) structured phosphate framework compounds are attracting a great deal of interest as suitable electrode materials for “rocking chair” type batteries. Manganese-based electrode materials are among the most favored due to their superior stability, resource non-criticality, and high electrode potentials. Although a large share of research was devoted to Mn-based oxides for Li- and Na-ion batteries, the understanding of thermodynamics and phase formation in Mn-rich polyanions is still generally lacking. In this study, we investigate a bifunctional Na-ion battery electrode system based on NASICON-structured Na1+2xMnxTi2–x(PO4)3 (0.0 ≤ x ≤ 1.5). In order to analyze the thermodynamic and phase formation properties, we construct a composition–temperature phase diagram using a computational sampling by density functional theory, cluster expansion, and semi-grand canonical Monte Carlo methods. The results indicate finite thermodynamic limits of possible Mn concentrations in this system, which are primarily determined by the phase separation into stoichiometric Na3MnTi(PO4)3 (x = 1.0) and NaTi2(PO4)3 for x < 1.0 or NaMnPO4 for x > 1.0. The theoretical predictions are corroborated by experiments obtained using X-ray diffraction and Raman spectroscopy on solid-state and sol–gel prepared samples. The results confirm that this system does not show a solid solution type behavior but phase-separates into thermodynamically more stable sodium ordered monoclinic α-Na3MnTi(PO4)3 (space group C2) and other phases. In addition to sodium ordering, the anti-bonding character of the Mn–O bond as compared to Ti–O is suggested as another important factor governing the stability of Mn-based NASICONs. We believe that these results will not only clarify some important questions regarding the thermodynamic properties of NASICON frameworks but will also be helpful for a more general understanding of polyanionic systems.

Despite the development of medicine, as well as robotics aimed at helping people with disabilities, most disabled people with missing limbs cannot hope for a full life with the help of bionic prostheses, since these products are made strictly individually, and their control requires expensive surgeries to implant sensors. In addition, such prostheses themselves are very expensive. An alternative solution is to reduce the cost of such prostheses as much as possible and abandon the implantable sensors of signals of the human nervous system. The paper proposes a method for solving such a problem using a variety of sensors that remove signals from the skin surface without invasive surgery.

The paper provides a rationale for the use of low-power wireless communication to create non-invasive bionic limb prostheses. The need for such a connection is dictated, on the one hand, by the expediency of using sensors located in various parts of the body, often very remote from each other, on the other hand, by the desire to free the disabled person’s body from the wires that would envelop it like a spider web. For each of these positions, arguments are given.

Cobalt based perovskites have great potential for numerous applications. Contrary to a generally assumed hexagonal space group (SG P63/mmc) model as the ground state of BaCoO3 (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. Rev. B, 2019, 100, 205139]. We found for the monoclinic BCO that the C-type anti-ferromagnetic low-spin (AFM LS) state (SG P2/c) is energetically only slightly more preferential at 0 K than the ferromagnetic (FM) LS state (SG C2/c). In turn, these monoclinic structures are energetically more favourable than the hexagonal ones, due to slight z-axis tilting. The analysis of density of states (DOS) and crystal orbital overlap population (COOP) shows a significant (almost 2 eV) separation between occupied and empty t2g states (in the spin-down channel and corresponding anti-bonding states) induced by the z-axis tilting.