Ordered phases of lithium nickelite Li<Subscript>1−<Emphasis Type="Italic">x</Emphasis>−<Emphasis Type="Italic">z</Emphasis></Subscript>Ni<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript>

A symmetry analysis of ordering in lithium nickelite Li_1?x?zNi_1+xO₂ (Li_1?x?z□_yNi_1+xO₂) was performed with regard to the substitution of Li and Ni atoms and the occurrence of structural vacancies □ in the metal sublattice. For all the ordered phases, the k ₉ ⁽³⁾ ray of the Lifshitz {k₉} star is present in the order-disorder transition channel. This ray determines the consecutive alternation of atomic planes filled with only Ni atoms or only Li atoms and vacancies in the \([1\bar 11]_{B1} \) direction. It was shown that the rhombohedral ordered LiNiO₂ phase is formed in the defect-free lithium nickelite, whereas a family of three monoclinic Li₃□Ni₄O₈ (C2/m space group) and Li₂□Ni₃O₆ (C2/m and C2 space groups) superstructures arises as the concentration of structural vacancies increases. For all the superstructures, the order-disorder phase-transition channels were determined and the distribution functions of Li and Ni atoms have been calculated. The long-range order parameters describing each superstructure were found as functions of the Li_{1?x? z}Ni_1+xO₂ composition.     

Neutron-diffraction study of the crystal structure of BaTiO<Subscript>3</Subscript> ferroelectric doped with iron

The crystal structure of iron-doped barium titanate BaTi_1–x Fe _x O₃ is studied by neutron diffraction in the range of 0 ≤ x ≤ 0.12. At low concentrations of iron, x < 0.01, and at room temperature, these compounds have a polar structure with tetragonal symmetry with space group P4mm. The temperature of the transition of the tetragonal ferroelectric phase into the cubic paraelectric phase with space group Pm \(\bar 3\) m for an iron concentration of x = 0.01 is 390 K (for pure BaTiO₃, it is 410 K). At an iron concentration of x = 0.07, the crystal structure of the studied compounds varies, and it is described by the centrosymmetric hexagonal space group P6₃/mmc. The structural parameters of various phases of compound BaTi_1–x Fe _x O₃ are determined from the experimental data.     

Ab initio calculations of the stability and structural defects of the <Emphasis Type="Italic">B</Emphasis>2 Cu<Subscript>x</Subscript>Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Al phases

The stability of the B2 Cu_xFe_1?x Al phases and the energy of defect formation are studied using ab initio band calculations. For B2 Cu_xFe_1?x Al alloys, vacancies in the 3d-metal sublattice and configurations with the minimum number of Fe-Cu bonds in the first coordination shell (including Fe antisite defects, which have a high local magnetic moment) are most stable. Complicated defect complexes with vacancies and displaced atoms, which are close to the atomic configurations of vacancy-ordered AlCu phases, can exist near the composition Cu_0.875Fe_0.125Al.     

Structure,microstructure, and dielectric responses of (1–<Emphasis Type="Italic">х</Emphasis>)BiFeO<Subscript>3</Subscript>–<Emphasis Type="Italic">x</Emphasis>BaTiO<Subscript>3</Subscript> solid solutions

The structure, microstructure, and dielectric responses of (1–х)BiFeO₃–xBaTiO₃ (0.0 < x ≤ 0.5) solid solutions prepared as ceramics are studied. Patterns in their dielectric parameters and the formation of their crystal and grain structures are investigated in a wide range of temperatures and frequencies.     

Oxygen vacancies effects on phase diagram of epitaxial La<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> thin films

We investigated the effects of oxygen vacancies on the structural, magnetic, and transport properties of La_1–x Sr _x MnO₃ (x=0.1, 0.2, 0.33, 0.4, and 0.5) grown around a critical point (without/with oxygen vacancies) under low oxygen pressure (10 Pa) and high oxygen pressure (40 Pa). We found that all films exhibit ferromagnetic behavior below the magnetic critical temperature, and that the films grown under low oxygen pressures have degraded magnetic properties with lower Curie temperatures and smaller magnetic moments. These results show that in epitaxial La_1–x Sr _x MnO₃ thin films, the magnetic and transport properties are very sensitive to doping concentration and oxygen vacancies. Phase diagrams of the films based on the doping concentration and oxygen vacancies were plotted and discussed.     

Symmetry and lattice parameters of SrTiO<Subscript>3</Subscript>-PbTiO<Subscript>3</Subscript> solid solutions

The lattice parameters of ceramic samples of (1 ? x)SrTiO₃-xPbTiO₃ solid solutions are measured at room temperature. It is found that the samples have cubic symmetry in the concentration range x = 0?0.3 and tetragonal symmetry for x > 0.3. The lattice parameter a is virtually independent of temperature for x < 0.8 and slightly decreases in the range x = 0.8?1.0, while the lattice parameter c increases with increasing x. The reduced cubic parameter varies nonlinearly and deviates from Vegard’s linear law as the concentration x increases.     

Electro-optic effect in SrTiO3 and Sr1–x Ca x TiO3 (x = 0.014)

The results of the investigation of the quadratic electro-optic effect in Sr_1–x Ca _x TiO₃ with x = 0.014 (SCT) and in nominally pure SrTiO₃ (STO) at room temperature in applied direct-current (dc) and alternating-current (ac) electric fields have been presented. It has been shown that the quadratic (in polarization) electro-optic coefficients of STO and SCT crystals coincide within the accuracy of the determination (±5%). It has been found that, in nominally pure STO measured in a dc electric field, there is a relaxation of the electro-optic effect with a relaxation time τ ≈ 30 s due to the formation of a space charge in the sample. No similar effect in SCT has been observed. A possible mechanism for the formation of a space charge in STO and SCT has been discussed.     

Ab initio calculations of Born charges in ferroelectrics with a perovskite structure

A method has been proposed for calculating Born effective charges in compounds with a cubic perovskite structure. The method is based on the first-principles calculation of individual contributions from the short-range interaction and the intercell dipole-dipole interaction to the Born tensor Z _ii (s) for crystalline dielectrics. It has been shown that the contribution from the short-range interaction Z _ii ^sr (s) to the Born tensor components can be derived from ab initio calculations performed for polyatomic clusters. The results of the calculations of the short-range interactions Z _ii ^sr (s) for the cubic phases of the BaTiO₃, SrTiO₃, CaTiO₃, PbTiO₃, BaZrO₃, PbZrO₃, KNbO₃, and KTaO₃ compounds with the use of the electronic structure calculations within the Hartree-Fock approximation and the density functional theory are presented. For the BaTiO₃, SrTiO₃, CaTiO₃, PbTiO₃, KNbO₃, and KTaO₃ compounds, the components of the complete Born tensor have been also calculated. The obtained values of Z _ii (s) are in good agreement with the results of the calculations in terms of the linear response theory and the Berry phase approach.     

High-temperature complex impedance and modulus spectroscopic studies of doped Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-BaTiO<Subscript>3</Subscript> ferroelectric ceramics

Lead-free Na_0.5Bi_0.5TiO₃ (NBT) and (1 ? x)Na_0.5Bi_0.5TiO₃ + xBaTiO₃ with x = 0.1 and 0.2 (where x = 0.1 and 0.2 are named as NBT1 and NBT2, respectively), (1 ? y)Na_0.5Bi_0.5TiO₃ + yBa_0.925Nd_0.05TiO₃ with y = 0.1 and 0.2 (where y = 0.1 and 0.2 are named as NBT3 and NBT4, respectively)-based relaxor ferroelectric ceramics were prepared using the sol-gel method. The crystal structure was investigated by X-ray diffraction (XRD) at room temperature (RT). The XRD patterns confirmed the presence of the rhombohedral phase in all the samples. The electrical properties of the present NBT-based samples were investigated by complex impedance and the modulus spectroscopy technique in the temperature range of RT–600 °C. The AC conductivity was found to increase with the substitution of Ba²⁺ ions to the NBT sample whereas it significantly decreased with the addition of Nd³⁺ ions. The more anion vacancies in Ba-added samples and the lower anion vacancies in Nd-added samples were found to be responsible for higher and lower conductivities, respectively.     

Structural phase transition in (1−<Emphasis Type="Italic">x</Emphasis>)SrTiO<Subscript>3</Subscript>+<Emphasis Type="Italic">x</Emphasis>SrMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript> solid solutions

The temperature dependences of the shear modulus and internal friction in ceramic samples of (1?x)SrTiO₃+xSrMg_1/3Nb_2/3O₃ solid solutions were studied by the torsional vibration method in the range 80–300 K. It was established that the temperature T _a of the O _h ¹ -D _4h ¹⁸ structural phase transition in these solid solutions increases with increasing x, although the lattice constant also increases. A discussion is presented of the contributions to the T _a (x) dependence due to a change in the volume and changes caused by the presence of a second solid-solution component (the impurity contribution). It is also shown that the defect-induced relaxation revealed earlier in a study of the dielectric properties of these materials manifests itself in internal friction peaks.     

Mechanical and Dynamic Stability of Complete and Nonstoichiometric 3<Emphasis Type="Italic">C</Emphasis>-Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>C<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> from Ab Initio Calculations

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-Si_xC_y (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-SiC_0.875 to 3C-SiC_0.75. It is established from the analysis of the phonon dispersion curves that the 3C-SiC_0.875 and 3C-SiC_0.75 phases, in contrast to the complete 3C-SiC phase, are dynamically unstable at T = 0 K.     

Hall effect in the Ce(Al<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript> heavy-fermion system

The influence of substitution on the binding energy of many-body states and the formation of the magnetically ordered state in a heavy-fermion compound (CeAl₂) have been studied by measuring the transport characteristics (Hall effect, resistivity) in intermetallic compounds of the Ce(Al_1?x M _x )₂ system (M = Ni, Co; x ≤ 0.08). It is established that the Hall coefficient R _H in Ce(Al_1?x Co _x )₂ intermetallides with x = 0.05 and 0.08 grows by more than an order of magnitude as the temperature decreases from 1.8 to 300 K. The experimental data are used to estimate the effective mass of charge carriers, the relaxation time, and the localization radius of many-body states.     

Calorimetry of (1 − <Emphasis Type="Italic">x</Emphasis>)BaTiO<Subscript>3</Subscript>-<Emphasis Type="Italic">x</Emphasis>Ba(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> solid solutions

Phase transitions in ceramic samples of (1 ? x)BaTiO₃-xBa(Mg_1/3Nb_2/3)O₃ solid solutions were studied by differential scanning calorimetry for x = 0, 0.01, 0.02, and 0.03 in the temperature ranges 255–290 K and 310–410 K. The experimental data obtained were used to derive the thermodynamic parameters of the phase transitions occurring at T _c ≈ 400?300 K and T ₁ ≈ 290?300 K and construct the concentration phase diagram.     

Thermooptical and Dielectric Studies of a Calcium-Induced Ferroelectric Phase in a SrTiO<Subscript>3</Subscript> Incipient Ferroelectric

We have examined temperature changes of the light refraction, birefringence, dielectric permittivity, and dielectric hysteresis loops in Sr_{1 – x}Ca_xTiO₃ single crystals with x = 0.014 (SCT-1.4). The dielectric properties of Sr_{1 – x}Ca_xTiO₃ with x = 0.007 (SCT-0.7) have been studied. We have performed ab initio calculations of equilibrium structures and total energies for three low-temperature phases of SrTiO₃ and CaTiO₃, based on which we have determined an expected symmetry of the ground state of their solid solution and spontaneous polarization directions in a calcium-induced ferroelectric phase in Sr_{1 – x}Ca_xTiO₃. In SCT-1.4, we have separated a spontaneous contribution to the light refraction, which arises due to the spontaneous electrooptical effect caused by the spontaneous polarization and its fluctuations. From the spontaneous contribution to the light refraction, based on a previously developed our phenological approach, we have quantitatively determined for the first time the values and temperature dependences of root-mean-square fluctuations of the order parameter—the polarization P_sh = 〈P _fl ² 〉^1/2(short-range, local polar order) in the ferroelectric phase. From optical and dielectric measurements in SCT-1.4, the average value of spontaneous polarization P_s (the contribution from the long-range order) has been determined. We have estimated the values of P_sh and P_s, which characterize the short- and long-range orders in the ferroelectric phase of SCT-0.7. Separate determination of the values and temperature dependences of P_s and P_sh (which considerably exceeds the value of P_s in the ordered phase) has allowed us to reveal on a quantitative level new particular features of the formation of the induced polar phase in Sr_{1 – x}Ca_xTiO₃.     

Effect of cation substitution on the lattice dynamics and ferroelectric instability of cubic BaTiO<Subscript>3</Subscript> and BaZrO<Subscript>3</Subscript> doped with Bi and La ions

The spectra of lattice vibration frequencies of solid solutions Ba_{1 ? x} Bi_2x/3? _x/3Ti(Zr)O₃ and Ba_{1 ? x} La _x Ti(Zr)_{1 ? x/4}? _x/4O₃ are calculated in terms of a generalized Gordon-Kim model with inclusion of the dipole and quadrupole polarizabilities. Over the entire concentration range, the calculated phonon spectra contain a ferroelectric soft mode. The effect of various interactions on the ferroelectric instability of these solid solutions is studied. It is shown that the character of ferroelectric instability is largely determined by the mechanism of charge compensation.     

Electron gas induced in SrTiO<Subscript>3</Subscript>

This mini-review is dedicated to the 85th birthday of Prof. L.V. Keldysh, from whom we have learned so much. In this paper, we study the potential and electron density depth profiles in surface accumulation layers in crystals with a large and nonlinear dielectric response such as SrTiO₃ (STO) in the cases of planar, spherical, and cylindrical geometries. The electron gas can be created by applying an induction D₀ to the STO surface. We describe the lattice dielectric response of STO using the Landau–Ginzburg free energy expansion and employ the Thomas–Fermi (TF) approximation for the electron gas. For the planar geometry, we arrive at the electron density profile n(x) ∝ (x + d)^–12/7, where d ∝ D₀^–12/7. We extend our results to overlapping electron gases in GTO/STO/GTO heterojunctions and electron gases created by spill-out from NSTO (heavily n-type doped STO) layers into STO. Generalization of our approach to a spherical donor cluster creating a big TF atom with electrons in STO brings us to the problem of supercharged nuclei. It is known that for an atom with a nuclear charge Ze where Z > 170, electrons collapse onto the nucleus, resulting in a net charge Zn < Z. Here, instead of relativistic physics, the collapse is caused by the nonlinear dielectric response. Electrons collapse into the charged spherical donor cluster with radius R when its total charge number Z exceeds the critical value Z_c ≈ R/a, where a is the lattice constant. The net charge eZ_n grows with Z until Z exceeds Z* ≈ (R/a)^9/7. After this point, the charge number of the compact core Z_n remains ≈ Z*, with the rest Z* electrons forming a sparse TF atom with it. We extend our studies of collapse to the case of long cylindrical clusters as well.     

Spectroscopy of charge transfer states in Mg<Subscript>1 – <Emphasis Type="Italic">x</Emphasis> </Subscript>Ni<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>O

Photoluminescence and photoluminescence excitation spectra of solid solution Mg_1–xNi_xO (x = 0.008) have been analyzed. The contributions of charge transfer electronic states and nonradiative Auger relaxation to the formation of the photoluminescence spectrum are discussed.     

On the origin of the shallow and “replica” bands in FeSe monolayer superconductors

We compare the electronic structures of single FeSe layer films on SrTiO₃ substrate (FeSe/STO) and K _x Fe_2-y Se₂ superconductors obtained from extensive LDA and LDA + DMFT calculations with the results of ARPES experiments. It is demonstrated that correlation effects on Fe-3d states are sufficient in principle to explain the formation of the shallow electron-like bands at the M(X)-point. However, in FeSe/STO these effects alone are apparently insufficient for the simultaneous elimination of the hole-like Fermi surface around the Γ-point which is not observed in ARPES experiments. Detailed comparison of ARPES detected and calculated quasiparticle bands shows reasonable agreement between theory and experiment. Analysis of the bands with respect to their origin and orbital composition shows, that for FeSe/STO system the experimentally observed “replica” quasiparticle band at the M-point (usually attributed to forward scattering interactions with optical phonons in SrTiO₃ substrate) can be reasonably understood just as the LDA calculated Fe-3d _xy band, renormalized by electronic correlations. The only manifestation of the substrate reduces to lifting the degeneracy between Fe-3d _xz and Fe-3d _yz bands near M-point. For the case of K _x Fe_2-y Se₂ most bands observed in ARPES can also be understood as correlation renormalized Fe-3d LDA calculated bands, with overall semi-quantitative agreement with LDA + DMFT calculations.     

Mechanism of charge transfer in <Emphasis Type="Italic">n</Emphasis>-CdS/<Emphasis Type="Italic">p</Emphasis>-CdTe heterojunctions

The capacitance-voltage and current-voltage characteristics of the n-CdS/p-CdTe heterosystem are investigated. Analysis of these characteristics demonstrates that the CdTe_1?x S _x solid solution formed at the n-CdS/p-CdTe heterointerface is inhomogeneous in both the conductivity and composition. The thickness of solid solutions is estimated from the capacitance-voltage characteristics. It is shown that, for the n-CdS/p-CdTe heterosystem, the current-voltage characteristic in the current density range 10^?8-10^?5 A cm^?2 is governed by the thermal electron emission, whereas the current in the heterostructure at current densities in the range 10^?4-10^?2 A cm^?2 is limited by recombination of charge carriers in the electroneutral region of the CdTe_1?x S _x solid solution. The lifetime and the diffusion length of minority charge carriers in the CdTe_1?x S _x solid solution and the surface recombination rate at the interface between the CdS layer and the CdTe_1?x S _x solid solution are determined. It is demonstrated that the n-CdS/p-CdTe heterostructure operates as a p-i-n structure in which CdTe is a p layer, CdTe_1?x S _x is an i layer, and CdS is an n layer.     

Sodium–cesium molybdenum-oxide bronzes

The region of the electrolytic deposition of bialkaline sodium–cesium molybdenum-oxide bronzes is established. A correlation between the nonstoichiometry of bronzes and Mo_nO_3n–x oxides with respect to molybdenum charge density, crystal structure, and type of electrical conduction is found. The importance of the Mo_nO_3n–x homologous series in the formation of the nonstoichiometry of bronzes and their physical properties is shown. The electrophoretic deposition of coatings with binary bronzes for the anticorrosion protection of metals is investigated.