Effect of nonstoichiometry on the lattice constant of cubic vanadium carbide VC<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

The effect of nonstoichiometry on the lattice constant of cubic vanadium carbide VC _y (0.65 < y < 0.875) is studied. It is found that the ordering of vanadium carbide VC _y with the formation of superstructures V₆C₅ and V₈C₇ leads to an increase in the base lattice constant in comparison with disordered carbide. Taking into account the change in the lattice constant, the direction of the static displacements of atoms near the vacancy is discussed.     

Ordered monoclinic vanadium suboxide V<Subscript>14</Subscript>O<Subscript>6</Subscript>

The monoclinic (space group C2/m) superstructure of the suboxide V₁₄O₆, which is formed as a result of the atomic and vacancy ordering of the tetragonal solid solution of oxygen in vanadium, is investigated using X-ray diffraction and symmetry analysis. The monoclinic suboxide V₁₄O₆ is observed in the vanadium oxide samples VO_0.57, VO_0.81, and VO_0.86 synthesized at 1770 K and the samples VO _y (0.87 ≤ y ≤ 0.98) additionally annealed at 1470 K after the synthesis. It is established that the channel of the disorder-order phase transition associated with the formation of the monoclinic suboxide V₁₄O₆ includes six superstructure vectors belonging to three non-Lifshitz stars of one type {k ₁}. The distribution function of the oxygen atoms in the monoclinic superstructure of the suboxide V₁₄O₆ is calculated. It is demonstrated that the displacements of vanadium atoms distort the body-centered tetragonal metal sublattice, thus preparing the formation of the facecentered cubic sublattice and the transition from the suboxide V₁₄O₆ to the cubic vanadium monoxide with the B1 structure.     

Magnetic properties of the Gd<Subscript>2</Subscript>V<Subscript>0.67</Subscript>Mo<Subscript>1.33</Subscript>O<Subscript>7</Subscript> and Y<Subscript>2</Subscript>VMoO<Subscript>7</Subscript> pyrochlore compounds

It is demonstrated that 50% substitution of vanadium for molybdenum in the pyrochlore lattice of the complex oxide Y₂(V _x Mo_{1 ? x} )₂O₇ results in a transition from the spin-glass ground state (at x = 0) to the ferromagnetic state in Y₂VMoO₇ (a = 10.1645(2) Å, T _C = 55 K). The Gd₂V_0.67Mo_1.33O₇ compound (a = 10.2862(3) Å) is a ferromagnet with T _C (84 K) exceeding that of undoped Gd₂MnO₂O₇.     

V<Subscript>52</Subscript>O<Subscript>64</Subscript> tetragonal superstructure of cubic vanadium monoxide with vacancies in the metal sublattice

The atom-vacancy ordering of cubic vanadium monoxide VO_1.29, which has basis cubic structure B1 and structural vacancies in the metal sublattice, has been studied using the x-ray diffraction method. It has been shown that the formation of the tetragonal (space group I4₁/amd) ordered phase V₅₂O₆₄ of cubic vanadium monoxide VO_y proceeds as a first-order phase transition through the disorder-order channel including 22 nonequivalent superstructure vectors of four stars {k ₁₀}, {k ₄}, {k ₃}, and {k ₂}. The distribution function of the vanadium atoms in the V₅₂O₆₄ tetragonal superstructure has been calculated.     

Effect of ordering on the structure and heat capacity of cubic vanadium carbonitrides VC<Subscript>x</Subscript>N<Subscript>y</Subscript>

Experimental results are presented on measurements of the crystal structure and heat capacity of nonstoichiometric cubic vanadium carbonitrides VC_xN_y (x + y = 0.85) in the region of disorder-order phase transitions. It is found that ordered phases V₆(C,N)₅□ and V₈(C,N)₇□ with the structures of the V₆C₅ and V₈C₇ types form in vanadium carbonitrides at a temperature of ～1100 K through the first-order phase transition mechanism. The channels of disorder-order transitions are determined. It is found that, in the nonmetal sublattice of the detected ordered phases, C and N atoms form one sublattice and structural vacancies □ form another sublattice. C and N atoms are randomly distributed in their sublattice.     

Structure,ionic conduction,and phase transformations in lithium titanate Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript>

The spinel structure of lithium titanate Li₄Ti₅O₁₂ is refined by the Rietveld full-profile analysis with the use of x-ray and neutron powder diffraction data. The distribution and coordinates of atoms are determined. The Li₄Ti₅O₁₂ compound is studied at high temperatures by differential scanning calorimetry and Raman spectroscopy. The electrical conductivity is measured in the high-temperature range. It is shown that the Li₄Ti₅O₁₂ compound with a spinel structure undergoes two successive order-disorder phase transitions due to different distributions of lithium atoms and cation vacancies (□, V) in a defect structure of the NaCl type: (Li)_8a[Li_0.33Ti_1.67]_16dO₄ → [Li□]_16c[Li_1.33Ti_1.67]_16dO₄ → [Li_1.33□_0.67]_16c[Ti_1.67□_0.33]_16dO₄. The low-temperature diffusion of lithium predominantly occurs either through the mechanism ... → Li(8a) → V(16c) → V(8a) → ... in the spinel phase or through the mechanism ... → Li(16c) → V(8a) → V(16c) → ... in an intermediate phase. In the high-temperature phase, the lithium cations also migrate over 48f vacancies: ... Li(16c) → V(8a, 48f) → V(16c) → ....     

Magnetic phase diagram of the Bi<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript>x</Subscript>MnO<Subscript>3</Subscript> manganites

The magnetic and elastic properties of the Bi_1-xCa_xMnO₃ manganites are studied. The phase transformations revealed are ferromagnet-spin glass (x≥0.15) and spin glass-charge-ordered antiferromagnet (x≥0.25). The ferromagnetic state is characterized by ordering of the Mn³⁺d _x ²-y orbitals. It is suggested that thespin glass state originates from local static Jahn-Teller distortions. The antiferromagnetic charge-ordered and the spin-glass disordered phases coexist in samples with 0.25<x<0.32, which may be due to the charge order-disorder phase transformation being martensitic in character. The magnetic phase diagram is constructed.     

X-ray study of [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> at low temperatures

The unit cell parameters a, b, and c of [N(CH₃)₄]₂ZnCl₄ have been measured by x-ray diffraction in the temperature range 80–293 K. Temperature dependences of the thermal expansion coefficients α_a, α_b, and α_c along the principal crystallographic axes and of the unit cell thermal expansion coefficient α_V were determined. It is shown that the a=f(T), b=f(T), and c=f(T) curves exhibit anomalies in the form of jumps at phase transition temperatures T₁=161 K and T₂=181 K and that the phase transition occurring at T₃=276 K manifests itself in the a=f(T) and b=f(T) curves as a break. A slight anisotropy in the coefficient of thermal expansion of the crystal was revealed. The phase transitions occurring at T₁=161 K and T₂=181 K in [N(CH₃)₄]₂ZnCl₄ were established to be first-order.     

Dependence of the magnetic and superconducting characteristics of fine-grained HTSCs YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> on the degree of structural disorder and type of oxygen vacancy ordering in chain planes

The characteristics determining different contributions to the magnetic susceptibility at T > T _C (Pauli susceptibility, coherence length at T = 0, and Curie constant) as functions of the degree of structural disorder have been analyzed for high-temperature superconducting YBa₂Cu₃O _y samples ( y ≈ 6.92, T _C ≈ 92 K) with micrometer and submicron average grain sizes D _av. It is shown that the decrease in these characteristics, which is observed in fine-grained samples with a decrease in D _av, occurs in various ways, depending on the number and type of oxygen vacancy ordering in chain planes.     

Anisotropy of the Complex Permittivity of the Kagome-Staircase Compounds Co<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript> and Ni<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript>: Experiment and Ab Initio Calculations

The anisotropy of the components of the complex permittivity of vanadate Co₃V₂O₈ and Co₃V₂O₈ single crystals in the paramagnetic phase are studied by optical ellipsometry in the spectral region 0.5–5.0 eV. Our experimental results support the weak anisotropy of the optical response detected earlier for axes a and c. The optical properties are also investigated along axis b. The properties of both compounds are compared. The optical spectra of both compounds along axis b are shifted toward low energies as compared to axes a and c. The maximum of the main interband absorption band of Co₃V₂O₈ is shifted toward low energies by 0.25–0.3 eV as compared to Co₃V₂O₈. The electronic structure parameters of both compounds are determined. Optical function spectra are analyzed using the results of ab initio band calculations.     

Antisite defects and valence state of vanadium in Na<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

Electron paramagnetic resonance (EPR) studies have been performed with the aim of determining the valence state and local crystal structure of the nearest environment of vanadium ions in the initial, charged, and discharged samples of the cathode material Na_xV₂(PO₄)₃ (1 ≤ x ≤ 3). It has been found that the charged sample (x = 1) is characterized by an intense signal corresponding to V⁴⁺ ions located in a highly distorted octahedral crystal field. An EPR signal with the g-factor close to the g-factor of the V⁴⁺ ion has also been observed in the initial sample (x = 3), where the intensity of the resonance signal is one order of magnitude lower than that in the charged sample. It has been revealed that the resonance signal under consideration is associated with the formation of antisite defects when a part of vanadium ions are located in sites of sodium ions. It has also been found that the intensity of this signal increases after a complete charge–discharge cycle (x = 3).     

Nonstoichiometric high-pressure phase of Tm<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>3</Subscript>V<Subscript>4</Subscript>O<Subscript>12</Subscript>

Tm _x Cu₃V₄O₁₂, a perovskite-like oxide (space group, Im-3; Z = 2; a = 7.279–7.293 Å) containing vacancies in its cationic sublattice, was obtained barothermally (P = 7.0–9.0 GPa, t = 1000–1100°C) for the first time. The temperature dependences on the electrical resistivity (10–300 K) and the magnetic susceptibility (0–300 K) were investigated. It was shown that the oxide Tm _x Cu₃V₄O₁₂ is characterized by metal-type conductivity and paramagnetic properties.     

Effect of carbon vacancies on the electric resistivity of nonstoichiometric VC<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> vanadium carbide

The influence of the temperature, concentration, and distribution of structure vacancies of the carbon sublattice on the electric resistivity ρ of nonstoichiometric VC _y vanadium carbide (0.66 ≤ y ≤ 0.875) has been studied in the temperature range of 300–1200 K. The symmetry and structure characteristics of the ordered V₆C₅ and V₈C₇ phases formed owing to low-temperature annealing on various sections of the homogeneity region of the VC _y carbide. The dependence of the residual electric resistivity on the content of the disordered vanadium carbide is explained by the atom-vacancy interaction and the change in the carrier concentration in the homogeneity region of VC _y .     

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.     

A theoretical investigation of structural,mechanical, electronic and thermoelectric properties of orthorhombic CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>PbI<Subscript>3</Subscript>

The structural, mechanical, electronic and thermoelectric properties of the low temperature orthorhombic perovskite phase of CH₃NH₃PbI₃ have been investigated using density functional theory (DFT). Elastic parameters bulk modulus B, Young’s modulus E, shear modulus G, Poisson’s ratio ν and anisotropy value A have been calculated by the Voigt–Reuss–Hill averaging scheme. Phonon dispersions of the structure were investigated using a finite displacement method. The relaxed system is dynamically stable, and the equilibrium elastic constants satisfy all the mechanical stability criteria for orthorhombic crystals, showing stability against the influence of external forces. The lattice thermal conductivity was calculated within the single-mode relaxation-time approximation of the Boltzmann equation from first-principles anharmonic lattice dynamics calculations. Our results show that lattice thermal conductivity is anisotropic, and the corresponding lattice thermal conductivity at 150 K was found to be 0.189, 0.138, and 0.530 Wm^?1K^?1 in the a, b, and c directions. Electronic structure calculations demonstrate that this compound has a DFT direct band gap at the gamma point of about 1.57 eV. The electronic transport properties have been calculated by solving the semiclassical Boltzmann transport equation on top of DFT calculations, within the constant relaxation time approximation. The Seebeck coefficient S is almost constant from 50 to 150 K. At temperatures 100 and 150 K, the maximal figure of merit is found to be 0.06 and 0.122 in the direction of the c-axis, respectively.     

Neutron diffraction analysis of a defect vanadium monoxide close to the equiatomic vanadium monoxide

Neutron and X-ray diffraction analyses are applied to studying the defect structure of synthesis-temperature quenched and low-temperature annealed vanadium monoxides VO _y (0.90 ≤ y ≤ 0.97) close to the equiatomic monoxide VO_1.0. It is found that the monoxides VO_0.90 and VO_0.97 contain structural vacancies not only in the oxygen sublattice, but also in the metal sublattice. In addition to the cubic disordered phase VO _y with the structure B1, the monoclinic superstructure V₁₄O₆ with space group C2/m is present in the synthesized VO_0.90 sample and in the annealed VO_0.90 and VO_0.97 samples. The formation of the V₁₄O₆ superstructure is attributed to the ordering of oxygen atoms and nonmetal vacancies in the lattice of the tetragonal solid solution of oxygen in vanadium. No simultaneous ordering of metal and oxygen vacancies in two sublattices of the cubic vanadium is observed.     

In-plane torque and gap symmetry of untwinned YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript> crystals

The reversible magnetic torque of untwinned YBa₂Cu₃O₇ single crystals shows the four-fold symmetry in thea-b plane. The irreversible torque indicates evidence for a novel intrinsic pinning along thea andb axes. These facts mean that the free energy of the four-fold symmetry has a minimum when the field is applied along thea orb axis. The results are consistent with those expected from thed _x ²?y ² symmetry and rule out the possibility of thed _xy symmetry. The Fermi surface anisotropy is not responsible for the observed anisotropy. This is firstbulk evidence for thek-dependent gap anisotropy on the Fermi surface. The two-fold anisotropy parameter is found as\(\gamma _{ab} = \sqrt {{{m_a } \mathord{\left/ {\vphantom {{m_a } {m_b }}} \right. \kern-\nulldelimiterspace} {m_b }}} = 1.18 \pm 0.14\).     

Magnetic structure of Er<Subscript>5</Subscript>Ge<Subscript>3</Subscript> at 4.2 K

A symmetry analysis of the possible magnetic structures of Er₅Ge₃ in the ground state is performed using the results of measurements of elastic magnetic neutron scattering at 4.2 K. It is shown that the minimum discrepancy factor R _m ≈9.5% corresponds to a modulated collinear magnetic structure in which the magnetic moments of erbium atoms are oriented along the a ₃ axis of the unit cell of the crystal structure and induce an antiferromagnetic longitudinal spin wave (AFLSW). The magnetic structure is characterized by the wave vector k=2π(0, 0, μ /a ₃) (where μ≈0.293) and the modulation period λ≈3.413a ₃. The magnetic ordering temperature T _N ≈38 K is determined from the temperature dependence of the intensity of magnetic reflections. __________ Translated from Fizika Tverdogo Tela, Vol. 45, No. 9, 2003, pp. 1653–1659. Original Russian Text Copyright ? 2003 by Vokhmyanin, Dorofeev.     

The oxygen content and its ordering in the chain planes in fine-grained HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

A comparative analysis of the results of the X-ray and Mösbauer studies of the high-temperature superconductor (HTSC) YBa₂Cu₃O _y and YBa₂Cu_{3 ? x} ⁵⁷Fe _x O _y (x = 0.015, T _c ≈ 91.5 K) samples with different average grain sizes <D> in the micron and submicron ranges has been performed. The regularities in the change in the lattice parameter c and in the degree of occupation of different oxygen sites in the CuO_δ chain planes taking place at the decrease in <D> have been studied. The quantitative interrelation between the parameter c and the oxygen content δ in the CuO_δ planes exceeding the amount of the mobile oxygen due to the interplane oxygen redistribution is established.     

Magnetic properties of single crystals of a new cobaltite TbBaCo<Subscript>4</Subscript>O<Subscript>7+<Emphasis Type="Italic">x</Emphasis></Subscript>

The temperature and field dependences of the magnetization of a single crystal of a new class of layered cobaltites, TbBaCo₄O_7+x , with a structure containing a Kagomé lattice and a triangular lattice were measured. The measurements were performed on a SQUID magnetometer at temperatures in the range 2–300 K and in magnetic fields of up to 55 kOe for two field orientations. The anisotropy of the magnetization was studied, and the presence of antiferromagnetic ordering in fields H < H _c and a weak magnetic-field-induced (H > H _c ) ferromagnetic component in the low-temperature range was demonstrated. The magnetic characteristics of the initial TbBaCo₄O_7+x single crystal and the single crystal annealed in an O₂ atmosphere were compared.