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1.
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 VO0.57, VO0.81, and VO0.86 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.  相似文献   

2.
The atom-vacancy ordering of cubic vanadium monoxide VO1.29, 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 VOy 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.  相似文献   

3.
The effect of nonstoichiometry and ordering on the lattice constant a B1 of the basic lattice of vanadium carbide VC y (0.65 < y < 0.875) is studied. A change in the lattice constant of disordered carbide VC y at the reduction of the carbon content is considered using the direction of static displacements of atoms near a vacancy. A model for the calculation of the basic lattice constant a B1 of vanadium carbide is proposed taking into account nonstoichiometry and ordering. It is shown that the ordering of vanadium carbide VC y with the formation of V6C5 and V8C7 superstructures results in an increase in the basic lattice constant as compared to disordered carbide.  相似文献   

4.
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.  相似文献   

5.
It is demonstrated that 50% substitution of vanadium for molybdenum in the pyrochlore lattice of the complex oxide Y2(V x Mo1 ? 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 Gd2V0.67Mo1.33O7 compound (a = 10.2862(3) Å) is a ferromagnet with T C (84 K) exceeding that of undoped Gd2MnO2O7.  相似文献   

6.
A. I. Gusev 《JETP Letters》2009,90(3):191-196
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 .  相似文献   

7.
Structural neutron diffraction studies indicate that only one ordered phase arises after the disorder-order transition in nonstoichiometric cubic tantalum carbide TaCy. This phase arises in the composition range y = 0.79–0.89 due to long-term annealing with a decrease in temperature from 1600 to 300 K. It is incommensurate in the [1–11]B1 direction, but it is close to commensurate M6C5 structures (C2/m and P31 space groups) in mutual arrangement of atoms and vacancies in nonmetallic (1–11)B1 planes. The disorder-order transition channel that is associated with the formation of the incommensurate superstructure in TaCy carbide includes two arms k 5 (6) ≈ 0.473b2 and k 5 (5) = ?k 5 (6) of the {k5} star and arms of the {k4} and {k3} stars. The translation period of the incommensurate phase in the [1–11]B1 direction is 8.9–9.1 nm, which is larger than that in the commensurate phase M6C5 by a factor of about 18.  相似文献   

8.
A. I. Gusev 《JETP Letters》2009,90(5):376-381
Atomic displacements in the lattice of the tetragonal V52O64 superstructure have been experimentally determined. It has been found that atomic displacement waves, which are attributed to the formation of the short-range displacement order, appear in the vanadium and oxygen sublattices of this superstructure. It has been shown that the V52O64 superstructure is formed on the basis of disordered superstoichiometric cubic vanadium monoxide with the short-range order in the metallic sublattice. The character of the short-range order is such that vanadium atoms occupying tetrahedral positions are in the environment of four vacant sites of the vanadium sublattice. This means that the superstoichiometric VO>1.0 vanadium monoxide has a cubic structure differing from the B1-type structure characteristic of most of the strongly nonstoichiometric cubic compounds MX y (X = C, N, O) of transition metals.  相似文献   

9.
The disorder-order transition in cubic vanadium monoxide VOy (y = 1.29, 1.30) possessing a B1 type structure and containing vacancies only in the metal sublattice has been studied by X-ray diffraction and symmetry analyses. It is established that the formation of a tetragonal (space group I41/amd) ordered V52O64 phase in cubic vanadium monoxide VOy proceeds in the form of the first-order phase transition via a channel involving 22 nonequivalent superstructural vectors of four stars ({k 10}, {k 4}, {k 3}, and {k 2}). The distribution function of V atoms in the tetragonal V52O64 superstructure is calculated, and it is found that the real ordered V51.6O64 phase exhibits significant atomic displacements. The boundaries of the domain of existence of the V52O64 phase at 54–60% O are determined in the phase diagram of the V-O system.  相似文献   

10.
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 NaxV2(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).  相似文献   

11.
An X-ray diffraction study indicates that nonstoichiometric vanadium monoxide VO y ≡ V x O z (y = z/x) has a cubic structure of the DO3 type (space group Fm $ \bar 3 $ \bar 3 m), where vanadium atoms are not only at the 4(a) sites of the metal fcc sublattice, but also at the tetrahedral 8(c) sites. This circumstance fundamentally distinguishes monoxide VO y from strongly nonstoichiometric MX y compounds with the B1 structure and the same space group, where atoms M and X and structural vacancies ▪ and ▭ of the metal and nonmetal sublattices, respectively, are distributed over the 4(a) and 4(b) sites. The dependence of the filling factor q of the tetrahedral interstices by vanadium atoms on the relative content y of oxygen in VO y has been obtained. It has been shown that the composition of cubic vanadium monoxide should be represented as VO y ≡ V x O z ≡ V x − 2q V2q (t)1 − x + 2q O z 1 − z , taking into account the structure.  相似文献   

12.
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.  相似文献   

13.
Li-ion battery cathode material lithium-vanadium-phosphate Li3V2(PO4)3 was synthesized by a carbon-thermal reduction method, using stearic acid, LiH2PO4, and V2O5 as raw materials. And stearic acid acted as reductant, carbon source, and surface active agent. The effect of its content on the crystal structure and electrochemical performance of Li3V2(PO4)3/C were characterized by XRD and electrochemical performance testing, respectively. The results showed that the content of carbon source has no significant effect on the crystal structure of lithium vanadium phosphate. Lihtium vanadium phosphate obtained with 12.3% stearic acid demonstrated the best electrochemical properties with a typical discharge capacity of 119.4 mAh/g at 0.1 C and capacity retention behavior of 98.5% after 50 cycles. And it has high reversible discharge capacity of 83 mAh/g at 5 C with the voltage window of 3 to 4.3 V.  相似文献   

14.
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.  相似文献   

15.
The thermoelectric properties of n-Bi2 ? x Sb x Te3 ? y ? z Se y S z solid solutions are studied in the temperature range 300–550 K. It is shown that an increase in the parameter β determining the figure-of-merit Z of the material is observed in compositions with the optimally related effective mass of the density of states m/m 0, the carrier mobility μ0, and the lattice thermal conductivity κ L . Within the temperature range 300–350 K, the parameter β and the figure-of-merit Z are found to increase in solid solutions with substitutions in both bismuth telluride sublattices Bi → Sb and Te → Se, S (x = 0.16, y = z = 0.12) for optimum electron concentrations. An increase in the electron concentration and substitutions of atoms only in the tellurium sublattice bring about an increase in the β parameter and the value of Z at higher temperatures. Within the range 350–450 K, the parameters β and Z are observed to increase in a solid solution with a low content of substituted atoms in the tellurium sublattice Te → Se, S for y = z = 0.09 and, at higher temperatures up to 550 K, in compositions with tellurium substituted by selenium only, with increasing content of substituted atoms.  相似文献   

16.
The temperature dependence of the Nernst-Ettingshausen coefficient Q(T) in the normal phase of doped HTSCs of the yttrium system was studied. The main features characterizing the behavior of this coefficient were revealed, and the character and mechanism of the effect that various nonisovalent substituents exert on the Q(T) dependence were analyzed. It is shown that the narrow-band model permits one not only to describe all the specific features observed in the Q(T) curves but also to perform a simultaneous quantitative analysis of the temperature dependences of four kinetic coefficients (the electrical resistivity and the Seebeck, Hall, and Nernst-Ettingshausen coefficients) with the use of a common set of model parameters characterizing the band structure and carrier system in the normal phase of an HTSC. This approach was employed to determine the carrier mobilities and the asymmetry of the dispersion curve in the systems studied (YBa2Cu3Oy, y = 6.37–6.91; YBa2Cu3?xCoxOy, x = 0–0.3; Y1?xCaxBa2Cu3Oy, x = 0–0.25; Y1?xCaxBa2?xLaxCu3Oy, x = 0–0.5) and to analyze the effect of the substitutions involved on the variation of these parameters.  相似文献   

17.
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[Li0.33Ti1.67]16dO4 → [Li□]16c[Li1.33Ti1.67]16dO4 → [Li1.330.67]16c[Ti1.670.33]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) → ....  相似文献   

18.
The results of x-ray structural studies of the [N(C2H5)4]2CdBr4 crystal at low temperatures are presented. The unit cell parameters and the thermal expansion coefficients along the main crystallographic directions are measured at temperatures in the range from 90 to 320 K. The integrated intensities of the diffraction reflections are investigated as a function of the temperature. It is shown that the curves a = f(T), c = f(T), I 500 = f(T), and I 006 = f(T) at temperatures T 1 ≈ 174 K and T 2 ≈ 226 K exhibit anomalies in the form of abrupt changes in the lattice parameters and the diffraction reflection intensities. This indicates that the [N(C2H5)4]2CdBr4 crystal undergo phase transitions at these temperatures. Moreover, there is an anomaly in the form of a small maximum at the temperature T 3 = 293 K.  相似文献   

19.
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 VO1.0. It is found that the monoxides VO0.90 and VO0.97 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 VO0.90 sample and in the annealed VO0.90 and VO0.97 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.  相似文献   

20.
The charge storage behavior of nanostructures based on Si1?x Ge x (0 ≤ x ≤ 1) nanocrystals (NCs) in an Al2O3 matrix was investigated. The structures have been grown by RF magnetron sputtering and subsequently annealed at temperatures ranging from 700 °C to 1000 °C for 30 min in nitrogen ambient. The stoichiometry of the SiGe NCs and the alumina crystalline structure were found to be significantly dependent on the RF power and the annealing temperature. The sizes of the SiGe NCs and their distribution were investigated by grazing incidence small angle X-ray scattering (GISAXS). The capacitance-voltage (C-V) and conductance-voltage (G-V) measurements were performed to investigate the charge trapping characteristics of the memory structures. The C-V hysteresis width depends on variations in the crystalline structure resulting from different annealing temperatures. It is also shown that charge injection is governed by the Fowler-Nordheim tunnel mechanism for higher electric fields.  相似文献   

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