Effect of ordering on the structure and specific heat of nonstoichiometric titanium carbide

The experimental results on the change in the crystal structure and specific heat of nonstoichiometric titanium carbide TiC_y (0.5<y<0.65) near disorder-order phase transitions are reported. It is established that at temperatures below 1000 K the ordered phases Ti₂C with cubic and trigonal symmetry and an orthorhombic ordered phase Ti₃C₂ form in titanium carbide by a close-to-first-order phase transition mechanism. The temperatures and heats of order-disorder phase transformations are determined. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 9, 631–637 (10 May 1999)     

Effect of nonstoichiometry on the structure and dielectric properties of bismuth ferrite

Nonstoichiometric bismuth ferrite Bi _x FeO₃ 0.88 ≤ x ≤ 1.04) fabricated via solid-state synthesis is investigated by means of X-ray diffraction and electrical methods. The periodicity of the formation of impurity phases and variations in the structural parameters and densities of ceramic samples is established. The correlation between the number of impurity phases and characteristics of the dielectric spectra in the cation/anion-deficient (0.88 ≤ x < 1.00) and cation/anion-excess (1.00 < x ≤ 1.04) nonstoichiometry regions of Bi _x FeO₃ is confirmed. The observed effects are explained for the actual (defect) structure of bismuth ferrite.     

Effect of high pressure on the period of the basis lattice and concentration of vacancies in titanium monoxide TiO

The results of the experiment on the thermobaric annealing of titanium monoxide with a B1 cubic structure at a temperature of 2273 K have been analyzed. The analysis of these results has shown that an increase in the initial period of the lattice from 417.6 to 418.0 pm after processing at a pressure of 3 GPa and to 418.5 pm at a pressure of 6 GPa is due to the filling of vacant sites of the crystal structure with atoms. The ab initio quantum calculations indicate that the equilibrium period of the lattice should increase monotonically by 2% at a decrease in the concentration of vacancies from 16.7 to 0%. After aging the samples under normal conditions, the period of the lattice is no more than 421.0 pm and the concentration of vacancies is no lower than 11% because of the instability of the structure at lower concentrations of vacancies.     

Effect of the degree of ordering of structural vacancies on the fine structure of the valence-band top in cubic boron nitride

The electronic energy structure of the defect system of c-BN_x with ZnS-type structure is calculated in the multiple-scattering theory by the local coherent potential method. The cluster version of the MT approximation is used to calculate the crystal potential. The effect of the relaxation of the crystal lattice on the electronic structure of nonstoichiometric boron nitride c-BN_0.75 is studied and a comparison is made with the electronic energy structure of c-BN in the same approximation. Fiz. Tverd. Tela (St. Petersburg) 39, 1064–1065 (June 1997)     

The influence of a metalloid on ordered structure in titanium carbide

X-ray and neutron diffraction were used to investigate the effect of oxygen and nitrogen atoms on the formation of the intermediate δ′-cubic (sp. gr. Fd3m) and δ″-tetragonal (sp. gr. P3₁21) ordered structures in titanium carbide. In TiC_0.545O_0.08, TiC_0.545N_0.09 samples that were investigated, the metal atoms were shifted from ideal positions in the direction from vacancies to metalloid atoms. In the intermediate cubic phase (δ′), the free-parameter values of titanium atoms in the two samples are the same, but are different from their analogous value in titanium carbide. Fiz. Tverd. Tela (St. Petersburg) 39, 2207–2209 (December 1997)     

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.     

Bulk and surface ordering in cubic crystals

The order-disorder transition of face and body centred cubic crystals is studied by a Monte Carlo simulation of a two-component lattice gas with nearest neighbour interactions, and of different compositions. It is shown that the degree of surface order may both be higher and lower than that of the bulk. General guide lines are drawn from the results to predict such behaviour in experimental situations.     

Specific features of morphology and the structure of nanocrystalline cubic silicon carbide films grown on a silicon surface

Physics of the Solid State - The composition, surface morphology, and crystal structure of nanocrystalline cubic silicon carbide films grown on a silicon surface through chemical conversion from...     

Influence of atomic vacancy ordering in the nonmetallic sublattice on the electronic structure of titanium hydride

We present a calculation of the electronic density of states in TiH_x alloys (x=2.0, 1.7, and 1.5) with various degrees of long-range atomic order, η, in the subsystem of hydrogen and vacancies in the nonstoichiometric hydrides. In comparing the calculated results with the features of the photoelectron spectrum of TiH_1.5 we find agreement for η=0.5. We determine the temperature range for the establishment of atomic vacancy order. Siberian V. D. Kuznetsov Physicotechnical Institute, Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 7–14, October, 1998.     

Nanostructure and atomic ordering in vanadium carbide

Nanostructured nonstoichiometric vanadium carbide VC_0.87 was obtained in powdered form using the ordering effect. The composition, structure, and properties of the carbide were studied by chemical and thermogravimetric analysis, gas chromatography, x-ray diffraction, optical and electronic microscopy, electron-positron annihilation, magnetic susceptibility, and microhardness methods. Nanostructured vanadium carbide VC_0.87 possesses the crystal structure of the cubic ordered phase V₈C₇ with space group P4₃32. Vanadium carbide nanocrystallites are shaped in the form of 400–600 nm in diameter and 15–20 nm thick curved petals. The surface layer of the nanocrystallites contains defects of the vacancy agglomerate type. The microhardness of vanadium carbide, obtained by vacuum sintering of VC_0.87 nanopowder was 60–80 GPa, which is 3–4 times greater than the microhardness of coarse-grained vanadium carbide with the same composition and close to the hardness of diamond. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 6, 436–442 (25 March 1999)     

Magnetoresistance in the cubic modification of silicon carbide

The results of an investigation into the angular and field dependences of the magnetoresistance in the cubic modification of silicon carbide are presented in this paper. From an analysis of the temperature dependence of the Hall mobility of the electrons and the magnetoresistive mobility it is concluded that the magnetoresistance in the specimens under investigation is conditioned by the presence of inhomogeneities.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 90–94, May, 1977.     

Nature of chemisorption on titanium carbide and nitride

Extensive density-functional calculations are performed to understand atomic chemisorption on the TiC(1 1 1) and TiN(1 1 1) surfaces, in particular the calculated pyramid-shaped trends in the adsorption energies for second- and third-period adatoms. Our previously proposed concerted-coupling model for chemisorption on TiC(1 1 1) is tested against new results for adsorption on TiN(1 1 1) and found to apply on this surface as well, thus reflecting both similarities and differences in electronic structure between the two compounds.     

Vacancy induced changes in the electronic structure of titanium carbide—I. Band structure and density of states

Results of a self-consistent “Augmented Plane Wave” (APW) band-structure calculation are presented for substoichiometric titanium carbide with 25% vacancies on the carbon sublattice sites (TiC_{0 75}) assuming a model structure with ordered vacancies Comparison with an earlier APW study on stoichiometric TiC reveals that the carbon vacancies induce two pronounced peaks in the density of states (DOS), 0.4 eV below and 0.8 eV above the Fermi energy E_γ, thus forming electronic states in a region where the DOS for stoichiometric TiC exhibits a minimum So-called “vacancy states” with an important amount of charge on the vacancy site are found to be derived from Ti 3d states extending into the vacancy muffin tin sphere An angular momentum decomposition with respect to the center of the vacancy muffin tin sphere shows that the s character predominates for the occupied and the p character for the unoccupied “vacancy states” The theoretical findings explain features near E_γ, observed in recently published X-ray emission spectra Furthermore, we find a slight increase of electronic charge in the carbon muffin tin spheres as compared with stoichiometnc TiC.     

Atomic-vacancy ordering and magnetic susceptibility of nonstoichiometric hafnium carbide

Experimental data on the magnetic susceptibility of nonstoichiometric hafnium carbide HfC_y(0.6< y<1.0) are presented. Anomalies are found in the temperature dependences of the magnetic susceptibility of the carbides HfC_0.71, HfC_0.78, and HfC_0.83 in the temperature range 870–930 K. These anomalies are due to the formation of a superstructural short-range order in their nonmetallic sublattice. It is shown that the short-range order in the carbides HfC_0.71 and HfC_0.78 corresponds to the ordered phase Hf₃C₂, while in the carbide HfC_0.83 it corresponds to the ordered phase Hf₆C₅. It is found that the magnetic susceptibility of the carbide HfC_0.78 in the temperature interval 910–980 K is zero. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 296–301 (25 February 1999)     

Influence of doping with titanium on the electronic structure of Ni3Fe and Ni3Mn in ordering

The specific electrical resistance , mean magnetic moment of an atom of the alloy, and the anomalous Hall (R_s) and Nernst-Ettinghausen (Q_s) constants of ordered alloys Ni₃(Fe, Ti) and Ni₃(Mn, Ti) are investigated. It is suggested that there is an additional degree of delocalization of the 3d electrons of the system Ni₃(Mn, Ti) and increase in the degree of localization of 3d electrons of the system Ni₃(Fe, Ti) with small additions of Ti, with increase in the long-range-order parameter. A model of the electronic structure of the given alloys is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 56–59, October, 1981.     

Trends in atomic adsorption on titanium carbide and nitride

Extensive density-functional calculations on atomic chemisorption of H, B, C, N, O, F, Al, Si, P, S, and Cl on the polar TiC(1 1 1) and TiN(1 1 1) yield similar adsorption trends for the two surfaces: (i) pyramid-like adsorption-energy trends along the adatom periods; (ii) strongest adsorption for O, C, N, S, and F; (iii) large adsorption variety; (iv) record-high adsorption energy for O (8.4-8.8 eV). However, a stronger adsorption on TiN is found for elements on the left of the periodic table and on TiC for elements on the right. The results support that a concerted-coupling model, proposed for chemisorption on TiC, applies also to TiN.     

Precursor phenomena at the magnetic ordering of the cubic helimagnet FeGe

We report on detailed magnetic measurements on the cubic helimagnet FeGe in external magnetic fields and temperatures near the onset of long-range magnetic order at T(C)=278.2(3) K. Precursor phenomena display a complex succession of temperature-driven crossovers and phase transitions in the vicinity of T(C). The A-phase region, present below T(C) and fields H<0.5 kOe, is split in several pockets. The complexity of the magnetic phase diagram is theoretically explained by the confinement of solitonic kinklike or Skyrmionic units that develop an attractive and oscillatory intersoliton coupling owing to the longitudinal inhomogeneity of the magnetization.     

Features and mechanisms of growth of cubic silicon carbide films on silicon

The mechanisms and specific features of the growth of silicon carbide layers through vacuum chemical epitaxy in the range of growth temperatures from 1000 to 700°C have been considered. The structure of the heterojunction formed has been studied using the results of the performed investigations of photoluminescence spectra in the near-infrared wavelength range and the data obtained from the mass spectrometric analysis. It has been found that, in the silicon layer adjacent to the 3C-SiC/Si heterojunction, the concentration of point defects significantly increases and the dislocation structure is not pronounced. According to the morphological examinations of the surface of the growing film, by analogy with the theory of thermal oxidation of silicon, the theory of carbidization of surface silicon layers has been constructed. A distinctive feature of the model under consideration is the inclusion of the counter diffusion fluxes of silicon atoms from the substrate to the surface of the structure. The growth rate of films and the activation energy of diffusion processes have been estimated. The performed experiments in combination with the developed growth model have explained the aggregates of voids observed in practice under the silicon carbide layer formed in the silicon matrix and the possibility of forming a developed surface morphology (the island growth of films) even under conditions using only one flow of hydrocarbons in the reactor.     

Effect of metal and carbon vacancies on the band structure of hexagonal tungsten carbide

The band structure of hexagonal tungsten carbide (β-WC) containing vacancies in metal and carbon sublattices is investigated within the first-principles full-potential linear muffin-tin orbital (LMTO) approach for a model of 16-atom supercells. Specific features of the formation of “vacancy” states are discussed, and the formation energies of defects and their charge states are estimated. The results obtained are compared with previous calculations and available experimental data.