Order-disorder phase transformations and specific heat of nonstoichiometric vanadium carbide

AbstractA study has been made of the order-disorder phase transformations in the homogeneity region of the VC_y nonstoichiometric cubic vanadium carbide (0.66<y<0.88). It has been established that an ordered V₆C₅ phase with monoclinic (space group C2/m) or trigonal (P3₁) symmetry, and a cubic (space group P4₃32) ordered V₈C₇ phase can form in the VC_y carbide below 1450 K, depending on the actual composition. The effect of off-stoichiometry and structural vacancy ordering on the specific heat of the VC_y carbide has been investigated. The temperatures and heats of the reversible order-disorder equilibrium transitions have been determined. The ordering in the VC_y carbide is shown to be a first-order phase transition. An equilibrium diagram of the V-C system taking into account ordering in the nonstoichiometric vanadium carbide has been constructed. Fiz. Tverd. Tela (St. Petersburg) 41, 529–536 (March 1999)     

Effect of nonstoichiometry and ordering on the period of the basis structure of cubic titanium carbide

We have examined the influence of nonstoichiometry and order-disorder phase transformations on the basis period (of type B1) of the structure of titanium carbide TiC_y (0.5<y<1.0). We found that ordering of titanium carbide TiC_y with formation of superstructures of the type Ti Ti₂C and Ti₃C₂ leads to growth of the period of the basis crystal lattice in comparison with the disordered carbide. Taking the change in the lattice period into account, we discuss the question of the directions of the static displacements of atoms near a vacancy. Fiz. Tverd. Tela (St. Petersburg) 41, 1134–1141 (July 1999)     

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)     

Disorder-order phase transformations and electrical resistivity of nonstoichiometric titanium carbide

Phase transformations of the disorder-order type in the homogeneity region of nonstoichiometric titanium carbide TiC_y (0.5<y<1.0) have been studied. It has been established that, depending on the actual composition of TiC_y, there may form in it for T<980–1000 K a cubic or a trigonal ordered Ti₂C phase (space groups Fd3m and , respectively) and a rhombic ordered Ti₃C₂ phase (space group C222₁). The effect of ordering on the electrical resistivity of the nonstoichiometric carbide TiC_y was studied, and the temperatures of the reversible disorder-order equilibrium transitions determined. The ordering in titanium carbide is shown to be a first-order phase transition. Fiz. Tverd. Tela (St. Petersburg) 40, 1332–1340 (July 1998)     

ESCA studies of core level shifts and valence band structure in nonstoichiometric single crystals of titanium carbide

Core level shifts and valence band structure from six single crystals of TiC_x have been studied by X-ray photoelectron spectroscopy (ESCA). The     

Dependence of the resistivity of nonstoichiometric titanium carbide TiCy on the density and distribution of carbon vacancies

The influence of temperature and of the density and distribution of structural vacancies in the carbon sublattice on the resistivity of nonstoichiometric titanium carbide TiC_y(0.5⩽y⩽0.98) is studied. It is shown that in titanium carbide TiC_y with y<0.7 reversible disorder-order structural phase transitions occur at temperatures below 1000 K. The temperatures of order-disorder phase transformations are determined. The dependence of the residual resistivity on the composition of the disordered titanium carbide is explained by the change in the carrier density in the region of homogeneity of the carbide TiC_y, on the one hand, and the atom-vacancy interaction, on the other. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 4, 284–289 (25 August 1999)     

Theory of nonstoichiometric ordering of Pb-containing relaxors with perovskite structure

Conditions that are imposed on the interatomic interaction and under which a certain ordered state of AB _x ^′ B _1?x ^″ perovskites arises are determined with allowance for effectively pairwise interactions and the configuration entropy of interchanges of B′ and B″ ions. It is shown that, for the interaction potential u(R) = u ₀/R ⁶, the highest temperature of ordering of the 1: 2 type (T _ord (1: 2)) corresponds to a structure observed at A ≡ Ba. The highest temperature of ordering of the 1: 1 type (T _ord (1: 1)) corresponds to a structure that typically occurs in the case of A ≡ Pb. Within the approximation used, it is found that T _ord (1: 1) > T _ord (1: 2) for all compositions and that the 1: 1-ordered phase is most stable. For models with u(R) = u ₀/R ⁿ (n = 1–6) including the interaction in the first m coordination shells (m = 3, 6, 8, 11), it is shown that the ground state of AB _x ^′ B _1?x ^″ O₃ corresponds either to a decomposed solid solution or to an ordered state similar to that observed in PbMg_1/3Nb_2/5O₃.     

A thermodynamic model of ordering of nonstoichiometric titanium monoxide TiO y with structural vacancies in the oxygen and titanium sublattices

A thermodynamic model of ordering of nonstoichiometric titanium monoxide TiO _y with structural vacancies in the oxygen and titanium sublattices has been developed taking into account the long-range Coulomb interactions. Methods for calculating the internal energy and entropy of this compound have been proposed. The free energy is represented in the form of a function of long-range order parameters, stoichiometry, and temperature. It has been found that the order-disorder phase transition in the titanium monoxide should occur according to the mechanism of a first-order phase transition. The calculated dependences of the critical values of the long-range order parameters and temperatures on the composition y have demonstrated that, when the composition deviates from stoichiometry, the disordering of structural vacancies in different sublattices should proceed at different temperatures.     

The cluster component method and ordering in nonstoichiometric monoxides with the rocksalt structure

Several nonstoichiometric metal monoxides have structures typical ofthat of defective rocksalt-type. Their physical behaviour has been shown to depend on the degree of nonstoichiometry. In particular, wüstite and manganosite are known to behave in different ways according to the subdomains of their stability fields, and one major problem is to relate their thermodynamic properties with their actual defect structures. This is accomplished here by means of the cluster component method (CCM). The three kinds of behaviour of wüstite are accounted for by a structural model using five sublattices of the spinel lattice. A similar situation is predicted for manganosite. Finally the CCM allows a rationalization of the relationship between composition and physical properties.     

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)     

Microstructure of nanocrystalline nonstoichiometric vanadium carbide VC0.875

A nanocrystalline powder of nonstoichiometric vanadium carbide VC_0.875 has been prepared by the high-energy ball milling method. The crystal structure, microstructure, morphology, and size distribution of particles of the initial and milled powders have been investigated using X-ray diffraction, laser diffraction, and scanning electron microscopy. For vanadium carbide, the model calculation of the particle size of a VC_0.875 nanopowder as a function of the milling duration has been performed for the first time. A comparison of the experimental and theoretical results has demonstrated that a nanopowder with an average particle size of 40–80 nm can be obtained by a 10-h high-energy ball milling of the initial vanadium carbide powder with an average particle size of ～6 μm.     

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.     

Electronic structure and stability of nonstoichiometric titanium monoxide TiO y with structural vacancies in one of the sublattices

The electronic structure of nonstoichiometric titanium monoxide TiO _y with different compositions y, which contains structural vacancies either in the metallic sublattice or in the nonmetallic sublattice, has been investigated using the supercell method within the DFT-GGA approximation with pseudopotentials. The cases of ordered and disordered arrangements of vacancies have been considered. It has been found that the complete removal of vacancies from the sublattice is energetically unfavorable, and the ordering of oxygen vacancies according to the type of the Ti₆O₅□₁ superstructure, as well as titanium vacancies according to the Ti₅?₁O₆ type, does not lead to the stabilization of the B1 basic structure of titanium monoxide.     

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)     

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.     

The specific heat of iron titanium hydride at room temperature

Specific heat measurements were used to determine vibrational properties of hydrogen interstitials in the intermetallic alloy Fe_{0, 49}Mn_{0, 01}Ti_{0, 5}H_x. The specific heats are 0, 465 J/gK for x = 0, 0, 486 for x = 0, 31, 0, 515 for x = 0, 47, 0, 557 for x = 0, 66 at 280 K. The localized vibrational excitations of hydrogen have an energy of 100 meV for x = 0, 5 and 77 meV for x = 1. The Debye temperature at x = 0 is 300 K at a temperature of 280 K.     

Role of structural vacancies in the stabilization of the basic B1 structure in nonstoichiometric titanium monoxide TiO y

The electron structures and formation enthalpies of vacancy-free cubic TiO, vacancy-ordered monoclinic Ti₅O₅, and vacancy-disordered cubic TiO _y have been investigated using DFT+GGA calculations. Ti₅O₅ was found to be the stablest phase and TiO was found to be the least stable. The reason for the stability of the titanium and oxygen vacancies in the basic B1 structures of Ti₅O₅ and TiO _y has been deduced. The titanium vacancies lead to a decrease in the Fermi energy. Oxygen vacancies cause strengthening in covalent Ti-Ti bonding.     

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.     

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.