Simulation of the short-range order in disordered cubic titanium monoxide TiO1.0

Abstract

616Oxides, carbides, and nitrides of transition metals having a wide range of homogeneity at high tempera ture owing to the high concentration of structural vacancies are conventionally singled out into a sepa rate class of strongly nonstoichiometric compounds [1]. An important feature of such compounds is the atomic vacancy ordering, which leads to the forma tion of new phases. The effect of ordering on the prop erties of the compound in some cases is comparable with the effect of nonstoichiometry. The structures of the ordered phases have been studied rather thor oughly to date. Their description can be found in [1].At the same time, the features of the structure of the disordered phases have been studied insufficiently. The assumption that the vacancies are distributed sta tistically in the disordered phases, i.e., the short range order is absent, is used most often. However, this assumption is not substantiated theoretically and experimentally. Effects of diffuse scattering indicating the existence of the short range order were found in experiments on X ray and electron diffraction on the samples of the disordered oxides, carbides, and nitrides of transition metals [2][3][4][5][6]. However, no mod els of the disordered structures with the short range order were proposed. The effect of the appearance of the short range order in the distribution of vacancies on the properties of strongly nonstoichiometric com pounds remains unknown.This work is aimed at the simulation of the short range order in the distribution of vacancies for the dis ordered cubic phase of titanium monoxide of the equi atomic composition TiO 1.0 and the study of the effect of the short range order on the electronic structure and total energy of this compound.It is known that titanium monoxide TiO y has the basic structure of the B1 type. In contrast to the major ity of other strongly nonstoichiometric compounds, it contains vacancies in both metal and nonmetal sublat tices. In the case of equiatomic composition (y = 1.0), their fraction is about 15% in each sublattice [7][8][9][10]. The concentration of vacancies can be decreased only by annealing at high pressures and temperatures [11]. Vacancies can be distributed over the sites of the basic structure regularly or randomly. The disordered state is thermodynamically equilibrium at temperatures above 1263 K. The ordered state is formed as a result of slow annealing at temperatures below 1263 K [8,9]. It was earlier accepted that the ordering of vacancies in tita nium monoxide of the stoichiometric composition leads to the formation of only the monoclinic ordered phase, considered in detail in [8][9][10]. In [12], an alter native variant of the ordering was analyzed and the model of the ordered phase with the cubic symmetry was proposed.All these features make titanium monoxide the most interesting object among other strongly nonsto ichiometric compounds for the study of the features of the ordering, simulation of the short and long range orders, and analysis of the effect of the ordering on...

Stable Ti9O10 nanophase grown from nonstoichiometric titanium monoxide TiOy nanopowder

“…Note that the dip in the density of Ti 3d states distribution for Ti 9 O 10 does not become more profound as compared with the disordered phase, which, according to [25][26][27][28], is typical of energetically favorable structures of titanium monoxide. On the other hand, complete disappearance of the dip occurring during the formation of energetically unfavorable local atomic vacancy correlations in the structure [29] is not observed either. The presence of the electronic states at the Fermi level is indicative of the metallic character of the considered phases.…”