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

Abstract

Titanium monoxide TiO y is one of the few nonsto ichiometric compounds with a high concentration of structural vacancies in both sublattices. Their percent age can be up to 32 at % [1][2][3][4]. In the case of an equi atomic composition, the amount of vacancies is about 14-15 at % in each sublattice [3,4]. When the compo sition deviates from stoichiometric, the concentration of vacant sites increases in one sublattice and decreases in another [1]. The high content of vacan cies results in a wide homogeneity of titanium monox ide, which is in the range of y = 0.7-1.25 [3].Three stoichiometric crystal modifications of tita nium monoxide composition are known. They all have the basic B1 structure, but they differ from each other by the presence of vacancies and their specific loca tion. One can obtain a phase without structural vacan cies by annealing at high pressures [5]. At low pres sures, the thermodynamic equilibrium phases are those with vacancies. Vacancies may be located over sites of the basic В1 structure either randomly or in an ordered manner. The disordered state is thermody namically stable at temperatures above 1300 K, but it can be obtained at room temperature by quenching [3,4]. Since the vacancies are chaotically located, the X ray diffraction patterns of disordered samples exhibit only characteristic of the cubic B1 structure. The vacancies are ordered during slow annealing from the temperature of the order-disorder phase transi tion; the crystal symmetry is lowered to monoclinic, so a Ti 5 О 5 phase is formed [3,4].Thus, the presence of vacancies and their ordering is energetically favorable for titanium monoxide. However, the role of vacancies in stabilization of the basic B1 structure is poorly understood. There are many theoretical studies dedicated to this problem [6][7][8][9][10][11][12][13]. For example, the electron structure of the TiO and Ti 5 O 5 phase and disordered stoichiometric TiO 1.0 phase have been calculated in [12]. However, the ordered and disordered phases have been studied by various methods, which has made it impossible to compare their total energies adequately. In [13], only TiO and Ti 5 O 5 were considered. It was shown that a crucial role in stabilizing the ordered phase is played by oxygen vacancies; however, the reasons for the sta bility of the disordered state have not been considered. Thus, what the role of metal and nonmetal vacancies in disordered and ordered phase are remains an open question. This paper is aimed at determining the contribu tion of titanium and oxygen vacancies in the stabiliza tion of the basic B1 structure and to investigate the sta bility of crystal titanium monoxide modifications. To do this, we calculated the electron structures and total energies of the three phases: vacancy free cubic TiO, vacancy ordered monoclinic Ti 5 O 5 , and vacancy dis ordered cubic TiO y ; analyzed the features of the chem ical bond; and found the stablest crystal modification.The electron structure was calculated with the den sity functional theory and account f...