Vacancy distribution in nonstoichiometric vanadium monoxide

Structural vacancy distribution in the crystal lattice of the tetragonal V52O64 superstructure which is formed on the basis of disordered superstoichiometric cubic vanadium monoxide VOy≡VxOz is experimentally determined and the presence of significant local atomic displacements and large local microstrains in a crystal lattice of real ordered phase is established. It is shown that the relaxation of local microstrains takes place owing to the basic disordered cubic phase grain refinement and a formation of ordered phase domains. The ordered phase domains grow in the direction from the boundaries to the centre of grains of the disordered basic cubic phase. Isothermal evolution at 970 K of the average domain size 〈D〉 in ordered VO1.29 vanadium monoxide is established. It is shown that the short-range order presents in a metal sublattice of disordered cubic VOy vanadium monoxide. 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 disordered VOy vanadium monoxide, especially superstoichiometric VOy>1.0 monoxide, has a cubic (space group Fm3¯m) D03-type structure differing from the B1-type structure typical for most of the strongly nonstoichiometric cubic compounds carbides, nitrides and oxides.

Figure optionsDownload as PowerPoint slideResearch highlights▶ A certain fraction of vanadium atoms in disordered cubic vanadium monoxide VOy and ordered tetragonal phase V52O64 is located in tetrahedral positions of a basic cubic lattice. ▶ These positions are never occupied by any atoms in other strongly nonstoichiometric carbides, nitrides and oxides. ▶ Both disordered and ordered structures of vanadium monoxide are characterized by the presence of short-range order of displacements in the oxygen sublattice and short-range order of substitution in the metal sublattice. ▶ The short-range order of displacement is caused by the local displacements of O atoms from V(t) atoms occupying tetrahedral positions. The short-range order of substitution appears because V(t) atoms in the tetrahedral positions are always in the environment of four vacancies ■ of the vanadium sublattice.