First-principles study of titanium oxides

Titanium forms a wide range of stoichiometric oxides. The low-temperature monoclinic phase of titanium monoxide ${\mathrm{Ti}}_5$${\mathrm{O}}_5$ has an ordered array of vacancies in an otherwise simple NaCl structure. Using the variable cell shape method and the local-density approximation, all 12 parameters of the crystal structure are relaxed. Comparisons with other possible structures and other transition-metal monoxides suggest that there is a competition between kinetic- and Madelung-energy contributions and that this low symmetry structure is stabilized in part by the overlap of occupied Ti-ion orbitals via the vacant oxygen site. Resistivities of the low-temperature ${\mathrm{Ti}}_5$${\mathrm{O}}_5$ phase fitted using the Bloch-Grüneisen formula imply that the monoxide is a borderline quasiparticle gas. © 1996 The American Physical Society.