FIRST-PRINCIPLES STUDY OF TITANIUM-OXIDES

Titanium forms a wide range of stoichiometric oxides. The low-temperat ure monoclinic phase of titanium monoxide Ti5O5 has an ordered array o f vacancies in an otherwise simple NaCl structure. Using the variable cell shape method and the local-density approximation, all 12 paramete rs of the crystal structure are relaxed. Comparisons with other possib le structures and other transition-metal monoxides suggest that there is a competition between kinetic- and Madelung-energy contributions an d that this low symmetry structure is stabilized in part by the overla p of occupied Ti-ion orbitals via the vacant oxygen site. Resistivitie s of the low-temperature Ti5O5 phase fitted using the Bloch-Gruneisen formula imply that the monoxide is a borderline quasiparticle gas.