Stability of Defectless Structures of Titanium Monoxide at High Pressures

Titanium monoxide with the basis crystal structure B1 is of interest because of an anomalously high concentration of vacancies and diverse effects of atom-vacancy ordering. It was previously believed that the application of relatively low pressures to such systems does not change the crystal structure type, and the crystal structure is densified through a decrease in the equilibrium concentration of defects. The genetic algorithm to search for optimal structures and calculations by the electron density functional method have demonstrated that phases with a structure derivative of the B1 structure should be metastable in a wide pressure range from 0 to 100 GPa. Two defectless hexagonal modificationsthe epsilon-TiO and H-TiO phasesare thermodynamically stable at P < 28 GPa and P > 28 GPa, respectively. These phases demonstrate a pronounced pseudogap at the Fermi level and, thereby, have a low electrical conductivity.