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 modifications — the $\varepsilon$-TiO and H-TiO phases — are 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.