Ten-fold coordinated polymorph and metallization of TiO2 under high pressure

Abstract

Titanium dioxide (TiO₂) has a wide range of industrial applications (e.g., in photocatalysis and solar cells). Pressure causes profound structural and electronic changes in TiO₂, leading to the fundamental modification of its physical properties. We report the metallization of TiO₂ at high pressures through first-principles swarm structure searching calculations. Metallization accompanies the stabilization of a body-centered tetragonal CaC₂-type structure (space group I4/mmm), which is more stable than the Fe₂P-type structure above 690 GPa. This phase adopts a ten-fold coordinated structure consisting of face-sharing TiO₁₀ dodecahedrons, the highest coordination number among all TiO₂ phases known so far. In contrast to the nine-fold Fe₂P-type structure, the higher coordination and denser polyhedral packing makes the CaC₂-type structure energetically favorable. Our work enables an opportunity to understand the structure and electronic properties of TiO₂ at high pressures.