Crystal Structures and Electronic Properties of Oxygen-rich Titanium Oxides at High Pressure

Pressure is well-known to significantly change the bonding patterns of materials and lift the reactivity of elements, leading to the synthesis of unconventional compounds with fascinating properties. Titanium-oxygen (Ti-O) compounds (e.g., TiO₂) are attracting increasing attention due to their attractive electronic properties and extensive industrial applications (e.g., photocatalysis and solar cells). Using the effective CALYPSO structure searching method combined with first-principles calculations, we theoretically explored various oxygen-rich Ti-O compounds at pressures ranging from 0 to 200 GPa. Our results revealed, unexpectedly, that pressure stabilizes two hitherto unknown stoichiometric oxygen-rich Ti₂O₅ and TiO₃ compounds. Ti₂O₅ crystallized in P-42₁ c structure, whose remarkable feature is that it contains a peroxide group (O2^2-) with an O-O distance of 1.38 Å at 150 GPa. The trioxide TiO₃ is an ionic metal and is the oxygen-richest compound known thus far in the Ti-O system. It adopts a high symmetry (space group Pm-3 n) structure consisting of a 12-fold coordinated face-sharing TiO₁₂ icosahedron, where Ti has the highest coordination number with O among all Ti-O structures. The underlying mechanisms for the stabilization of Ti₂O₅ and TiO₃ lie in the higher coordination number and denser structure packing. Our current results unravel the unusual oxygen-rich stoichiometry of Ti-O compounds and provide further insight into the diverse electronic properties of Ti oxides under high pressure.