Observation of 9-Fold Coordinated Amorphous TiO₂ at High Pressure

Knowledge of the structure in amorphous dioxides is important in many fields of science and engineering. Here we report new experimental results of high-pressure polyamorphism in amorphous TiO₂ (a-TiO₂). Our data show that the Ti coordination number (CN) increases from 7.2 ± 0.3 at ∼16 GPa to 8.8 ± 0.3 at ∼70 GPa and finally reaches a plateau at 8.9 ± 0.3 at ≲86 GPa. The evolution of the structural changes under pressure is rationalized by the ratio (γ) of the ionic radius of Ti to that of O. It appears that the CN ≈ 9 plateau correlates with the two 9-fold coordinated polymorphs (cotunnite, Fe₂P) with different γ values. This CN-γ relationship is compared with those of a-SiO₂ and a-GeO₂, displaying remarkably consistent behavior between CN and γ. The unified CN-γ relationship may be generally used to predict the compression behavior of amorphous AO₂ compounds under extreme conditions.