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 similar to 16 GPa to 8.8 +/- 0.3 at similar to 70 GPa and finally reaches a plateau at 8.9 +/- 0.3 at less than or similar to 86 GPa. The evolution of the structural changes under pressure is rationalized by the ratio (gamma) of the ionic radius of Ti to that of O. It appears that the CN approximate to 9 plateau correlates with the two 9-fold coordinated polymorphs (cotunnite, Fe2P) with different gamma values. This CN gamma relationship is compared with those of a-SiO2 and a-GeO2, displaying remarkably consistent behavior between CN and gamma. The unified CN-gamma relationship may be generally used to predict the compression behavior of amorphous AO(2) compounds under extreme conditions.