Observation of nine-fold coordinated amorphous TiO₂ at high pressures

Understanding pressure-induced structural changes in amorphous dioxides (a-AO₂) is of great importance in many fields of science. Here we report new experimental results of high pressure polyamorphism in amorphous TiO₂ (a-TiO₂) with the Ti-O coordination number (CN) close to 9. Our experimental data show that CN increases from 7.2 at 15.7 GPa, to 8.8 at 70.2 GPa, and finally reaches a plateau ~8.9 at pressures up to 85.7 GPa. We find that CN of both crystalline TiO₂ and a-TiO₂ follows a similar and systematic dependence on the ratio (γ) of the ionic radii of Ti and O. The γ of a-TiO₂ is 0.614 at 15.7 GPa, which is similar to that of baddeleyite-type TiO₂ (~0.61), and increases continuously with pressure. At 70.2 GPa, γ of a-TiO₂ is 0.701, which is similar to that of cotunnite-type TiO₂ (~0.693). It appears that the CN≈9 plateau of a-TiO₂ correlates to the cotunnite-type and Fe₂P-type polymorphs, which have the same CN=9 but correspond to different γ values. This CN-γ relationship is applicable to other a-AO₂ of a-SiO₂ and a-GeO₂. All three compounds show surprisingly consistent between CN and γ, implying a unified relation between CN and γ in a-AO₂. The established CN-γ relationship may be used to predict the compression behavior of a-AO₂ compounds to extreme conditions.