Observation of 9-Fold Coordinated Amorphous TiO₂ at High Pressure

Abstract

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 TiO 2 (a-TiO 2 ). 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, Fe 2 P) with different γ values. This CN−γ relationship is compared with those of a-SiO 2 and a-GeO 2 , displaying remarkably consistent behavior between CN and γ. The unified CN−γ relationship may be generally used to predict the compression behavior of amorphous AO 2 compounds under extreme conditions.

Probing Medium-Range Order in Oxide Glasses at High Pressure

“…In particular, extreme densification provides a new opportunity to tailor material properties that may not be achievable through conventional glass processing under ambient and low-pressure conditions. ,− Despite the potential importance, the structural origin of the mechanical response of noncrystalline oxides during deformation under extreme compression has remained a challenging problem in physical chemistry and materials research. − Furthermore, the changes in the structures of noncrystalline oxides upon compression have implications for the properties of dense magmatic liquids in Earth and planetary interiors. ,− The changes in melt properties at high pressure are often assumed to be due primarily to structural changes on the short-range scale (coordination number and degree of network polymerization). Such is often the case for oxide glasses under high pressures above 4 to 5 GPa, where overall densification has often been attributed to an increase in the average coordination number (e.g., refs and − ). Nevertheless, the experimental measurement of the elastic properties and the melt viscosity often suggests that changes in short-range structures alone cannot fully explain the observed pressure-induced changes in these properties. , Pressure-induced structural adaptation of the amorphous network beyond the second coordination shell may contribute to overall changes in their properties.…”

Degree of Permanent Densification in Oxide Glasses upon Extreme Compression up to 24 GPa at Room Temperature

“…For example, a significant change in the cation coordination numbers in densified Li-aluminoborate glasses only up to 2 GPa was observed, accounting for the improved crack resistance and the enhanced residual density . Therefore, the nature of the structural transformation in prototypical aluminosilicate glasses that is expected to occur under much higher pressure conditions (>20 GPa) at room temperature needs to be confirmed because the ability of glasses to deform is inextricably related to the changes in coordination numbers that occur under extreme compression. − The potential changes in coordination environments under permanent densification at room temperature also provide improved insights into crack initiation and its propagation during the indentation and ductile deformation of noncrystalline materials at low temperature. , …”

Overview of HPCAT and capabilities for studying minerals and various other materials at high-pressure conditions

“…Benefitting from the broad energy range of the beamline and the large opening access of the PE press, the energy dispersive x-ray diffraction (EDXD) setup at the beamline covers a large momentum transfer (large-Q), allowing for the collection of structure factor data (q) between 1 and 30 Å −1 . A large q coverage is essential for reliable structural information in particular of amorphous materials and melts (Kono et al 2016 , 2020b ; Ohira et al 2019 ; Shu et al 2020b ; Eastmond et al 2021 ). PE press inside the 16-BM-B station and the schematic of the setup is shown in Fig.…”