Dimension-Dependent Stability of an Ultra-high Pressure TiO2 Phase in Rutile Matrix

Macroscopic TiO2 with α-type PbO2 structure is stable at ultra-high pressure. However, stability fields for nano-phases or nano-precipitates in their host rutile crystals will differ greatly from those for their macroscopic phases. To use nano-phases in host minerals to evaluate formation conditions such as pressure and temperature, we must first understand the effects of size, shape, and interface structures on the stabilities of the nano-phases. TEM results show that ball-milling can introduce micro-twinning. The structure at the twin boundary corresponds to unit-cell scale lamella with α-type PbO2 structure. Results also indicate that mechanical crushing will introduce artifacts in rutile crystals. Computer modelling using Density Functional Theory (DFT) within the Generalized-Gradient Approximation (GGA) was used to examine the thermodynamic stability of the α-PbO2-type lamella in rutile TiO2. These results show that 2-dimensional interface energy is directly related to lamella thickness; correspondingly, stability of TiO2 lamellae with α-PbO2-type structure might be also directly relate to thickness of the lamellae. It is proposed that nano-lamellae of TiO2 with α-type PbO2 structure may form at relatively low pressure conditions compared to its macroscopic phase. Integrating TEM study of nano-phases with theoretical modelling of nano-phase lamellae stability in host minerals will help us to understand formation conditions and evolutionary history of minerals and their host rocks.