Mechanism of anatase-to-columbite TiO2 phase transformation via sheared phases: first-principles calculations and high-pressure torsion experiments

TL;DR

This paper investigates how shear strain accelerates the anatase-to-columbite phase transformation in TiO2, combining first-principles calculations and high-pressure torsion experiments to reveal the underlying mechanism.

Contribution

It proposes and validates a mechanism involving sheared crystal structures as intermediate phases, explaining shear's role in phase transformation under high pressure.

Findings

Sheared structures act as intermediate phases in the transformation.

DFT calculations support the viability of sheared structures.

Experimental HRTEM observations confirm metastable sheared phases.

Abstract

High-pressure torsion (HPT) can facilitate phase transformations in titanium dioxide (TiO2) and stabilize its high-pressure columbite phase, as an active photocatalyst, by shear straining under high pressure. This study aims to understand the mechanism underlying the acceleration of the anatase-to-columbite phase transformation by shear strain. A mechanism by considering sheared crystal structures as intermediate phases was proposed and examined using quantum mechanics in the framework of density functional theory (DFT) and HPT experiments. DFT energy and phonon calculations demonstrated the viability of the sheared structures as intermediate phases. Furthermore, the sheared structures were observed experimentally as new metastable phases using high-resolution transmission electron microscopy. These findings can explain the significant effect of shear strain on pressure-induced phase transitions, reported during severe plastic deformation of various metals and ceramics.