Two pressure-induced transitions in TiOCl: Mott insulator to anisotropic metal

TL;DR

This study uses molecular dynamics to reveal two pressure-induced phase transitions in TiOCl, transforming it from a Mott insulator to an anisotropic metal, with the second transition being newly predicted.

Contribution

It uncovers a second, previously unreported metallic transition in TiOCl under pressure, driven by bandwidth expansion and electronic redistribution.

Findings

First transition: Mott insulator to anisotropic metal with Ti-Ti dimerization

Second transition: disappearance of dimerization, uniform metallic phase

Pressure-dependent lattice parameters match experimental data

Abstract

Using Car-Parrinello molecular dynamics calculations we investigate the behavior of the low-dimensional multiorbital Mott insulator TiOCl under pressure. We show that the system undergoes {\it two} consecutive phase transitions, first at $P_\text{c}$ from a Mott-insulator to a metallic phase in the $ab$ plane with a strong Ti-Ti dimerization along $b$. At a pressure $P'_\text{c} > P_\text{c}$ the dimerization disappears and the system behaves as a uniform metal. This second transition has not yet been reported experimentally. We show that the insulator-to-metal transition at $P_\text{c}$ is driven by the widening of the bandwidth rather than structural changes or reduction of crystal field splittings and it shows a redistribution of the electronic occupation within the $t_{2g}$ bands. Our computed pressure-dependent lattice parameters are consistent with experimental observations and the existing controversy on the change of crystal symmetry at high pressures is discussed.