Two pressure-induced structural phase transitions in TiOCl

TL;DR

This study investigates the structural changes of TiOCl under high pressure, revealing two phase transitions with new superstructures and anomalies, highlighting complex pressure-dependent behavior in this material.

Contribution

First detailed high-pressure x-ray diffraction study revealing two structural phase transitions in TiOCl up to 25 GPa.

Findings

Emergence of a 2x2x1 superstructure at 15 GPa

Anomalous lattice parameter behavior at 22 GPa

Persistence of orthorhombic phase across all pressures

Abstract

We studied the crystal structure of TiOCl up to pressures of $p$=25~GPa at room temperature by x-ray powder diffraction measurements. Two pressure-induced structural phase transitions are observed: At $p_{c1}$$\approx$15~GPa emerges an 2$a$$\times$2$b$$\times$$c$ superstructure with $b$-axis unique monoclinic symmetry (space group P2$_1$/$m$). At $p_{c2}$$\approx$22~GPa all lattice parameters of the monoclinic phase show a pronounced anomaly. A fraction of the sample persists in the ambient orthorhombic phase (space group $Pmmn$) over the whole pressure range.