Mott-Hubbard gap closure and structural phase transition in the oxyhalides TiOBr and TiOCl under pressure

TL;DR

This study investigates how applying pressure causes the Mott-Hubbard gap to close and induces structural phase transitions in TiOBr and TiOCl, revealing the interplay between electronic and structural changes.

Contribution

It provides the first detailed pressure-dependent optical and structural data showing gap closure and phase transitions in TiOBr and TiOCl.

Findings

Mott-Hubbard gap closes at 14 GPa in TiOBr and 16 GPa in TiOCl.

Structural phase transition coincides with gap closure.

Similar effects observed at low temperature in TiOBr.

Abstract

Pressure-dependent transmittance and reflectance spectra of TiOBr and TiOCl single crystals at room temperature suggest the closure of the Mott-Hubbard gap, i.e., the gap is filled with additional electronic states extending down to the far-infrared range. According to pressure-dependent x-ray powder diffraction data the gap closure coincides with a structural phase transition. The transition in TiOBr occurs at slightly lower pressure ($p$=14 GPa) compared to TiOCl ($p$=16 GPa) under hydrostatic conditions, which is discussed in terms of the chemical pressure effect. The results of pressure-dependent transmittance measurements on TiOBr at low temperatures reveal similar effects at 23 K, where the compound is in the spin-Peierls phase at ambient pressure.