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.
@article{arxiv.0806.2396,
title = {Mott-Hubbard gap closure and structural phase transition in the oxyhalides TiOBr and TiOCl under pressure},
author = {C. A. Kuntscher and A. Pashkin and H. Hoffmann and S. Frank and M. Klemm and S. Horn and A. Schönleber and S. van Smaalen and M. Hanfland and S. Glawion and M. Sing and R. Claessen},
journal= {arXiv preprint arXiv:0806.2396},
year = {2018}
}