  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.
