Effect of pressure on the electrical transport and structure of TiOCl

We have investigated the effect of pressure on the electrical transport and the structural properties of TiOCl. The temperature dependence of the electrical resistivity up to 24 GPa shows no indication of an insulator-to-metal transition. However, the analysis of the pressure dependence of the energy gap (E-g) around 300 K reveals first a strong decrease of E-g with increasing pressure, followed by a much weaker decrease of E-g above 13 GPa, indicating a change of the electronic structure. The analysis of the pressure dependence of the lattice parameters up to 18 GPa reveals an extreme anisotropic and anomalous change of the lattice parameters with increasing pressure. While the lattice parameters b and c smoothly decrease up to the highest pressure, the lattice parameter a remains nearly pressure independent above about 11 GPa. It is shown that such anomalous change of the lattice parameters with pressure strongly affects the relative strength of the effective direct and indirect hopping along the b and a axis, respectively, and therefore it is responsible for the observed pressure-induced change of the electronic properties of TiOCl above 13 GPa.