BiTeCl and BiTeBr: A comparative high-pressure optical study

We here report a detailed high-pressure infrared transmission study of BiTeCl and BiTeBr. We follow the evolution of two band transitions: the optical excitation $\ensuremath{\beta}$ between two Rashba-split conduction bands, and the absorption $\ensuremath{\gamma}$ across the band gap. In the low-pressure range, $p<4$ GPa, for both compounds $\ensuremath{\beta}$ is approximately constant with pressure and $\ensuremath{\gamma}$ decreases, in agreement with band structure calculations. In BiTeCl, a clear pressure-induced phase transition at 6 GPa leads to a different ground state. For BiTeBr, the pressure evolution is more subtle, and we discuss the possibility of closing and reopening of the band gap. Our data is consistent with a potential Weyl phase in BiTeBr at $5--6$ GPa, followed by the onset of a structural phase transition above 7 GPa.