Pressure-induced phase transitions and shifts in the absorption edge of CuCl

The dependence of the optical absorption edge on hydrostatic pressure has been investigated in CuCl up to 15 GPa. The absorption edge shifts abruptly at 4.8 and 9.0 GPa, as reported by Edwards and Drickamer, due to phase transitions from zinc blende→tetragonal→NaCl-type structure. From the shape of the absorption edge we conclude that in the zinc-blende and tetragonal phases, the lowest energy gap is direct and in the NaCl type it is indirect. In all the three phases the absorption edge shifts to higher energy with pressure at the rates: zinc blende, 1.78 × ${10}^{-2\mathrm{}}$ eV/GPa; tetragonal, 2.1 × ${10}^{-2\mathrm{}}$ eV/GPa; and NaCl type, 4.3 × ${10}^{-2\mathrm{}}$ eV/GPa. These values yield -1, -1.65, and -5 eV for the deformation potentials of the gaps in the three phases, respectively. We compare our experimental values of these deformation potentials with the calculated ones for the zinc-blende phase. The energy gaps and deformation potentials for AgCl are discussed to obtain some insight into the electronic band structure of the NaCl phase of CuCl. Our data have revealed the new fact that the lowest energy gap (indirect gap) in the NaCl phase of CuCl is ∼ 3.0 eV, which is lower than that in the zinc-blende phase. This is consistent with the behavior of other zinc-blende-type compounds undergoing a similar transition.