Raman-scattering study of pressure-induced phase transitions in CuI

The pressure-induced phase transitions: zinc-blende (ZB) → rhombohedral → tetragonal in CuI were studied by Raman scattering. The rhombohedral primitive cell is presented as a slightly distorted ZB unit cell that is twice as long as the primitive cell in the [111] direction. By folding back the ZB Brillouin zone, an approximation to the phonon dispersion curves of the rhombohedral structure in the [111] direction can be obtained. A comparison can then be made between the rhombohedral Raman frequencies and those obtained by neutron scattering at the ZB zone boundary at $L$. The agreement was found to be satisfactory. A similar analysis is made of the tetragonal phase; it can be related to the zinc-blende structure by means of a doubled primitive cell which involves a slight distortion of the iodine sublattice but a large rearrangement of the copper sublattice. The Brillouin zone must be folded along one cubic axis. The mode Grüneisen parameters were calculated for the rhombohedral phase and all of them were found to be positive including that of the $E$ mode that corresponds to TA ($L$) of the ZB structure.