Vibrational Properties of InSe under Pressure: Experiment and Theory

The pressure dependence of the phonon modes in the layered semiconductor γ‐InSe has been investigated experimentally and theoretically for pressures up to 11 GPa. The mode Grüneisen parameters of all Raman‐active zone‐center phonons have been determined by Raman scattering under pressure. In addition, features corresponding to second and third‐order scattering processes are apparent in the Raman spectra under resonance conditions, from which information about zone‐edge modes can be obtained. For the assignment of the observed Raman features to vibrational modes we have calculated the phonon dispersion curves using a rigid‐ion model including couplings to first‐nearest neighbors and long‐range Coulomb interaction. At about 7 GPa the sample turns from transparent to opaque and a new Raman mode appears in the spectra at around 165 cm−1. This is evidence of a pressure‐induced structural instability of γ‐InSe, which is optically detected but not by X‐ray diffraction.