Abstract
Raman scattering and infrared studies are reported for the layer compound GaSe under hydrostatic pressures up to 4 GPa. The rigid-layer mode shifts towards higher frequencies with an initial pressure coefficient of 0.234 . The overall behavior is explained in terms of the volume anharmonicity characteristic of the van der Waals bonding. The internal bond-bending mode softens and the Born effective charge decreases linearly. By adopting the single-layer compressibility ≃0.015 we obtain a Grüneisen parameter of -1.9 for the bond-bending mode and -0.55±0.05 for the effective charge, comparable to those of a tetrahedral semiconductor. Coexistence of such molecular and covalent characters leads to a nonlinear shift of the energy, , of the Penn-Phillips oscillator. This behavior is shown to be described well by Δ=(Δa/) +(Δc/) with the deformation potentials =-7.6±1.0 eV for the a axis and =1.06±0.16 eV for the c axis. These deformation potentials reflect the dimensionality of bonding network as well as the nature of the electronic structure.
- Received 17 October 1986
DOI:https://doi.org/10.1103/PhysRevB.35.3860
©1987 American Physical Society