Lattice-dynamical and photoelastic properties of GaSe under high pressures studied by Raman scattering and electronic susceptibility

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 ${\mathrm{GPa}}^{\mathrm{-}1}$. 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 ${\kappa }_l$≃0.015 ${\mathrm{GPa}}^{\mathrm{-}1}$ 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, $E_g$, of the Penn-Phillips oscillator. This behavior is shown to be described well by Δ$E_g$=$D_a$(Δa/$a_0$) +$D_c$(Δc/$c_0$) with the deformation potentials $D_a$=-7.6±1.0 eV for the a axis and $D_c$=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.