Phonon modes and stability of GaS up to 200 kilobars

Raman scattering and absorption measurements of GaS under high pressure are reported. The pressure coefficients of one-phonon modes, as well as 2-LA and 2-TA combinations, have been measured. The latter is negative. The anomalous pressure coefficient of the intralayer $E_{1g}$ vibration is discussed in terms of its shear character. Accidental energetic degeneracy, at ≃ 100 kbars, of an $A_{1g}$ mode with a 2-LA density of states broadens the Raman line. Other line broadenings which are induced by pressure are discussed. This behavior is shown to be an intrinsic property of the crystal and not inhomogeneous strain broadening, since the original method we use minimizes stress gradients even above 100 kbars in the diamond anvil cell. A structural phase transition above 190 kbars is reported. The new phase behaves like a three-dimensionally bonded network.