Raman scattering studies on mercuric iodide at high pressures and at low temperatures

Raman scattering measurements have been performed on single crystal red mercuric iodide under high-pressures up to ∼5.5GPa and at low-temperatures down to 40K. Our low-temperature measurements help clarify the issues related to the accidental degeneracy of the Eg1 and B1g2 modes in the tetragonal phase. Raman modes stiffen and their widths reduce with the lowering of the temperature. All the Raman active mode frequencies increase with pressure up to 1.3GPa, beyond which the modes of the tetragonal phase disappear and new modes appear supporting the earlier reported red -> high-pressure yellow structural transition at this pressure. Knowledge of both temperature and pressure dependences has allowed us to separate, for all the Raman active modes, the explicit phonon-excitation contribution and the implicit volume-driven contribution to the observed temperature coefficient dω/dT for both the phases