Raman Scattering and Fundamental Absorption in Red-HgI₂ under Hydrostatic Pressure

Abstract

The Raman scattering and optical absorption spectra have been measured under hydrostatic pressure up to 1.67 GPa in the layer compound red-HgI₂. The Raman spectrum shows that the bending force of HgI₄ tetrahedra is too weak for a layer to vibrate rigidly in the so-called rigid-layer mode. The bending force increases rapidly with pressure, whereas the van der Waals force remains unchanged. These characteristics originate from the fact that the bond ionicity is extremely high and sensitive to pressure. The fundamental absorption edges exhibit red shifts, giving an evidence that the ionicity decreases with pressure. Anisotropy in elastic constants is also discussed in relation to the nature of chemical bondings.