EFFECT OF PRESSURE ON THE CRYSTAL-STRUCTURE, VIBRATIONAL-MODES, AND ELECTRONIC EXCITATIONS OF HGO

We have investigated the effect of pressure on structural properties, lattice vibrations, and electronic excitations of HgO using x-ray-diff raction and optical spectroscopies. At 14(1) GPa orthorhombic HgO unde rgoes a first-order phase transition. The diffraction diagram of the h igh-pressure phase can be indexed assuming a tetragonal cell. Its stru cture is a distorted variant of the NaCl-type structure. Pronounced so ftening of oxygen-related Raman modes is observed in both the low-and high-pressure phases. Photoluminescence spectra indicate that the band gap of orthorhombic HgO decreases with increasing pressure, in accord ance with the redshift of band-edge-related features seen in optical r eflectivity. Above 28 GPa, the optical reflectivity of the high-pressu re phase shows an increase of the near-infrared oscillator strength, i ndicating the onset of a metallic regime.