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-diffraction and optical spectroscopies. At 14(1) GPa orthorhombic HgO undergoes a first-order phase transition. The diffraction diagram of the high-pressure phase can be indexed assuming a tetragonal cell. Its structure is a distorted variant of the NaCl-type structure. Pronounced softening 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 accordance with the redshift of band-edge-related features seen in optical reflectivity. Above 28 GPa, the optical reflectivity of the high-pressure phase shows an increase of the near-infrared oscillator strength, indicating the onset of a metallic regime.