Effect of pressure on the crystal structure, vibrational modes, and electronic excitations of HgO

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Abstract
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.