Structural and optical properties of HgO at high pressure

We have investigated high-pressure properties of HgO by optical spectroscopies and powder X-ray diffraction. At pressures up to 14 GPa we observe a pronounced softening of two oxygen-related Raman modes of orthorhombic HgO which is interpreted as a signature of increasing coupling between -O-Hg-O- chains. The lattice parameters reveal a strongly anisotropic compressibility in this pressure range. At a pressure of 14 GPa, discontinuous changes of Raman active modes indicate a structural phase transition, in agreement with changes in the X-ray diffraction diagrams. The high-pressure phase is assigned as tetragonal with the mercury partial structure being close to face centered cubic. An increase in near-infrared oscillator strength, which starts at 28 GPa as seen in optical reflectivity, is attributed to an insulator-metal transition resulting from a monotonic increase of valence and conduction band overlap.