We have investigated high-pressure properties of HgO by optical spectr
oscopies and powder X-ray diffraction. At pressures up to 14 GPa we ob
serve a pronounced softening of two oxygen-related Raman modes of orth
orhombic HgO which is interpreted as a signature of increasing couplin
g between -O-Hg-O- chains. The lattice parameters reveal a strongly an
isotropic 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 m
ercury partial structure being close to face centered cubic. An increa
se in near-infrared oscillator strength, which starts at 28 GPa as see
n in optical reflectivity, is attributed to an insulator-metal transit
ion resulting from a monotonic increase of valence and conduction band
overlap.