HgO at high pressures: the transition to the NaCl structure (HgO-III) and the equation of state of tetragonal HgO-II

Abstract

The high-pressure behavior of HgO-montroydite was investigated up to 36.5 GPa using angle-dispersive X-ray diffraction. The tetragonal phase of this material (HgO-II), a distortion of the NaCl structure, transforms into the cubic NaCl structure (HgO-III) above *31.5 GPa. The transformation of mercury oxide from the orthorhombic Pnma (HgO-I) structure to a tetragonal I4/mmm structure (HgO-II) is confirmed to occur at 13.5 ± 1.5 GPa. Neither of the high-pressure phases, HgO-II nor HgO-III, is quenchable in pressure. The derived isothermal bulk modulus of HgO-II and its pressure derivative strongly depend on the assumed zero-pressure volume of this phase, but our elasticity results on HgO-II nevertheless lie significantly closer to theoretical calculations than prior experimental results, and the measured pressure of the phase transformation to the NaCl structure is also in agreement with recent theoretical results. The general accord with theory supports the existence of significant relativistic effects on the high-pressure phase transitions of HgO.