Crystal Structure of the High-Pressure γ Phase of Mercury: A Novel Monoclinic Distortion of the Close-Packed Structure

The crystal structure of the high-pressure γ phase of solid mercury, stable between 12 and 37 GPa, has been determined by powder x-ray diffraction experiments. The structure is monoclinic C 2/ m with six atoms in the unit cell. A mercury atom is coordinated by 10 to 11 atoms, which are distributed at distances up to 10% longer than the first nearest one. The structure is intimately related to α (simple rhombohedral), β (body-centered-tetragonal) and δ (hexagonal-close-packed) phases of mercury, where the stacking of pseudohexagonal layers is systematically modified. This novel monoclinic distortion of the close-packed structure serves as a new class of dense structures for metallic elements.