Structural consideration of phase transitions in Zn(OH)2 under high pressure

Abstract

Pressure-induced phase transitions of Zn(OH)2 were investigated under hydrostatic condition at room temperature by an in-situ X-ray powder diffraction method and a visible observation method for a single-crystalline specimen. Two phase transitions were observed in the pressure range up to 4GPa. The first transition occurred at 1.1 GPa and it was a quick single crystal – single crystal transition. The high-pressure phase 1 with a tetragonal α-cristobalite-related structure was stable up to 2.1GPa. The coordination number of zinc atoms was 4+2. The second phase transition was observed above 2.1GPa. The high-pressure phase 2 was quenchable to ambient condition. The crystal structure has not been determined yet because the X-ray powder diffraction pattern was too broad to be even indexed. Comparison of polymorphs in Zn(OH)2 suggests that the volume per chemical formula of the high-pressure phase 2 is in 40.0 – 44.5 Å3 at 2.1 GPa and room temperature and that the coordination number of zinc atoms in the phase is probably 4+2 or 5.