Pressure-induced phase transitions in gypsum

We report high-pressure Raman scattering spectroscopy and energy dispersive X-ray diffraction investigations on gypsum, CaSO4. 2H(2)O, at room temperature in a diamond cell. With increasing pressure, measurements indicate that CaSO4. 2H(2)O undergoes two stages of crystalline-state phase transitions at 5 and 9 GPa, and then converts to a disordered phase above 11 GPa. The structures of the three high-pressure phases of gypsum have not been determined yet. These phases are tentatively named as "post-gypsum-I" (PG-T), "post-gypsum-II" (PG-II) and "disordered" according to the sequence of their appearance with pressure. Gypsum shows anisotropic compressibility along three crystallographic axes with b > c > a below 5 GPa. The difference in the behavior of the two OH stretching modes in gypsum is attributed to the different reduction rate in the hydrogen bonding distances by the anisotropic axial compressibility.