Static compression of Ca(OH)₂ at room temperature: Observations of amorphization and equation of state measurements to 10.7 GPa

We report x‐ray diffraction measurements on Ca(OH) 2 ‐portlandite as it is compressed to 37.6 gigapascals (GPa) in the diamond cell at room temperature. Between 10.7 and 15.4 GPa crystalline Ca(OH) 2 transforms to a glass, and on decompression the glass recrystallizes between 3.6 and 5.1 GPa. Below pressures of 10.7 GPa we measure the elastic compression of crystalline Ca(OH) 2 . A finite strain analysis of these data shows that the isothermal bulk modulus and its pressure derivative are K 0 = 37.8 (±1.8) GPa and K 0 ' = 5.2 (±0.7) at zero pressure. From the change in the unit cell dimensions, we find that the linear incompressibilities of Ca(OH) 2 differ by a factor of three. These data show that hydrogen bonding can produce large anisotropies in the elastic properties of hydroxides. We infer from the amorphization that dense, high‐pressure phases of Ca(OH) 2 exist at pressures of 11 GPa and above.