We investigate by means of first-principles methods the structural response of gypsum (CaSO4ยท2H2O) to pressures within and above the stability range of gypsum-I, P <= 4 GPa. Structural and vibrational properties calculated for gypsum-I are in excellent agreement with experimental data. Compression within gypsum-I takes place predominantly through a reduction in the volume of the CaO8 polyhedra and through a distortion of the hydrogen bonds. The distance between CaSO4 layers becomes increasingly incompressible, indicating a mechanical limit to the packing of water molecules between the layers. We find that a structure with collapsed interlayer distances becomes more stable than gypsum-I above about 5 GPa. The collapse is concomitant with a rearrangement of the hydrogen-bond network of the water molecules. Comparison of the vibrational spectra calculated for this structure with experimental data taken above 5 GPa supports the validity of our model for the high-pressure phase of gypsum.
[ Phys. Rev. B. 81, 064103 (2010) [link] ]

Stretching of water at about 3400 cm-1