Temperature related structural variation of the hydrous components in gypsum

Single crystal neutron diffraction studies have been carried out on a natural crystal of gypsum (CaSO 4 · 2H 2 0) at 50, 115 and 200 K. The structural refinements converged to Rw(F) = 0.072, 0.068 and 0.065 respectively and complement well previous refinements at 294 K. Large changes in the intermolecular hydrogen bonds is observed as a function of temperature. Significant variation between the two intramolecular O(w)-H bonds is evident at all temperatures. The distortion of the water molecules and the hydrogen bond angles display temperature independence. The thermal ellipsoids of the protons determined in our single crystal neutron diffraction experiment are strongly perpendicular to the O(w)-H···O(1) hydrogen bonds, with their orientations temperature independent and with no evidence of directionality of the protons along the hydrogen bond below 294 K.