Temperature related structural variation of the hydrous components in gypsum

Single crystal neutron diffraction studies have been carried out on a natur al crystal of gypsum (CaSO4.2 H2O) at 50, 115 and 200 K. The structural ref inements converged to Rw(F) = 0.072, 0.068 and 0.065 respectively and compl ement well previous refinements at 294 K. Large changes in the intermolecul ar hydrogen bonds are observed as a function of temperature. Significant va riation between the two intramolecular O(w)-H bonds is evident at all tempe ratures. The distortion of the water molecules and the hydrogen bond angles display temperature independence. The thermal ellipsoids of the protons de termined in our single crystal neutron diffraction experiment are strongly perpendicular to the O(w)-H . . .O(1) hydrogen bonds, with their orientatio ns temperature independent and with no evidence of directionality of the pr otons along the hydrogen bond below 294 K.