Thermal expansion of gypsum investigated by neutron powder diffraction

The cell parameters of deuterated gypsum have been extracted from Rietveld analysis of powder neutron diffraction data within the temperature range 4.2-320 K. The thermal expansion of gypsum is highly anisotropic along the b axis principally because of the effect of the D2.. .01 hydrogen bond. The high-temperature limits for the expansion coefficients for the cell edges a, b, and care 3.98 x 10-6,4.36 X 10-5, and 2.53 x 10-5 K-l, respectively, and for the cell volume the limit is 6.96 x 10-5 K-l. The (3angle displays oscillatory variation, reflecting a change in the influence of at least two coupled processes. Empirical data analysis, within this temperature range, results in a~ = 1.251 x 10-6 sin(0.0116T + 0.311) K-l.