HYDROGEN THERMAL MOTION IN CALCIUM HYDROXIDE - CA(OH)2

Calcium hydroxide, Ca(OH)2, M(r) = 74, trigonal, P3m1BAR, a = 3.589 (8 ), c = 4.911 (14) angstrom, V = 54.8 (4) angstrom3, D(x) = 2.26 g cm-3 at room temperature, Z = 1, lambda = 0.8330 (5) angstrom, mu = 1. 195 cm-1, F(000) = 9.028 fm, final R = 0.0 18, wR = 0.0 1 9, for 105 uniq ue reflections, at 80 K; R = 0.025, wR = 0.027, for 211 unique reflect ions, at room temperature. The structure of calcium hydroxide has been refined using neutron data collected at 80 K and at room temperature. Thermal motion of the H atom has been refined with anharmonic thermal -motion parameters at room temperature and at 80 K. Corresponding prob ability-density functions have been calculated. A three-site disordere d model for the H atom is also proposed and discussed. These new struc tural models allow us to give a new interpretation of the apparent sho rtness of the O-H distance reported in the literature.