STRUCTURAL AND THERMODYNAMIC EVIDENCE OF A CHANGE IN THERMAL MOTION OF HYDROGEN-ATOMS IN CA(OH)(2) AT LOW-TEMPERATURE

Accurate measurements of lattice parameters and heat capacity of Ca(OH ,) were performed in the temperature ranges 8-450 K and 12-300 K, resp ectively. No apparent phase transition was evident but thermal expansi on and heat capacity anomalies were detected above about 180 K. Based on structural and NMR results, we have attributed these anomalies to a change in the thermal motion of hydrogen atoms; namely, that above 18 0 K OH-ions have a reorientational motion between three equivalent ori entations.