Orientational ordering and tunneling states in solid CH2D2 as deduced from low temperature heat capacity and neutron cross section measurements

Abstract

Heat capacity measurements have been performed on isotopically purified solid CH2D2 over the temperature range 0.12⩽T<2.7 K to characterize a large Schottky anomaly. In addition, the total neutron cross section of condensed CH2D2 was measured over the temperature range 0.75⩽T⩽100 K. While a change in neutron cross section consistent with conversion between nuclear spin symmetry species was observed in the region T<4.2 K, no measurable thermal relaxation was encountered in the calorimetric experiments. On the basis of the latter result and of the magnitude of the entropy at the lowest temperatures, it is argued that conversion probably does not occur in this methane. It seems probable that the results should be interpreted as indicating incomplete orientational ordering of the CH2D2 molecules in phase III. A model is developed for the manifold of low-lying tunneling states.