Tunnel splittings in solid CD4 estimated from heat capacity data

Heat capacity measurements have been made on CD4, isotopically purified by gas chromatography, in the temperature region 0.15–4.0 K. For T<0.5 K, the heat capacity is found to increase with decreasing temperature by much more than can be accounted for by contributions from impurities such as CHD3. An analysis shows that the results can be reproduced by an array of low-lying tunneling states with maximum splitting of 0.042±0.010 K. States in this region have been predicted by theory and found recently by measurements of inelastic neutron scattering. Enhanced thermal relaxation is observed in the calorimetric measurements in the region T<0.25 K. The onset of conversion between nuclear spin symmetry species is a possible cause.