The low-temperature heat capacity of the (Th,Pu)O2 solid solution

The low-temperature heat capacity of the (Th1−y,Puy)O2 solid solution with y=0.03, 0.08, 0.30, 0.54 and 0.85 was determined from 4 K up to about 300 K with a PPMS instrument using the hybrid adiabatic relaxation method. In addition, the heat capacity of 239PuO2 end member was also measured and compared with available literature data for 242PuO2, observing an indistinguishable match. The obtained heat capacity values of the intermediate compositions are greater than the data obtained from Neumann-Kopp's molar additivity rule based on the ThO2 and PuO2 end-members, and a substantial excess capacity is observed for all intermediate compositions, particularly at the lowest temperatures.