The Entropy and Low Temperature Heat Capacity of Neptunium Dioxide

Abstract

The heat capacity of a two-gram sample of neptunium dioxide was determined from 10 to 315°K in an adiabatic calorimeter. At 298.16°K the molal entropy and enthalpy calculated from the heat capacity data are 19.19±0.1 cal deg−1 and 2770±15 cal, respectively. A pronounced hump was found in the heat capacity curve of neptunium dioxide at 25.3°K similar to that found by Jones, Gordon, and Long in uranium dioxide at 28.7°K. This anomaly in the heat capacity is suggestive of a cooperative phenomenon and is attributed to the incidence of antiferromagnetism below the temperature of the maximum in the heat capacity. A calorimeter for gram scale samples and a modification of the cryostat design of Ruehrwein and Huffman so as to employ only liquid helium and liquid nitrogen as refrigerants are described.