Lambda-type heat capacity anomalies in U3O8

The heat capacity of U3O8 was measured in the temperature range from 310 to 970 K using an adiabatic scanning calorimeter. Three lambda-type heat capacity anomalies were observed at 483, 568 K and 850 K, indicating second order phase transitions. The enthalpy and entropy changes for these anomalies were 135, 148 and 314J mol-1, and 0.28, 0.26 and 0.37 J mol-1 K-1, respectively. A high temperature X-ray diffraction study has also been undertaken to confirm these phase transitions. The first anomaly at 483 K involves the structural change from orthorhombic to pseudohexagonal (strictly speaking orthorhombic). The origin of the other two phase transitions is not yet clear, but some order-disorder process would be involved in these phase transitions. (Auth.)