Phase transitions in U3O8−z: I, heat capacity measurements

The heat capacity of U₃O_8?z with various O/U ratios was measured in the range from 250 to 750 K, and λ-type heat capacity anomalies were found in each sample. The transition temperatures were 487 and 573 K for UO_2.663, 490 and 576 K for UO_2.656 and 508, 562 and 618 K for UO_2.640. The entropy changes of the transitions were 0.44 and 0.39 J K^?1mol^?1 for UO_2.663, 0.58 and 0.47 J K^?1mol^?1 for UO_2.656 and 0.62, 0.51 and 0.25 J K^?1mol^?1 for UO_2.640, increasing as O/U decreases. The enthalpy change due to the transition varied linearly with the transition temperature except for UO_2.640, showing the presence of the same mechanism of phase transition among the samples with various O/U ratios. The mechanism of the phase transition was discussed on the assumption that the transition is originated from the order-disorder rearrangement of U⁵⁺ and U⁶⁺ with a consequent displacement of atoms, similarly to the case of U₄O_9?y.