Energetics of stepwise disordering transformation in pyrochlores, RE ₂Ti ₂O ₇ (RE = Y, Gd and Dy)

The capacity to incorporate actinide cations makes pyrochlore titanates first-choice phases in titanate-based waste form ceramics. Despite broad interest in the pyrochlore order-disorder transformation due to the cumulative effects of ²³⁸U, ²³⁵U and ²³²Th radioactive decay and their daughter products, only limited thermodynamic data, mainly based on simulations of ion-beam irradiation experiments, have been reported. In this work, for the first time, heavily disordered pyrochlores, RE ₂Ti ₂O ₇ (RE = Y, Gd and Dy), from mechanical milling of their constituent oxides, were thermochemically investigated. Two types of thermal events were identified using high-temperature differential scanning calorimetry and correlated to the structural disorder in the cation and anion sublattices. Moreover, the excess formation energy measured by oxide melt solution calorimetry shows that the smaller the ionic radius of the RE, the easier it is to remove damage domains.