Energetics of Formation and Disordering in Rare Earth Weberite RE₃TaO₇ Materials

The recent finding of local weberite-like ordered domains in disordered and radiation damaged pyrochlore oxides has sparked interest in studying the structure, stability, and order-disorder in compounds that form in the weberite structure. In order to understand the relationships among the energetics, structure, and disordering, weberites of the formula RE₃TaO₇ (RE = La, Nd, Sm-Yb) were synthesized by conventional solid-state techniques. High temperature oxide melt solution calorimetry was used to determine their enthalpies of formation. Rietveld refinement of PXRD patterns shows that the La compound forms in the weberite La₃NbO₇ (Cmcm) structure; the Nd compound has both Y₃TaO₇ (C222₁)-type and La₃NbO₇-type polymorphs; the Sm-Ho compounds crystallize in the weberite Y₃TaO₇ (C222₁) structure; and the Ho-Yb compounds adopt the defect fluorite (Fm3̄ m) disordered structure. Depending on the reaction temperature, Ho₃TaO₇ crystallizes in ordered Y₃TaO₇ (low temperature) or disordered defect fluorite (high temperature) structures. The formation enthalpy of weberites becomes more exothermic with increasing rare earth ionic radius, implying an increase in stability, i.e., La₃TaO₇ is most stable and Yb₃TaO₇ is least stable with respect to the component oxides. The calorimetric data also show that ordered Ho₃TaO₇ (Y₃TaO₇ structure) is energetically more stable by 9.2 ± 1.1 kJ/mol than disordered Ho₃TaO₇ (defect fluorite structure).