LAUSR

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 (Fm3̅m) 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).