Energetics of Formation and Disordering in Rare Earth Weberite RE 3 TaO 7 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 RE TaO (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 La NbO ( ) structure; the Nd compound has both Y TaO ( )-type and La NbO -type polymorphs; the Sm-Ho compounds crystallize in the weberite Y TaO ( ) structure; and the Ho-Yb compounds adopt the defect fluorite ( ) disordered structure. Depending on the reaction temperature, Ho TaO crystallizes in ordered Y TaO (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., La TaO is most stable and Yb TaO is least stable with respect to the component oxides. The calorimetric data also show that ordered Ho TaO (Y TaO structure) is energetically more stable by 9.2 ± 1.1 kJ/mol than disordered Ho TaO (defect fluorite structure).