The weberite structure, named for the mineral Na2MgAlF7, occurs for a number of oxides, especially niobates and tantalates of composition RE3NbO7 and RE3TaO7. We have synthesized RE3TaO7 (RE = La, Pr, Nd, Sm-Tm) by conventional solid state synthesis in order to understand relationships among the energetics, structure and disordering. Rietveld refinement of PXRD patterns show that RE = La and Pr weberites form in La3NbO7 (Cmcm) structure, RE = Nd, Sm-Ho weberites crystallize in Y3TaO7 (C2221) structure and RE = Ho-Tm compounds adopt the defect-fluorite type (Fm-3m) disordered structure. Depending on the reaction temperature, Ho3TaO7 crystallizes in both ordered Y3TaO7 (low temperature) and disordered defect-fluorite (high temperature) structures. The high temperature oxide melt solution calorimetry experiments on these weberites reveal that the formation enthalpy becomes more exothermic with increasing ionic radius, implying increase in the stability with increasing ionic radius. i.e. La3TaO7 is more stable and Tm3TaO7 is least stable among the investigated weberites. The calorimetric results also show that ordered Ho3TaO7 (Y3TaO7 structure) is energetically more stable by about 9.18 kJ/mol, than disordered Ho3TaO7 (defect-fluorite type structure).

Energetics of formation and disordering in RE3TaO7 weberites

2019 Spring National Meeting

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