Energetics of Formation and Disordering in Rare Earth Weberite RE₃TaO₇ Materials

Abstract

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 (C222 1 )-type and La3NbO7-type polymorphs; the Sm–Ho compounds crystallize in the weberite Y3TaO7 (C222 1 ) 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).

Disorder in Ho2Ti2−xZrxO7: pyrochlore to defect fluorite solid solution series

“…58 The nding of weberite-type short-range ordering in nominally disordered pyrochlores by Shamblin et al 42 has led to many thermodynamic studies aimed at understanding the mechanism and energetics of orderdisorder transitions in pyrochlores, weberites, and other compounds with uorite-derived structures. 52,[59][60][61][62] While the local atomic arrangement and long-range structure in fully disordered complex oxides has been studied in detail, little information is available on the behavior at both length scales in partially disordered materials. In this study, neutron and X-ray total scattering experiments were performed on nine members of the solid solution series Ho 2 Ti 2Àx Zr x O 7 (0 # x # 2.0) to comprehensively characterize the chemically induced order-disorder transformation across both short and long length scales.…”

A Comparison of Order-Disorder in Several Families of Cubic Oxides

“… Mielewczyk-Gryn and Navrotsky (2015) reported enthalpies of formation of RE 3 NbO 7 (RE = Y, La, Nd, Gd-Er, and Yb) with order-disorder occurring in compounds with RE ionic radii smaller than Tb. Energetics of another system, RE 3 TaO 7 (La, Nd, Sm-Yb), was reported by Subramani and Navrotsky (2019) . In both systems, enthalpies of formation became less exothermic as RE ionic radii decreased, showing a trend similar to RE 2 M 2 O 7 pyrochlores.…”

“…Three different compositions in RE titanate and stannate pyrochlore systems were studied to understand the energetics of disordering induced by radiation damage (Chung et al, 2018b). Study of radiation damage, structural evolution upon annealing, and energetics of Dy 2 Ti 2 O 7 pyrochlore used a combination of nondestructive methods [pair distribution function (PDF) analysis of neutron total scattering] followed by destructive (Saradhi et al, 2012;Subramani and Navrotsky, 2019). methods (high-temperature solution calorimetry and differential scanning calorimetry).…”

“…The data for RE 2 Hf 2 O 7 are from Navrotsky and Ushakov (2005) and Ushakov et al (2007) . The data for RE 3 MO 7 (M = Nb and Ta) are from ( Mielewczyk-Gryn and Navrotsky (2015) and ( Subramani and Navrotsky (2019) .…”

“…FIGURE 1 | Structure and energetics of order-disorder transitions between ordered pyrochlore, weberite, and disordered defect fluorite structures with RE 2 Zr 2 O 7 and RE 3 TaO 7 systems as examples. The figure has been made based on data from the literature(Saradhi et al, 2012;Subramani and Navrotsky, 2019).…”