Formation and stability regions of the high-temperature fluorite-related phase in the R₂O₃-Ta₂O₅ system (R = La, Nd, Sm, Ho, Er, and Yb)

Solid-state reaction and annealing of melts of samples in the R₂O₃-Ta₂O₅ system (R is a rare-earth element, i.e., La, Nd, Sm, Ho, Er, and Yb) revealed a defect-fluorite phase (F-phase) at high temperatures. The formation region of this F-phase was in the region of ∼70-80 mol% R₂O₃ for small rare-earth ions, such as erbium and ytterbium, but only in the region of ∼80 mol% R₂O₃ at temperatures of >1800°C for large rare-earth ions, such as the lanthanum-through-samarium series. This F-phase exhibited disordered cation and anion sublattices, such as (R_0.8Ta_0.2)(O_1.7□_0.3). The F-phase decomposed to R₂O₃ and an ordered phase - R₃TaO₇ (orthorhombic weberite) - through a eutectic reaction at temperatures in the range of 1500°-1700°C for gadolinium or larger rare-earth ions, whereas the F-phase was stable at 1500°C for the small rare-earth ions (the erbium-through-ytterbium series).