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 >1,800 C for large rare-earth ions, such as the lanthanum-through-samarium series. This F-phase exhibited disordered cation and anion sublattices, such as (R0.8Ta0.2)(O1.7□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 1,500--1,700 C for gadolinium or larger rare-earth ions, whereas the F-phase was stable at 1,500 C for the small rare-earth ions (the erbium-through-ytterbium series)