Formation and Stability Regions of the High‐Temperature Fluorite‐Related Phase in the R2O3‐Ta2O5 System (R = La, Nd, Sm, Ho, Er, and Yb)

Abstract

Solid‐state reaction and annealing of melts of samples in the R2O3‐Ta2O5system (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% R2O3for small rare‐earth ions, such as erbium and ytterbium, but only in the region of ˜80 mol% R2O3at 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 (R0.8Ta0.2)(O1.7box0.3). 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).