Polymorphic transformation of δ-Bi 2O 3 stabilized with Ln 2O 3 ( Ln = Sm, Eu, Gd, Tb and Dy) into a new phase with a C-type rare-earth oxide-related structure

Abstract

This paper describes the polymorphism of the novel phase Bi 1 − x Ln x O 1.5 ( Ln = Sm, Eu, Gd, Tb, and Dy with = 0.38, 0.375, 0.275-0.40, 0.275-0.35, and 0.30−0.35, respectively) which has a C-type rare-earth oxide-related structure. This C-type phase transformed without delay to the δ-Bi 2O 3 type high-temperature phase around 900 °C upon heating. Upon cooling, however, the δ-phase never returned to the original C-type phase. The irreversibility is only apparent, because the δ-phase transformed gradually to the C-type phase by annealing around 800 °C; in other words, the rate of transition from the δ to the C-type phase is extremely sluggish. The electrical conductivity of the C-type phase was not good ( σ = 10 −3.9 S cm −1 at 700 °C), though the oxide-ion transport number was 70% over a temperature range 600 to 850 °C.