Crystal Structure and Ionic Conductivity in Bi₂O₃-Rich Bi₂O₃–Ta₂O₅ Sintered Oxides

Abstract

Aside from the already known fluorite phase in (Bi 2 O 3 ) 1- x (Ta 2 O 5 ) x with 0.18≦ x ≦0.25, a second fluorite phase was found in heated samples (800°C) with x ≦0.07 and quenched samples with 0.05≦ x ≦0.07; we called these two phases F I and F II, respectively. The decrease in conductivity with increasing x at temperatures between 650°C and 800°C was classified into three degrees, (i) low, (ii) medium and (iii) high corresponding to F II, F I + F II and F I zones, respectively. The appearance of these degrees was explained in terms of the growth of low-conductivity F I domains within the high-conductivity F II matrix. The sample held at 630°C, 600°C or 550°C after slow cooling from 800°C showed a gradual decrease in conductivity depending on the holding temperature, and this phenomenon was interpreted by relating the growths of γ-Bi 2 O 3 -type structure (630°C and 600°C) and α-and β-Bi 2 O 3 -type structures (550°C).