Crystal structure and ionic conductivity in Bi2O3-rich Bi2O3-Ta2O5 sintered oxides

Aside from the already known fluorite phase in (Bi2O3)(1-x)(Ta2O5)(x) with 0.18 less than or equal to x less than or equal to 0.25, a second fluorite phase was found in heated samples (800 degrees C) with x less than or equal to 0.07 and quenched samples with 0.05 less than or equal to x less than or equal to 0.07; we called these two phases FI and FII, respectively. The decrease in conductivity with increasing x at temperatures between 650 degrees C and 800 degrees C was classified into three degrees, (i) low, (ii) medium and (iii) high corresponding to FII, FI + FII and FI zones, respectively. The appearance of these degrees was explained in terms of the growth of low-conductivity FI domains within the high-conductivity FII matrix. The sample held at 630 degrees C, 600 degrees C or 550 degrees C after slow cooling from 800 degrees C showed a gradual decrease in conductivity depending on the holding temperature, and this phenomenon was interpreted by relating the growths of gamma-Bi2O3-type structure (630 degrees C and 600 degrees C) and alpha-and beta-Bi2O3-type structures (550 degrees C).