As well as the known fluorite phase in (Bi2O3)1-x(Ta2O5)x, where x was between 0.18 and 0.25, a second fluorite phase was found in heated samples (800C) for x-values less than 0.07 and in quenched samples with x-values of between 0.05 and 0.07. These phases were termed FI and FII, respectively. The decrease in conductivity, with increasing x-value, at temperatures of between 650 and 800C was classified into 3 zones (low, medium, high) corresponding to FII, FI+FII and FI, respectively (figure 3). The appearance of these zones was explained in terms of the growth of low-conductivity FI domains within a high-conductivity FII matrix. Samples which were held at 630, 600 or 550C after slow cooling from 800C exhibited a gradual decrease in conductivity; depending upon the holding temperature.

Crystal Structure and Ionic Conductivity in Bi2O3-Rich Bi2O3-Ta2O5 Sintered Oxides. T.Saito, R.Miida: Japanese Journal of Applied Physics - 1, 1999, 38[8], 4838-42

Figure 3

Ionic Conductivity of (Bi2O3)1-x(Ta2O5)x at 800C