Thermal behavior of Bi5Pb3O10.5

In Bi5Pb3O10.5 there exist three polymorphs: the low-temperature stable βL, the high-temperature stable β which shows a high oxide-ion conduction, and the metastable β2 phases. This paper describes the phase transformations between these polymorphs using TG–DTA and dc conductivity measurements. The triclinic βL phase transforms to the anti-α-AgI-type bcc β phase at about 585°C; on cooling, at about 560°C the β phase changes to the tetragonal β2 phase which is kept to room temperature. The β2 phase reverts gradually to the βL phase during heating. The rate of this gradual β2-to-βL transition depends strongly on a heating rate. A very small weight change is observed with respect to the β phase: weight gain upon the βL-to-β transition in the heating direction and weight loss upon the β-to-β2 transition in the cooling direction. However, the TG–DTA results under N2 gas atmosphere suggest that this weight change seems to be due to the excessive oxygen vacancies of the β phase.