A reappraisal of fast ion conduction in Ta, Ga, and Al-substituted Aurivillius phases

Oxygen ion conduction in the Aurivillius phases Bi2Sr2(Nb or Ta)2(Ga or Al)O11.5 has been evaluated in detail. High-temperature X-ray diffraction studies, coupled with differential scanning calorimetry, electron microscopy, and conductivity measurements were used to clearly demonstrate that Bi2Sr2(NbxTa1−x)2(GayAl1−y)O11.5 with x = 0, 1 and y = 0, 0.5, and 1 form only multiphase specimens. The origin of the previously observed fast ion conduction is a result of interconnected Bi2O3 grains that undergo a high-temperature phase transformation to the δ-Bi2O3 polymorph. The Aurivillius phase component of the multiphase specimens does not demonstrate any type of order–disorder transformation, however, the phase transformations of the Bi2O3 are clearly observed using high-temperature diffraction and calorimetry.