Evidence for diffuse ferroelectric phase transition and cooperative tricritical freezing of random-site dipoles due to off-centered Bi3+ ions in the average cubic lattice of (Ba1-xBix)(Ti1-xFex)O3

The mixed solid solution (Ba1-xBix)(Ti1-xFex)O-3 (BT-xBF) system exhibits nonrelaxor-type diffuse ferroelectric phase transition whose diffuseness increases with increasing concentration of BiFeO3 for x <= 0.40. Rietveld analysis of the high-resolution synchrotron x-ray diffraction data reveals that the average cubic symmetry of BT-xBF for 0.15 <= x <= 0.40 is broken locally by the off-centering of the Bi3+ ions with respect to the Ba2+ positions at the perovskite cube corners in the < 001 > direction. We also show that the random-site dipoles generated by the off-centered Bi ions freeze out cooperatively with a critical exponent of 1/4 characteristic of a tricritical transition. It is argued that the diffuse ferroelectric phase transition in the BT-xBF system is due to the formation of random field domains. The static random fields are generated by the local charge imbalances, strain field, or both caused by the presence of off-valent substituents, even though the overall composition is charge neutral.