Multiferroic Bismuth Ferrite: Perturbed Angular Correlation Studies On Its Ferroic Alpha-Beta Phase Transition

Work of numerous research groups has shown different outcomes of studies of the transition from the ferroelectric alpha-phase to the high temperature beta-phase of the multiferroic, magnetoelectric perovskite Bismuth Ferrite (BiFeO3 or BFO). Using the perturbed angular correlation (PAC) method with Cd-111m as the probe nucleus, the alpha to beta phase transition was characterized. The phase transition temperature, the change of the crystal structure, and its parameters were supervised with measurements at different temperatures using a six detector PAC setup to observe the gamma-gamma decay of the Cd-111m probe nucleus. The temperature dependence of the hyperfine parameters shows a change in coordination of the probe ion, which substitutes for the bismuth site, forecasting the phase transition to beta-BFO by either increasing disorder or formation of a polytype transition structure. A visible drop of the quadrupole frequency omega(0) at a temperature of about T-c approximate to 820 degrees C indicates the alpha-beta phase transition. For a given crystal symmetry, the DFT-calculations yield a specific local symmetry and electric field gradient value of the probe ion. The Pbnm (beta-BFO) crystal symmetry yields calculated local electric field gradients, which very well match our experimental results. The assumption of other crystal symmetries results in significantly different computed local environments not corresponding to the experiment.