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 (BiFeO$_{3}$ or BFO). Using the perturbed angular correlation (PAC) method with $^{111m}$Cd 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 $^{111m}$Cd 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 ω0 at a temperature of about $T$$_{c}$ ≈ 820$^{∘}$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.