Speaker
Description
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. These are the first data on $^{111m}$Cd in
BFO so far. 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 is substituting the bismuth site,
forecasting the phase transition. A visible drop of the
quadrupole frequency $\omega_0$ at a temperature of about
$T_c\approx820^\circ$C is indicating the $\alpha$-$\beta$
phase transition. Matching results with Density Functional
Theory (DFT) simulations suggest orthorhombic $Pbnm$
crystal symmetry for the high temperature $\beta$-phase.
This structure is proven from a nuclear point of view.
Combined with high temperature x-ray diffraction (XRD)
measurements also showing the beta phase appearing in $Pbnm
$ setting, a general description of the $\beta$-phase could
be made.
