Differences in local structure around Co and Fe of the BiCo1-xFexO3 system determined by x-ray absorption fine structure

The local structures around Co and Fe in BiCo1-xFexO3 (BCFO) have been determined element-selectively by means of extended x-ray absorption fine structure (EXAFS) measurements at the Co and Fe K edges. In the tetragonal (T) phase at x <= 0.63, both Fe and Co ions are trivalent and locate in the O-5 pyramidal coordination. The four in-plane Fe bonds with transverse oxygen (Fe-O2) split into 2 x 2 bonds in the monoclinic (M) phase at 0.63 <= x <= 0.73 whereas the Co-O2 bonds do not show a split exceeding the resolution of our EXAFS analysis. This result is an element-specific interpretation of the polarization rotation in the M phase around the morphotropic phase boundary. The components of thermal vibration and static disorder are evaluated separately from the temperature dependence of the Debye Waller factor from T = 8 K to 300 K. The static disorder of the Fe site is large compared to that of the Co site, thereby indicating that Fe is unstable in the O-5 pyramidal coordination of the T and M phases. This study demonstrates that the instability of Fe ions plays an important role in the sequential structural distortion of the BCFO system.