Differences in local structure around Co and Fe of theBiCo1−xFexO3system determined by x-ray absorption fine structure

The local structures around Co and Fe in ${\mathrm{BiCo}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{3}$ (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 $x\ensuremath{\le}0.63$, both Fe and Co ions are trivalent and locate in the ${\mathrm{O}}_{5}$ pyramidal coordination. The four in-plane Fe bonds with transverse oxygen (Fe-O2) split into $2\ifmmode\times\else\texttimes\fi{}2$ bonds in the monoclinic (M) phase at $0.63\ensuremath{\le}x\ensuremath{\le}0.73$ 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 $T=8\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ 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 ${\mathrm{O}}_{5}$ 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.