Differences in local structure around Co and Fe of the ${\mathrm{BiCo}}_{1-x}{\mathrm{Fe}}_x{\mathrm{O}}_3$ system determined by x-ray absorption fine structure

The local structures around Co and Fe in ${\mathrm{BiCo}}_{1-x}{\mathrm{Fe}}_x{\mathrm{O}}_3$ (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 $x\le 0.63$, 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 $2\times 2$ bonds in the monoclinic (M) phase at $0.63\le x\le 0.73$ 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 $T=8\mathrm{K}$ 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.

Figure 1

(a)–(c) Crystal structure and space group of the ${\mathrm{BiCo}}_{1-x}{\mathrm{Fe}}_x{\mathrm{O}}_3$ system. (d) Schematic of the atomic arrangement viewed from an $a$ axis of the pseudocubic perovskite. Four equivalent transverse O2 sites of the tetragonal phase split to form two O21 and two O22 sites due to the symmetry of the monoclinic phase.

Figure 2

Raw absorption profiles of the BCFO sample taken at $T=8$ K. For clarity, each spectrum is shifted vertically. The vertical broken lines indicate position of the Fe and Co $K$ absorption edges.

Figure 3

Composition dependence of the XANES profile. (a) Fe and (b) Co $K$ edges. The insets show the composition dependence of the absorption edge $E_0$. The horizontal lines in the insets represent $E_0$ of the reference samples.

Figure 4

Comparison of XANES profiles and the absorption edges with those of reference samples at the (a) Fe and (b) Co $K$ edges.

Figure 5

$k^3$-weighted EXAFS profiles [$k^3\chi \left(k\right)$] of the three phases at the Fe and Co $K$ edges. The profiles measured at 8 K and 260 K are shown.

Figure 6

Solid lines indicate the magnitude of the Fourier transforms of $k^3\chi \left(k\right)$ around Fe ion (left panels) and Co ion (right panels) at each phase. The horizontal solid lines near the peaks of the TM-O and TM-Bi/TM bonds show the magnitude at highest and lowest temperatures. Broken lines represent the fitted $k^3\chi \left(k\right)$ curves at $T=8$ K (black lines) and $T=300$ or 260 K (grayish lines).

Figure 7

Bond length (left) and DWFs (right) of the pyramidal ${\mathrm{TMO}}_5$ structure in the T phase as a function of temperature. The panels (a) and (b) show the results of the TM-O1 and TM-O2 bonds, respectively. In panel (b), the large and small symbols are used to represent $2\times 2$ sets of the TM-O2 bond, i.e., TM-O21 and TM-O22 bonds. The solid and dashed lines in the panels (d) indicate fitted curves of the Einstein model.

Figure 8

(a) and (b) Composition dependence of the bond length, (c) ${\mathrm{\Theta }}_{\text{E}}$, and (d) ${\sigma }_{\text{static}}^2$ of the pyramidal ${\mathrm{TMO}}_5$ structure. The crosses in the panels (a) and (b) represent the bond lengths of ${\mathrm{BiCoO}}_3$ and ${\mathrm{BiFeO}}_3$ determined from NPD experiments at room temperature [8, 20]. The broken lines are guides to the eye.

Figure 9

The magnitude of the Fourier transforms of $k^3\chi \left(k\right)$ ($|\text{FT}{k}^3\chi \left(k\right)|$, thin solid lines) for the M phase at Fe content $x=0.63$, 0.67, 0.70, and 0.73. The vertical arrow in panel (a) indicates the shoulder structure in the $|\text{FT}{k}^3\chi \left(k\right)|$ profile at $x=0.63$. Thick broken lines represent the fitted $|\text{FT}{k}^3\chi \left(k\right)|$ curves at $T=300$ K (grayish lines).

Figure 10

${\sigma }_{\text{TM-Bi}}^2\left(T\right)$ of the T, M, and R phases at (a) Fe and (b) Co $K$ edges. The composition dependence of (c) ${\mathrm{\Theta }}_{\text{E}}$ and (d) ${\sigma }_{\text{static}}^2$. The solid lines in the panels (a) and (b) represent curves fitted using the Einstein model. The dashed lines are guides to the eye.

Figure 11

Comparison of the splitting of the TM-O2 bonds between BCFO and PZT systems. The data of the PZT system were obtained from Ref. [13]. The vertical lines represent the phase boundaries of the BCFO system.