Variations of ferroelectric off-centering distortion and 3d-4p orbital mixing in La-doped BiFeO3 multiferroics

The lanthanum (La) modification is known to improve dielectric and magnetic properties of BiFeO3 (BFO), a promising room-temperature multiferroic oxide. The effects of La doping on the variations of the off-centering distortion and the orbital mixing of BFO are experimentally studied, in conjunction with first-principles density-functional theory (DFT) calculations. Both the Fe-O bond anisotropy in the FeO6-octahedron cage and the off-centering ferroelectric polarization along the hexagonal [001](h) h are predicted to be substantially reduced by the La doping. These DFT predictions agree with the structural-refinement results obtained from high-resolution x-ray powder-diffraction data. We have shown that the apparent improvement of the polarization-field response is not intrinsic and can be attributed to the reduced leakage current by the La doping. X-ray absorption near-edge structure (XANES) spectroscopy study further indicates that the degree of Fe 3d-4p orbital mixing decreases with the La doping. The conclusion deduced from XANES study correlates well with the orbital-resolved density of states which predicts that the La doping increases the number of unoccupied states in the p orbital but suppresses the number of unoccupied states in the Fe 3d orbital.