Effect of pressure on the band gap and the local FeO 6 environment in BiFeO 3

Abstract

BiFeO 3 exhibits a complex phase-transition sequence under pressure associated with changes in octahedron tilts and displacements of Bi3 + and Fe3 + cations. Here, we investigate the local structure of Fe3 + as a function of pressure through absorption crystal-field spectroscopy in the 0-18 GPa range. We focus on the influence of phase transitions on the Fe3 + off-center displacement through the energy (E) and oscillator strength (f d-d) of the 4T 1 and 4T 2 Fe3 + (3d5) bands observed below the band gap (E gap = 2.49 eV) at 1.39 and 1.92 eV, respectively, at ambient conditions. Pressure induces linear redshift of both 4T 1 and 4T 2 bands, consistent with the compression of the FeO 6 octahedron under pressure. On the other hand, the transition oscillator strength (f d-d=3×10 -5), enabled by both the exchange mechanism and the off-center Fe3 + distortion, slightly increases with pressure. The absence of notable anomalies in the variation of E(P) and f d-d(P) through the phase sequence from the ferroelectric rhombohedral R3c phase to the nonpolar orthorhombic Pnma phase suggests a persisting off-center position of the Fe3 +. While this local polarity is correlated and expected in the ferroelectric R3c phase, its presence in the high-pressure nonpolar Pnma phase indicates the presence of local polar instabilities. © 2012 American Physical Society.