Effect of pressure on the band gap and the local FeO6 environment in BiFeO3

Abstract

BiFeO3 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 (fd−d) of the 4T1 and 4T2 Fe3+ (3d5) bands observed below the band gap (Egap = 2.49 eV) at 1.39 and 1.92 eV, respectively, at ambient conditions. Pressure induces linear redshift of both 4T1 and 4T2 bands, consistent with the compression of the FeO6 octahedron under pressure. On the other hand, the transition oscillator strength (fd−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 fd−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.