Order-Disorder Phase Stabilization by Pressure-Induced Charge Transfer Enhances the Ferroelectric Photovoltaic Effect...

Multiferroic ferroelectric photovoltaic (FPV) materials, combining magnetic and ferroelectric properties, are of paramount importance for optoelectronic and photovoltaic applications. However, optimizing both the remanent polarization and the optical bandgap-key factors for enhanced FPV performance-presents a significant challenge due to their trade-off. This work shows that pressure-induced charge transfer between different metal sites can break this trade-off. Above ≈20 GPa, charge transfer between different trivalent iron (Fe) sites in the multiferroic material BaFe₄O₇ leads to Fe valence disproportionation, FeO₄ tetrahedra disorder, and Jahn-Teller distortion of FeO₆ octahedra. These changes reduce the bandgap, lower resistivity, and enhance ferroelectric polarization, resulting in a 2.5-fold increase in photocurrent. Upon decompression, BaFe₄O₇ retains an order-disorder structure, optimal ferroelectric and optical properties at ambient conditions. This work provides a novel pathway to simultaneously optimizing ferroelectricity and bandgap via pressure-induced charge transfer, overcoming the traditional trade-off in FPV materials, and offers a promising approach for developing high polarization performance, narrow-bandgap FPV materials.