Pressure-induced phase transitions and structural evolution across the insulator-metal transition in bulk and nanoscale BiFeO3

The pressure-induced phase-transition sequences and structural evolution across the insulator-metal transition (IMT) in multiferroic BiFeO3 still remain unclear. Here we use a combination of high-pressure XRD, XAFS experiment and first principle calculation to investigate the pressure-derived structural transformations and structure-related properties in bulk and nanoscale BiFeO3 up to 55 GPa. A new Imma structure of BiFeO3 has been discovered in the pressure range of 48-52 GPa, which presents ferromagnetic (FM) metallic properties and therefore plays a key role in the IMT. Local structure study reveals that the Bi3+ cation gradually shifts toward the centrosymmetric position in BiO12 cluster during IMT. Besides, the detailed structural information of post-perovskite Cmcm phase has also been determined and thus the complete phase sequence up to 60 GPa is obtained. Our research provides a structural origin of the IMT and a new way to understand the FM release in BiFeO3 system.