Local insight into the La-induced structural phase transition in multiferroic BiFeO₃ ceramics by x-ray absorption...

Substitution of bismuth by rare earth (RE) ions is of great technological importance to develop room-temperature BiFeO₃-based multiferroic materials. Despite this interest, many fundamental properties and the structure-property correlations of RE-doped BiFeO₃ remain poorly understood. Here we report a systematical experimental and theoretical exploration on the structural phase transition in Bi_1-x La _x FeO₃ (0  ⩽  x  ⩽  0.2) ceramics. By using x-ray absorption fine structure spectroscopy, we for the first time show that the La³⁺ dopants in fact substitute the Bi site of secondary nanosized particles with orthorhombic Pbam symmetry instead of the long-believed parental rhombohedral R3c phase at all La³⁺ doping concentrations (0.001  ⩽  x  ⩽  0.2). This homogeneously mixed two-phase compound cannot be detected by the x-ray diffraction until La content approaching x  =  0.1. The finding is further supported by complementary studies of transmission electron microscopy and thermodynamic preference, and it casts serious challenges on the prevailing assumption of La³⁺ substitution on the Bi³⁺ site in R3c structure when x  ⩽  0.1 as well as the previously proposed origin of enhanced functional properties based on morphotropic phase boundary. This new insight may ignite a revival on exploring the underlying multiferroic mechanisms in BiFeO₃-based materials and facilitate the bottom-up design of novel multifunctional devices.