Phase transitions, domain structure, and pseudosymmetry in La- and Ti-doped BiFeO3

The phase transitions and domain structure of the promising PbO-free solid solution series, (0.95-x)BiFeO3-xLaFeO3-0.05La2/3TiO3, were investigated. X ray diffraction(XRD) revealed a transition from a ferroelectricR3c to a PbZrO3-like (Pbam) antiferroelectric (AFE) structure at x = 0.15 followed by a transition to a paraelectric (PE, Pnma) phase at x > 0.30. The ferroelastic/ferroelectric twin domain width decreased to 10–20 nm with increasing x as the AFE phase boundary was approached but coherent antiphase tilted domains were an order of magnitude greater. This domain structure suggested the local symmetry (20 nm) is lower than the average structure (R3c, a−a−a−) of the tilted regions. The PE phase (x = 0.35) exhibited a dominant a−a−c+ tilt system with Pnma symmetry but diffuse reflections at ∼1/4{ooe} positions suggest that short range antipolar order is residual in the PE phase. The complex domain structure and phase assemblage of this system challenge the conventional interpretation of phase transitions based on macroscopic symmetry. Instead, it supports the notion that frustration driven by chemical distributions at the nanometric level influences the local or pseudo-symmetry as well as the domain structure, with XRD giving only the average macroscopic structure.