Temperature-Driven Multiferroic Phase Transitions and Structural Instability Evolution in Lanthanum-Substituted Bismuth Ferrite

Structural phase transition behavior of a material under temperature plays a key role in understanding its physical properties, instabilities, and design of new functions. However, there is extreme shortage of investigations on the temperature-driven phase transition behaviors of bismuth ferrite (BiFeO ₃ ). Herein, magnetic phase transitions within low-temperature regimes, ferroelectric phase transitions under high temperature, structural instability evolution, and spin-phonon coupling in 5% La-doped BiFeO ₃ ceramic (BFLa05) were systemically studied by employing X-ray diffraction, differential scanning calorimetry, Raman spectroscopy, and magnetic measurement, as well as the phenomenological theoretical analysis. Thanks to the outstanding stability of BFLa05 ceramic under high temperature, a complete phase transition sequence of α ↔ β ↔ γ was observed, simultaneously determining the crystalline symmetries for β and γ phases as orthorhombic Pnma and cubic Pm3m, respectively. Octahedral tilt as a typical structure instability exhibited an anomalous change around the Néel temperature and a significant discontinuity in the vicinity of the Curie temperature, implying strong interplay or coupling between octahedral tilt and magnetic or ferroelectric ordering. Surprisingly, the intense spin-phonon coupling was observed not only around the Néel temperature, but also at two low temperatures of about 140 and 220 K. Furthermore, these two low-temperature magnetic phase transitions accompanying with spin-phonon interactions were well illustrated by a phenomenological theoretical model on the basis of the nearest-neighbors approximation and molecular field approximation.