Thermal evolution of the crystal structure of the orthorhombic perovskite LaFeO₃

The thermal evolution of the crystal structure of the prototypical orthorhombic perovskite LaFeO₃ has been studied in detail by powder neutron diffraction in the temperature range 25<T<1285 K. A conventional bond length/bond angle analysis, combined with an analysis in terms of symmetry-adapted modes, allows key aspects of the thermal behavior to be understood. In particular, the largest-amplitude symmetry modes (viz. in-phase and out-of-phase octahedral tilts, and A-site cation displacements) are shown to display relatively ‘normal’ behavior, increasing with decreasing temperature, which contrasts with the anomalous behavior previously shown by the derivative Bi_0.5La_0.5FeO₃. However, an unexpected behavior is seen in the nature of the intra-octahedral distortion, which is used to rationalize the unique occurrence of a temperature dependent crossover of the a and c unit cell metrics in this compound.