Non-linear thermal evolution of the crystal structure and phase transitions of LaFeO3 investigated by high temperature X-ray diffraction

Abstract

The crystal structure, anisotropic thermal expansion and structural phase transition of the perovskite LaFeO3 has been studied by high-temperature X-ray diffraction from room temperature to 1533K. The structural evolution of the orthorhombic phase with space group Pbnm and the rhombohedral phase with R3¯c structure of LaFeO3 is reported in terms of lattice parameters, thermal expansion coefficients, atomic positions, octahedral rotations and polyhedral volumes. Non-linear lattice expansion across the antiferromagnetic to paramagnetic transition of LaFeO3 at TN=735K was compared to the corresponding behavior of the ferroelectric antiferromagnet BiFeO3 to gain insight to the magnetoelectric coupling in BiFeO3, which is also multiferroic. The first order phase transition of LaFeO3 from Pbnm to R3¯c was observed at 1228±9K, and a subsequent transition to Pm3¯m was extrapolated to occur at 2140±30K. The stability of the Pbnm and R3¯c polymorphs of LaFeO3 is discussed in terms of the competing enthalpy and entropy of the two crystal polymorphs and the thermal evolution of the polyhedral volume ratio VA/VB.