Structural anomalies at the ferromagnetic transition and precursor effects in the vicinity of the structural phase transition of La0.815 Ba0.185 MnO3

Rhombohedral La0.815 Ba0.185 MnO3 undergoes a phase transition at Tc =251 K from a paramagnetic insulating state (PMI) toward a ferromagnetic metallic state (FMM) on cooling. At TS =187.1(3) K a first-order phase transition occurs toward a structure with monoclinic symmetry. Accurate crystal structures are reported for selected temperatures between TS and room temperature. Thermal expansion is found to be different in the PMI and FMM phases, with a larger thermal expansion coefficient in the FMM phase. This feature is related to incoherent distortions of the MnO6 octahedral groups, which are smaller in the FMM state than in the PMI state. Precursor effects are observed as anomalous structural behavior within the rhombohedral phase close to TS. They indicate two independent contributions to the driving force for the structural transition. Shear distortions of MnO6 octahedra in the rhombohedral structure are transformed into energetically more stable Jahn-Teller-type distortions in the monoclinic structure. And the larger number of degrees of freedom in monoclinic symmetry allows an energetically more stable coordination of La by oxygen in the monoclinic structure.