A neutron diffraction study and mode analysis of compounds of the system La_1−xSr_xFeO_3−xF_x (x=1, 0.8, 0.5, 0.2) and an investigation of their magnetic properties

We report here a detailed study of the system La1−xSrxFeO3−xFx, by neutron powder diffraction- and magnetic-measurements. All the compounds are robust antiferromagnetics with ordering temperatures well above room temperature. Magnetic moments are shown to align parallel to the c-axis. FC-ZFC measurements indicate a small canting of the magnetic moments, resulting in a ferromagnetic component with a maximum for La0.5Sr0.5FeO2.5F0.5. We show that the system exhibits a composition-driven transition from a phase, for low fluorination levels (x≤0.5), with Pnma symmetry and the usual system of octahedral tiltings, to a phase with space group Imma for higher fluorine contents, where a correlated distortion of the oxygen octahedra plays a significant role. The consistency of the structural models, with respect to the expected continuity of the amplitudes of the different distortion modes and the invariance of their internal form, was monitored through the symmetry mode decomposition of the structures.