Abstract
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.
Highlights
A variety of applications have been reported for the perovskitetype compounds La1ÀxSrxFeO3Àd, ranging from oxygen separation membranes to gas sensors [1,2,3,4,5,6]
The compounds 15R–BaFeO2F and 15R–BaFeO2.42F0.2 [28] only show a small difference in the average iron oxidation state, the orientation of the spins is different in that the spins align parallel to the c-axis for 15R–BaFeO2.42F0.2 [28] but are aligned in the a/b-plane for 15R–BaFeO2F [19]
O/F are indistinguishable by means of X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) experiments, but full occupancy of the anion sites was verified from the NPD diffraction data
Summary
A variety of applications have been reported for the perovskitetype compounds La1ÀxSrxFeO3Àd, ranging from oxygen separation membranes to gas sensors [1,2,3,4,5,6] These compounds show interesting magnetic properties, varying from antiferromagnetic G-type ordering in LaFeO3 [7,8] to antiferromagnetic ordering in rhombohedral La1/3Sr2/3FeO3, in which ferromagnetic interactions between the magnetic moments on Fe3 þ and Fe5 þ in neighbouring layers occur [9]. For SrFeO2F, which was previously reported to be a simple cubic perovskite by analysis of XRD data [14,15,20,21], it is shown that the structural arrangement of the ions has this lower orthorhombic symmetry, the cell parameters are pseudocubic Both the results from the structural study reported in [24] and those reported here have been analyzed and checked in terms of distortion modes with respect to the cubic perovskite.
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