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Cation order imaging and magnetic properties in the Ca2Fe2-xGaxO5 brownmillerite (0 <= x <= 1)

Abstract : Polycrystalline Ca2Fe2-xGaxO5 brownmillerite samples of nominal x = 0.5, 0.7, 0.9 and 1 compositions have been synthesized by solid state reaction under normal pressure conditions. Whatever x is, room temperature structural analyses by Rietveld refinements and electron microscopy techniques highlight a centrosymmetric Prima space group. Furthermore, STEM-HAADF observations point towards a preferential Ga occupation at the tetrahedral sites of the structure in agreement with the Rietveld analyses. This structure remains unchanged up to 1000 K as shown by thermo-diffraction analysis. As the Ga content x increases, the room temperature unit cell volume decreases gradually consistently with the fourfold coordination of the smallest Ga3+ ionic radius than that of high spin Fe3+ in the same tetrahedral coordination. This magnetic dilution by a diamagnetic cation is responsible for the T-N decrease revealed by magnetization measurements and allows a spin flop transition to be observed. Despite their similarities with the Ca2Fe2-xAlxO5 samples, no electric polarization could be evidenced for the homologous Ca2Fe2-xGaxO5 series.
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Submitted on : Friday, July 5, 2019 - 9:51:41 AM
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Ayman Moussa, L. Monnier, D. Pelloquin, A. Maignan. Cation order imaging and magnetic properties in the Ca2Fe2-xGaxO5 brownmillerite (0 <= x <= 1). Journal of Solid State Chemistry, Elsevier, 2018, 265, pp.129-134. ⟨10.1016/j.jssc.2018.05.032⟩. ⟨hal-02174380⟩



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