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

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.