Abstract
The synthesis and characterisation of LaCa2Fe2GaO8 is described. Neutron powder diffraction data reveal LaCa2Fe2GaO8 adopts an anion-vacancy ordered, ‘triple layer’ structure consisting of an OOTLOOTR stacking sequence of layers of (Fe/Ga)O6 octahedra (O) and (Fe/Ga)O4 tetrahedra (T), related to that of the 4-layer brownmillerite structures (a=5.4784(1)Å, b=22.6780(4)Å, c=5.6007(1)Å, space group Pbma). The apex-linked chains of tetrahedra within structure of LaCa2Fe2GaO8 exhibit a cooperative twisting distortion which alternates in direction between adjacent layers of tetrahedra, the first time such an arrangement has been seen in a triple-layer phase. Magnetisation and neutron diffraction data reveal that LaCa2Fe2GaO8 exhibits two magnetic ordering transitions. On cooling below 550K the phase adopts a state with short range antiferromagnetic coupling within each double layer of (Fe/Ga)O6 octahedra, but only weak coupling between layers. On cooling below 470K full three-dimensional, G-type antiferromagnetic order is observed.
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