Composition dependence of the structural chemistry and magnetism of Ca{sub 2.5}Sr{sub 0.5}(Ga,Co){sub 1+} {sub x} Mn{sub 2-} {sub x} O{sub 8}

Polycrystalline samples of bilayered brownmillerite-like Ca{sub 2.5}Sr{sub 0.5}GaCo{sub 0.15}Mn{sub 1.85}O{sub 8} and Ca{sub 2.5}Sr{sub 0.5}Ga{sub 1.2}Mn{sub 1.8}O{sub 8} have been prepared and characterised by magnetometry and neutron diffraction over a wide temperature range. The structural chemistry and magnetic properties are compared to those of Ca{sub 2.5}Sr{sub 0.5}GaMn{sub 2}O{sub 8}. Ga enrichment has a significant effect on the former but not on the latter, whereas changes in both occur when paramagnetic Co{sup 3+} cations enter the parent phase on the 4-coordinate sites. The coupling between the environment around the 4-coordinate cations and the transition to an antiferromagnetic ordered state that was observed in Ca{sub 2.5}Sr{sub 0.5}GaMn{sub 2}O{sub 8} is not apparent in the cation-substituted compositions, although both show long-range antiferromagnetic order at low temperatures.