Structure and magnetism of 4H-BaMnO3−x (image) and 4H-Ba0.5Sr0.5MnO3−x (image)

The synthesis and structure determination of 4H-BaMnO3−x (image) and 4H-Ba0.5Sr0.5MnO3−x (image) is described. Neutron powder diffraction data unambiguously show the anion vacancies in the oxygen-deficient phases lie in the hexagonal BaO3−x layers, rather than the cubic layers predicted by previous structural models. The localization of anion vacancies in the hexagonal layers is discussed on the basis of the local co-ordination polyhedra of the metal cations. Low-temperature neutron diffraction data show the oxidized materials adopt antiferromagnetic ordered structures (Ba0.5Sr0.5MnO2.99: image). Anion-deficient phases do not achieve magnetic order suggesting the Mn(III/IV) charge disorder suppresses long-range magnetic order.