Redox Properties of Hexagonal and Cubic Sr_0.4Ba_0.6Mn_0.94Ti_0.06O _3- Investigated by In Situ Neutron Powder Diffraction

The synthesis and structural properties of polycrystallinecubicand hexagonal Sr0.4Ba0.6Mn0.94Ti0.06O3-& delta; have been investigated usinga combination of in situ neutron diffraction and thermogravimetricanalysis. The experiments were conceived to replicate and explainkey synthesis processes involving switching between different reducingand oxidizing atmospheres designed to transform a hexagonal phaseinto an oxygen-deficient cubic perovskite phase in which Mn ions arein their trivalent oxidation state. Hydrogen reduction of the hexagonalphase first produces a heavily oxygen-deficient hexagonal phase precedingto full decomposition of the material. Remarkable reversible propertiesof the parent hexagonal structure are observed with the phase recoveredwithin minutes by reoxidation of the material after it was decomposedin hydrogen-containing atmospheres. The partial substitution of largeBa ions at the Sr sites enhances the strains and creates wide channels,enabling the rapid oxidation of the oxygen-deficient cubic phase.The fast oxygen intake results in a phase separation between oxygen-richand oxygen-poor phases with no evidence for any vacancy-ordered superstructuresas those previously seen in the parent Ba- and Ti-free SrMnO3-& delta; system.