Redox Properties of Hexagonal and Cubic Sr_0.4Ba_0.6Mn_0.94Ti_0.06O_3−δ Investigated by In Situ Neutron Powder Diffraction

Abstract

The synthesis and structural properties of polycrystalline cubic and hexagonal Sr0.4Ba0.6Mn0.94Ti0.06O3−δ have been investigated using a combination of in situ neutron diffraction and thermogravimetric analysis. The experiments were conceived to replicate and explain key synthesis processes involving switching between different reducing and oxidizing atmospheres designed to transform a hexagonal phase into an oxygen-deficient cubic perovskite phase in which Mn ions are in their trivalent oxidation state. Hydrogen reduction of the hexagonal phase first produces a heavily oxygen-deficient hexagonal phase preceding to full decomposition of the material. Remarkable reversible properties of the parent hexagonal structure are observed with the phase recovered within minutes by reoxidation of the material after it was decomposed in hydrogen-containing atmospheres. The partial substitution of large Ba 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-rich and oxygen-poor phases with no evidence for any vacancy-ordered superstructures as those previously seen in the parent Ba- and Ti-free SrMnO3−δ system.