Synthesis, crystal structure, and magnetic properties of a novel layered manganese oxide Sr2MnGaO5+δ

New Sr2MnGaO4.97 complex oxide was synthesized by solid state reaction in sealed silica tubes at 950-1000 degreesC. The Sr2MnGaO4.97 crystal structure was refined from X-ray powder diffraction data. Sr2MnGaO4.97 is based on the Ima2 brownmillerite-type structure with apically elongated MnO6 octahedra due to a Jahn-Teller effect. Electron diffraction and high-resolution electron microscopy showed that local ordering of the left- and right-hand chains of GaO4 tetrahedra in Sr2MnGaO4.97 leads to a superstructure with a doubling of the b parameter of the orthorhombic unit cell. The formal oxidation state of Mn (V-Mn) can be varied by thermal treatments at elevated oxygen pressure (450 degreesC, 20 bar of O-2). The oxygen insertion induces a structure transformation in oxidized Sr2MnGaO5.47 material with the formation of a tetragonal perovskite-like structure (a approximate to a(p), c approximate to 2c(p)) with oxygen vacancies located in the Ga layers. The oxidation is accompanied by a significant compression of the Mn-O apical distances and a suppression of the Jahn-Teller deformation. Both Sr2MnGaO4.97 and Sr2MnGaO4.97 can probably be treated as canted antiferromagnets with T-N similar to 150 and 80 K, respectively. (C) 2001 Academic Press.