Stöber, S.; Redhammer, G.; Schorr, S.; Prokhnenko, O.; Pöllmann, H.: Structure refinements of members in the brownmillerite solid solution series Ca2Alx(Fe0.5Mn0.5)2-xO5+δ with 1/2<=x<=4/3. Journal of Solid State Chemistry 197 (2013), p. 420-428
10.1016/j.jssc.2012.08.032

Abstract:
Four different brownmillerite solid solutions Ca2Alx(Fe0.5Mn0.5)2-xO5+delta with 1/2<x<4/3 were synthesized by a solid oxide ceramic method. The phases crystallize either in a primitive centered orthorhombic cell with space group Pnma or in a body centered cell with space group I2mb dependent on the aluminum concentration present in the solid solution. Mn3+ ions occupy exclusively site 4a coordinated by six oxygen anions. Increasing Mn3+ concentrations cause a remarkable distortion of the octahedron and indirectly of the tetrahedron, resulting in twisted and tilted octahedral layers as well as buckled tetrahedral chains. The influences are discussed on the site 4a of trivalent manganese due to its Jahn-Teller activity, with regard to the occupation of octahedron and tetrahedron with different sized iron and aluminum ions.