Synthesis and crystal structure of the Sr 2MnGa(O,F) 6 oxyfluorides

Abstract

The fluorine-containing derivatives of Sr 2MnGaO 5.5 were prepared by treatment with XeF 2 at temperatures ranging from 300°C to 600°C. The compounds crystallize in a tetragonal unit cell with a t≈ a p, c t≈2 a p ( a p—the parameter of the perovskite subcell). An increase in fluorine content is accompanied by a reduction of the Mn oxidation state due to a partial replacement of oxygen by fluorine. The crystal structure of Sr 2MnGaO 4.78F 1.22 was determined by electron diffraction and X-ray powder diffraction ( a=3.85559(2) Å, c=7.78289(6) Å, S.G. P4/ mmm, R I =0.012, R P=0.019). The structure consists of alternating (MnO 2), (SrO) and (GaO 0.78F 1.22) layers. The Ga atoms are situated in slightly elongated octahedra, the MnO 6 octahedra are characterized by two short apical Mn–O distances of 1.876(8) Å and four long equatorial ones of 1.9278(1) Å. This is interpreted as an “apically compressed” type of Jahn–Teller distortion, in contrast to the “apically elongated” one in the Sr 2MnGaO 5+ δ brownmillerites with different oxygen content. Possible structural reasons for the reversed Jahn–Teller effect are discussed.