Synthesis and crystal structure of the Sr2MnGa(O,F)6 oxyfluorides

The fluorine-containing derivatives of Sr2MnGaO5.5 were prepared by treatment with XeF2 at temperatures ranging froin 300degreesC to 600degreesC. The compounds crystallize in a tetragonal unit cell with a(1) approximate to a(p), c(1) approximate to 2a(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 Sr2MnGaO4.78F1.22 was determined by electron diffraction and X-ray powder diffraction (a = 3.85559(2) Angstrom. c = 7.78289(6) Angstrom, S.G. P4/mmm, R-1 = 0.012. R-p = 0.019). The structure consists of alternating (MnO2), (SiO) and (GaO0.78F1.22) layers. The Ga atoms are situated in slightly elongated octahedra, the MnO6 octahedra are characterized by two short apical Mn-O distances of 1.876(8) Angstrom and four long equatorial ones of 1.9278(1)Angstrom This is interpreted as ail "apically compressed" type of Jahn-Teller distortion, in contrast to the "apically elongated" one in the Sr2MnGaO5+delta brownmillimetres with different oxygen content. Possible structural reasons for the reversed Jahn-Teller effect are discussed. (C) 2003 Elsevier Inc. All rights reserved.