Synthesis and crystal structure of novel layered manganese oxide Ca2MnGaO5+δ

New layered oxides Ca(2)MnGaO(5+delta) and Ca(2)MnGa(1-x)Zn(x)O(5+delta) (x = 0.1, 0.2) were synthesized by solid-state reaction at 1000-1300 degreesC, The crystal structure of Ca(2)MnGaO(50.45) was studied using X-ray, neutron and electron diffraction, and high-resolution electron microscopy (HREM). The samples of Ca(2)MnGaO(5.045) consist of two closely intermixed brownmilierite-type phases with Pnma or Ima2 space symmetry that possess very closely related structures that differ mainly by the orientation of the Ga-O chains. The formal Mn valence (V(Mn)) in Ca(2)MnGaO(5+delta) can be varied either by heterovalent replacement of Ga(3+) for Zn(2+) or by oxygen insertion after treatment under 80 atm O(2). In the latter case the compound with the composition Ca(2)MnCaO(5.39) forms built of domains of initial reduced material and fully oxidized Ca(2)MnGaO(5.5) phase, The Ca(2)Mn GaO(5.5) structure was determined by HREM as belonging to the brownmillerite A(n)B(n-1)B'O(3n-1) (n = 4) homologous series, in which three layers of BO(6) (B = Mn, Ga) octahedra form blocks separated by a single layer of B'O(4) (B' = Ga) tetrahedra. The increase of V(Mn) from + 3.09 to + 3.78 is accompanied by a drastic compression of the structure along the b = 4a(per) axis due to a suppression of the Jahn-Teller deformation of the MnO(6) octahedra. (C) 2001 Academic Press.