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

New layered oxides Ca2MnGaO5+delta and Ca2MnGa1-xZnxO5+delta (x = 0.1, 0.2) were synthesized by solid-state reaction at 1000-1300 degreesC, The crysta l structure of Ca2MnGaO50.45 was studied using X-ray, neutron and electron diffraction, and high-resolution electron microscopy (HREM). The samples of Ca2MnGaO5.045 consist of two closely intermixed brownmilierite-type phases with Pnma or Ima2 space symmetry that possess very closely related structu res that differ mainly by the orientation of the Ga-O chains. The formal Mn valence (V-Mn) in Ca2MnGaO5+delta can be varied either by heterovalent rep lacement of Ga3+ for Zn2+ or by oxygen insertion after treatment under 80 a tm O-2. In the latter case the compound with the composition Ca2MnCaO5.39 f orms built of domains of initial reduced material and fully oxidized Ca2MnG aO5.5 phase, The Ca2Mn GaO5.5 structure was determined by HREM as belonging to the brownmillerite A(n)B(n-1)B'O3n-1 (n = 4) homologous series, in whic h three layers of BO6 (B = Mn, Ga) octahedra form blocks separated by a sin gle layer of B'O-4 (B' = Ga) tetrahedra. The increase of V-Mn from + 3.09 t o + 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 th e MnO6 octahedra. (C) 2001 Academic Press.