Synthesis and Crystal Structure of Novel Layered Manganese Oxide Ca2MnGaO5+δ

Abstract

New layered oxides Ca2MnGaO5+δ and Ca2MnGa1−xZnxO5+δ (x=0.1, 0.2) were synthesized by solid-state reaction at 1000–1300°C. The crystal structure of Ca2MnGaO5.045 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 brownmillerite-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 (VMn) in Ca2MnGaO5+δ can be varied either by heterovalent replacement of Ga3+ for Zn2+ or by oxygen insertion after treatment under 80 atm O2. In the latter case the compound with the composition Ca2MnGaO5.39 forms built of domains of initial reduced material and fully oxidized Ca2MnGaO5.5 phase. The Ca2MnGaO5.5 structure was determined by HREM as belonging to the brownmillerite AnBn−1B′O3n−1 (n=4) homologous series, in which three layers of BO6 (B=Mn, Ga) octahedra form blocks separated by a single layer of B′O4 (B′=Ga) tetrahedra. The increase of VMn from +3.09 to +3.78 is accompanied by a drastic compression of the structure along the b=4aper axis due to a suppression of the Jahn-Teller deformation of the MnO6 octahedra.