Coupled cation and charge ordering in the CaMn₃O₆ tunnel structure

The synthesis and crystal structure of a mixed valent manganite CaMn ₃O₆, equivalent to Ca_2/3Mn₂O ₄, are reported, along with the magnetic properties. The structure was determined using electron diffraction and high-resolution transmission electron microscopy and refined from X-ray and neutron powder diffraction data (a = 10.6940(3) Å, b = 11.3258(3) Å, c = 8.4881(2) Å, β= 122.358(2)°, space group P2₁/a, R_I 0.037, R _P = 0.118). The structure is based on a framework of double chains of edge-sharing MnO₆ octahedra. The corner-sharing chains form a framework with six-sided tunnels, identical to that of the CaFe ₂O₄ structure. The Ca²⁺ cations are located in the tunnels. Compared to CaFe₂O₄, one-third of the Ca positions in the tunnels remain vacant, with an ordered distribution of vacant and occupied sites. The empty sites in neighboring Ca chains are shifted relative to each other along the c-axis by one period of the CaFe ₂O₄ subcell, which results in a symmetry decrease from orthorhombic to monoclinic. Based on the interatomic Mn-O distances, the charge ordering in CaMn₃O₆ is discussed. The compound exhibits a strong anti-ferromagnetic character, and differences between the zfc and fc magnetization curves at low temperature suggest ferro- or ferrimagnetic interactions.