Coupled Cation and Charge Ordering in the CaMn₃O₆ Tunnel Structure

Abstract

The synthesis and crystal structure of a mixed valent manganite CaMn 3 O 6, equivalent to Ca 2/3 Mn 2 O 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 P 2 1 / a, R I = 0.037, R P = 0.118). The structure is based on a framework of double chains of edge-sharing MnO 6 octahedra. The corner-sharing chains form a framework with six-sided tunnels, identical to that of the CaFe 2 O 4 structure. The Ca 2+ cations are located in the tunnels. Compared to CaFe 2 O 4, 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 2 O 4 subcell, which results in a symmetry decrease from orthorhombic to monoclinic. Based on the interatomic Mn−O distances, the charge ordering in CaMn 3 O 6 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.