Calcium Insertion in the Na4Mn9O18 Tunnel Structure: Na1.1Ca1.8Mn9O18

A new mixed valent manganese oxide, Na1.1Ca1.8Mn9O18, with a tunnel structure closely related to that of Na4Mn9O18 has been synthesized. It crystallizes in the Pnma space group with a=26.418(2) ?, b=5.6468(4) ?, and c=9.1384(8) ?. Its structural study, combining X-ray powder diffraction and electron microscopy, shows that the [Mn9O18]infinity framework is similar to that of Na4Mn9O18, forming large S-shaped and six-sided tunnels running along b. In contrast, the positions and distributions of calcium in the tunnels are different from those obtained for sodium in Na4Mn9O18, explaining the doubling of the periodicity along the tunnel axis direction. The insulating properties of these oxides agree with a tendency to ordering of the Mn3 + and Mn4+ species in different sites.