Calcium insertion in the Na4Mn9O18 tunnel structure: Na1.1Ca1.8Mn9O18

A new mixed valent manganese oxide, Na1.1Ca1.8Mn9O18, with a tunnel structu re closely related to that of Na4Mn9O18 has been synthesized. It crystalliz es in the Pnma space group with a 26.418(2) Angstrom, b = 5.6468(4) Angstro m, and c = 9.1384(8) Angstrom. Its structural study, combining X-ray powder diffraction and electron microscopy, shows that the [Mn9O18](infinity) fra mework is similar to that of Na4Mn9O18, forming large S-shaped and six-side d tunnels running along b. In contrast, the positions and distributions of calcium in the tunnels are different from those obtained for sodium in Na4M n9O18, explaining the doubling of the periodicity along the tunnel axis dir ection. The insulating properties of these oxides agree with a tendency to ordering of the Mn3+ and Mn4+ species indifferent sites. (C) 2001 Academic Press.