Neutron and X-ray powder diffraction studies of LiMn2-yCryO4

The crystal structures of chromium-doped spinel-type lithium manganese oxid es, LiMn2-yCryO4 (y = 0, 1/9, 1/6, and 1/3), have been studied by neutron a nd X-ray powder diffraction. Rietveld refinements of their neutron diffract ion data revealed that Cr is substituted for Mn selectively and that Li occ upies only the 8a site. Thus, the above solid solutions can be expressed as Li(Mn2-yCry)O-4. The fractional coordinates of oxygen were almost the same in all the samples while the lattice parameter a decreased linearly with i ncreasing Cr content. The lengths of (Mn, Cr)-O bonds in (Mn, Cr)O-6 octahe dra changed in a manner expected from average ionic radii for Mn2-yCry. Rie tveld analyses of the X-ray diffraction data showed the crystallite size an d anisotropic strain to decrease with increasing Cr content. (C) 1999 Acade mic Press.