Crystal structure of an open-tunnel oxide α-MnO2 analyzed by Rietveld refinements and MEM-based pattern fitting

The crystal structure of an open-tunnel oxide, alpha-MnO2, free from any large stabilizing cations was analyzed by Rietveld refinement and whole-pattern fitting based on the maximum-entropy method (MEM). Rietveld refinement from neutron powder diffraction data for a partially deuterated specimen of MnO2 . 0.1 (D0.34H0.66)(2)O showed it to have a hollandite-type structure (tetragonal; space group I4/m: a = 9.777(2) and c = 2.8548(5) Angstrom; Z = 8; R-wp = 4.56%, R-P = 3.67% R-B = 1.52%, and R-F = 0.77%; S = 1.23). The bond valence sum of Mn was calculated at +4.04. The quadratic elongation and bond angle variance for the MnO6 octahedron proved that its distortion is relatively small even if small H2O molecules are contained in tunnels instead of large stabilizing cations. Electron-density distribution (EDD) in MnO2 . 0.15H(2)O was visualized by MEM-based pattern fitting from both synchrotron and conventional X-ray powder diffraction data. The resulting EDD images showed that the inner effective diameters of a cage in alpha-MnO2 are about 2.6 Angstrom for a bottleneck on the (002) plane and about 4.8 Angstrom for an inner space on the (001) plane. Thus, H2O molecules (2.2 Angstrom) can be trapped in the narrow tunnels of alpha-MnO2 whereas N-2 molecules (4.3 Angstrom) cannot penetrate the tunnel cavity. Elongation of electron densities for tunnel water along the tunnel direction was observed in the EDD images. Further, to obtain a reasonable isotropic atomic displacement parameter for the O-3 site in the tunnel cavity, O-3 had to be split into two pieces at the 4e site in the Rietveld refinement from the neutron diffraction data. These findings provide evidence that H2O molecules are not only vibrating markedly but also highly disordered, particularly along the [001] direction, near the center of the cage. (C) 2003 Elsevier Inc. All rights reserved.