CDW superstructures in hydrogen molybdenum bronzes HxMoO3

Hydrogen molybdenum bronzes HxMoO3 (0 < x < 2) are consistently described as low-dimensional mixed conductors, whose properties under ambient conditions are controlled by charge density wave modulations. Proton conduction pathways in the bronzes are modeled by a bond valence approach. The redistribution of hydrogen during the intercalation process between two types of potential proton sites is simulated in a molecular mechanics study. Therefrom a structure model for the bronze phase TI (0.85 < x < 1.04) is derived, which permits a Rietveld refinement of its previously unknown structure from powder X-ray data (space group I12/m1; a = 14.5191(6) Angstrom, b = 3.7944(1) Angstrom, 7.7248(3) Angstrom, beta = 93.743(2)degrees for x approximate to 0,9), Both the doubling of the host cell along the c-axis in phase II and the 6 x c superstructure found for phase I with x approximate to 1/3 meet the expectations for quasi-one-dimensional Peierls distorted systems, Modifications in the structure, proton ordering, and properties of the bronzes are studied as a function of temperature. A time-resolved powder XRD investigation on the oxidation of phase II indicates the existence of a intermediate phase H0.6MoO3. The powder structure determination of this metastable phase (space group C2/m, a = 14.543(2) Angstrom, b = 3.8520(4) Angstrom, c = 3.7691(4) Angstrom, beta = 90.73(1)degrees) indicates a redistribution of the protons during this oxidation step. (C) 2000 Academic Press.