Abstract :
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 II (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) ?, b=3.7944(1) ?, c=7.7248(3) ?, BETA=93.743(2)° for x~0.9). Both the doubling of the host cell along the c-axis in phase II and the 6×c superstructure found for phase I with x~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) ?, b=3.8520(4) ?, c=3.7691(4) ?, BETA= 90.73(1)°) indicates a redistribution of the protons during this oxidation step.
