Journal of Solid State Chemistry 149, 75 }87 (2000) Article ID jssc.1999.8498, available online at http://www.idealibrary.com on CDW Superstructures in Hydrogen Molybdenum Bronzes H x MoO 3 Stefan Adams Mineralogisch-Kristallographisches Institut, Universita ( t Go ( ttingen, D37077 Go ( ttingen, Germany Received April 21, 1999; in revised form August 27, 1999; accepted September 7, 1999 Hydrogen molybdenum bronzes H x MoO 3 (0 < x < 2) are con- sistently 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 redistribu- tion of hydrogen during the intercalation process between two types of potential proton sites is simulated in a molecular mech- anics study. Therefrom a structure model for the bronze phase II (0.85 < x < 1.04) is derived, which permits a Rietveld re5nement of its previously unknown structure from powder X-ray data (space group I12/m1; a  14.5191(6) A  , b  3.7944(1) A  , c  7.7248(3) A  ,   93.743(2)3 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 expecta- tions for quasi-one-dimensional Peierls distorted systems. Modi- 5cations 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 H 0.6 MoO 3 . The powder structure determination of this metastable phase (space group C2/m, a  14.543(2) A  , b  3.8520(4) A  , c  3.7691(4) A  ,   90.73(1)3) indicates a redistribution of the protons during this oxidation step.  2000 Academic Press Key Words: molybdenum bronze; charge density waves; Peierls transition; hydrogen intercalate. 1. INTRODUCTION The intercalation of hydrogen into the layered structure of orthorhombic MoO  produces "ve hydrogen molyb- denum bronze phases H  MoO  (0(x(2) (1}5). Technical applications of these low-dimensional mixed conductors as hydrogen transfer catalysts, electrodes in fuel cells, elec- trochromic devices, sensors, etc., require a detailed under- standing of how their transport properties depend on host structure modi"cations, hydrogen distribution, and order- ing among the two potential H intercalation sites: chan- nels within the MoO  layers or the van der Waals gaps (vdW gaps) between the layers (cf. Fig. 1). Besides recently reported spin glass states (6) and studies on charge density wave (CDW) states (7), which are the main topic of this work, a continued scienti"c interest in H  MoO  phases is raised by the possibility of controlling the dimensionality of the transport processes via the hydrogen content and hence via the applied electric potential. E!ects of the reduced dimensionality become the more important in low-dimen- sional samples such as the molybdenum bronze thin "lms used in electrochromic devices (8, 9). So far the complex interplay between CDW structure modulations and re- arrangements of the mobile protons has limited the under- standing of the hydrogen molybdenum bronzes when compared to the structurally related Magne H li phases Mo  O  and alkali molybdenum bronzes (cf. (10, 11)). 2. EXPERIMENTAL Powder samples of phase III were prepared by chemical reaction with &&nascent'' hydrogen following Glemser and coworkers (1, 2) and by electrochemical intercalation of hy- drogen (12}14). Phase II powders were produced by the equilibration of phase III with the appropriate amount of MoO  following Birtill and Dickens (3). Thermochemical investigations have shown that this comproportionation is energetically favorable (15). Single crystals of MoO  were grown by repeated sublima- tion of reagent grade MoO  powder. Homogeneous single crystals of phase I of up to 1.70.80.1 mm were produced in the course of about 1 year by the modi"ed spillover process described in (16). The reduction of phase I in the solid state electrochemical cell described in (16, 17) yielded crystals of higher hydrogen contents. With the help of these crystals it was possible to reveal the exist- ence of a superstructures in phase III (7), but because of the minor quality of these crystals no single crystal structure determination has been attempted for the monoclinic phases. The hydrogen content of the H  MoO  powders was con- trolled by redox titrations (14, 18). The change in the hydro- gen content of the molybdenum bronzes is re#ected by variations in their lattice constants. Thereby approximate compositions for bronze phases in multiphase samples and for the single crystals were estimated from a determination 75 0022-4596/00 $35.00 Copyright  2000 by Academic Press All rights of reproduction in any form reserved.