Evidence for a charge-density-wave instability in the purple bronze K0·9Mo6O17

The purple molybdenum bronze K0–9Mo6O17 is a trigonal, quasi-two-dimensional metal which shows an electronic transition at 120 K. In this paper are reported specific-heat data which provide evidence that a second-order phase transition takes place at this temperature, with an entropy change of the order of 0·5 J mol−1 K−1. Both X-ray diffuse scattering and electron diffraction studies have established that this transition is accompanied by a structural transition towards a commensurate 2a × 2a × c state. All the results are consistent with a model of a Fermi-surface-driven instability. The temperature dependence of the charge-density-wave gaps, as well as the nature of the nesting at the Fermi surface, are also discussed.