Structural phase transitions and superconducting transition temperatures of hexagonal MxWO3compounds

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Abstract
Neutron scattering experiments have been performed on non-stoichiometric compounds of the hexagonal tungsten bronzes MxWO3 (M=Rb and K). A nearly dispersionless phonon branch has been observed in Rb0.33WO3 at an energy omega M approximately=5.5 meV. This branch reflects the local-mode character of the Rb atoms, which are loosely trapped at their sites within the large channel along the c direction formed by the linkage of WO6 octahedra. The x dependence of the energy omega M of this low-lying phonon (LLP) has been determined by measurements of the peak position in the powder inelastic scattering spectra. Since omega M depends only weakly on x, the variation in omega M appears not to be related to the anomalous x dependences of the superconducting transition temperature Tc observed previously in RbxWO3 and KxWO3. Two kinds of structural phase transitions were observed using powder diffraction measurements. One transition, which takes place above room temperature at Tc1, is due to a distortion of the WO3 host cage of MxWO3. It is accompanied by an optical phonon softening at the Gamma point. The second transition is an order-disorder transition of the M atoms. It produces for a certain value of x a long-wavelength incommensurate structure as the low-temperature phase. This is in contrast with the other incommensurate phases such as the CDW states of two-dimensional conductors, where the incommensurate structures generally appear at high temperatures. A local structural excitation model is adopted in the discussion of the superconductivity, and the x dependences of Tc in CsxWo3 and RbxWO3 are consistently explained in relation to the ordering schemes of the M atoms.