Concentration dependence of superconductivity and the order-disorder transition in the hexagonal rubidium tungsten bronze RbxWO3:: Interfacial and bulk properties -: art. no. 144528

We revisited the problem of the stability of the superconducting state in RbxWO3 and identified the main causes of the contradictory data previously published. We have shown that the ordering of the Rb vacancies in the nonstoichiometric compounds have a major detrimental effect on the superconducting temperature T-c. The order-disorder transition is of first order only near x=0.25, where it cannot be quenched effectively and T-c is reduced below 1 K. We found that the high T-c's that were sometimes deduced from resistivity measurements and attributed to compounds with 0.25less than or similar toxless than or similar to0.30, are to be ascribed to interfacial superconductivity that generates spectacular nonlinear effects. We also clarified the effect of acid etching and set more precisely the low-rubidium-content boundary of the hexagonal phase. This work makes clear that T-c would increase continuously (from approximate to2 K up to approximate to5.5 K) as we approach this boundary (xapproximate to0.20), if no ordering would take place-as it is approximately the case in CsxWO3. This behavior is reminiscent of the tetragonal tungsten bronze NaxWO3 and asks the same question: what mechanism is responsible for this large increase in T-c despite the considerable associated reduction of the electron density of state D-FE? By reviewing the other available data on these bronzes we conclude that the theoretical models that are able to answer this question are probably those where the instability of the lattice plays a major role and, particularly, the model that calls upon local structural excitations, associated with the missing alkali atoms.