Structural phase transitions and superconductivity in hexagonal tungsten bronzes

The anomalous behaviour of the superconducting transition temperature Tc as a function of x in MxWO3 (hexagonal tungsten bronzes), where M = K, Rb or Cs and 0·16 < x < 0.33, is explained in terms of structural transitions involving WO6 octahedral chains which are subject to Jahn-Teller (J-T) effect. Above a critical concentration (xc ~ 0.24) there is cooperative J-T effect resulting in a structural transition. Below this, there will exist only local distortions. The effect of these is to modify the density of states and lead to shifting and splitting of the relevant peaks. It is shown that the density of states profile at the Fermi level controls the superconducting transition temperatures as a function of x. The conduction electrons in these systems are J-T polarons subject to competition between symmetry-breaking distortion and the band delocalizing effects