Electronic Band Structure Study of the Anomalous Electrical and Superconducting Properties of Hexagonal Alkali Tungsten Bronzes A x WO 3 (A = K, Rb, Cs)

The electrical and superconducting properties of hexagonal alkali tungsten bronzes A x WO 3 (A = K, Rb, Cs) were examined by calculating the electronic band structure of a representative hexagonal tungsten bronze and analyzing reported crystal structures of A x WO 3 (A = K, Rb, Cs). These bronzes possess one-dimensional (1D) and three-dimensional Fermi surfaces. The metal-to-semiconductor-to-metal transitions and superlattice reflections in K x WO 3 and Rb x WO 3 are explained by a charge density wave (CDW) associated with the 1D Fermi surface. There occurs a maximum in the plots of the CDW onset temperature T B versus x for K x WO 3 and Rb x WO 3 . The presence of this maximum and the absence of a CDW in Cs x WO 3 reflect the balance of two opposing energy factors, the electronic instability and lattice stiffness, in forming a CDW. The dependence of the superconducting transition temperature T C on x suggests that a CDW transition removes lattice phonons conducive for superconductivity.