Electronic band structure study of the anomalous electrical and superconducting properties of hexagonal alkali tungsten bronzes A(x)WO₃ (A = K, Rb, Cs)

The electrical and superconducting properties of hexagonal alkali tungsten bronzes A(x)WO₃ (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)WO₃ (A = K, Rb, Cs). These bronzes possess one-dimensional (ID) and three-dimensional Fermi surfaces. The metal-to-semiconductor-to-metal transitions and superlattice reflections in K(x)WO₃ and Rb(x)WO₃ 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)WO₃ and Rb(x)WO₃. The presence of this maximum and the absence of a CDW in Cs(x)WO₃ 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.