Exotic Doping Schemes in the Tungsten Bronze Family

Abstract

The tungsten bronze family is an incredibly diverse system of electronically tunable perovskite-derived materials. One of the many interesting properties observed in this system is type-II superconductivity. The tunability of tungsten bronzes makes them an ideal playground for studying type-II superconductivity and moving toward enhancement of the superconducting transition temperature, T({c}). This thesis seeks to build up a predictive understanding of type-II superconductivity by manipulating the basic tungsten bronzes under the complex doping schemes of poor metal intercalation and oxygen substitution for fluorine. In the course of these studies, I report T({c}) enhancement in the hexagonal tungsten bronze system In({x})WO({3}) and probable new superconducting materials in the poor metal intergrowth tungsten bronzes. On the other hand, the fluorine-doped perovskite phase NbO({2-x})2F({1-x}) is found to not be a metallic conductor at all, despite simple band structure considerations. In the course of studying why NbO({2-x})2F({1-x}) does not electrically conduct like other bronze phases do, numerous physical phenomena are uncovered in this system: a structural phase transition, a pronounced negative coefficient of thermal expansion in the range 10 K - 295 K, 3D variable range hopping electrical conduction, and probable spin-glass ordering. While alkali metal dopants are generally thought of as simple electron donors, poor metals and fluorine atoms are both found to participate in the band structures of bronzes at the Fermi level, leading to dramatic and complex dependence of physical properties on doping level.