Dynamics of Caged Hydronium Ions and Super-protonic Conduction in (H3O)SbTeO6

Abstract

Abstract The defect pyrochlores have become the focus of attention for the development and optimization of novel solid-state proton conductors as alternatives to costly polymer-based materials. A promising candidate is (H3O)SbTeO6, a super-proton conductor characterized by an outstanding conductivity of 10–1 S cm–1 at ambient conditions. The crystal structure of this compound has been known for some time yet, to date, the physical mechanism underpinning proton conduction has remained elusive. Structurally, (H3O)SbTeO6 consists of corner-shared (Sb/Te)O6 octahedra forming a cubic framework of three-dimensional interconnected cages partially occupied with hydronium H3O+ ions. These ions are connected to the inorganic framework by weak hydrogen bonds. In this work, we present the first characterization of this compound by means of quasielastic and inelastic neutron spectroscopy over a wide temperature range (T = 1.5–500 K), as well as by heat capacity measurements. Our results show that the onset of low-frequency stochastic motions of the hydronium moiety takes place around room temperature and persists up to the highest temperatures investigated. Unlike other inorganic proton conductors, these motions are effectively decoupled from vibrational motions at higher frequencies, as evidenced by the temperature dependence of the vibrational density of states. These findings have important implications for the exploitation of the defect pyrochlores as proton-conducting media for fuel-cell and gas-sensing applications.