SrFeO(2.5) and SrCoO(2.5) are able to intercalate oxygen in a reversible topotactic redox reaction already at room temperature to form the cubic perovskites Sr(Fe,Co)O(3), while CaFeO(2.5) can only be oxidized under extreme conditions. To explain this significant difference in low temperature oxygen mobility, we investigated the homologous SrFeO(2.5) and CaFeO(2.5) by temperature dependent oxygen isotope exchange as well as by inelastic neutron scattering (INS) studies, combined with ab initio (DFT) molecular dynamical calculations. From (18)O/(16)O isotope exchange experiments we proved free oxygen mobility to be realized in SrFeO(x) already below 600 K. We have also evidence that low temperature oxygen mobility relies on the existence of specific, low energy lattice modes, which trigger and amplify oxygen mobility in solids. We interpret the INS data together with the DFT-based molecular dynamical simulation results on SrFeO(2.5) and CaFeO(2.5) in terms of an enhanced, phonon-assisted, low temperature oxygen diffusion for SrFeO(3-x) as a result of the strongly reduced Fe-O-Fe bond strength of the apical oxygen atoms in the FeO(6) octahedra along the stacking axis. This dynamically triggered phenomenon leads to an easy migration of the oxide ions into the open vacancy channels and vice versa. The decisive impact of lattice dynamics, giving rise to structural instabilities in oxygen deficient perovskites, especially with brownmillerite-type structure, is demonstrated, opening new concepts for the design and tailoring of low temperature oxygen ion conductors.

We have measured the magnetic susceptibility of the Rb x WO 3 compound ͑0.20ഛ x ഛ 0.33͒ and examined its structural properties and lattice dynamics, using elastic and inelastic neutron scattering ͑INS͒ experiments, in order to gain further insight into the unusual features of its superconducting state, namely, ͑i͒ the stabilizing effect resulting from the reduction of rubidium content, i.e., of the conduction electron density ͓what we shall name the "T c ͑x͒ paradox"͔, and ͑ii͒ the destabilizing effect of the ordering of the Rb ions. We also performed density-functional calculations of the phonon dispersion in the "stoichiometric" Rb 0.33 WO 3 and Cs 0.33 WO 3 to identify the main features of the phonon spectra. These calculations give a very satisfactory description of the INS data and confirm the assignment to these bronzes of a lower ͑orthorhombic͒ symmetry than previously proposed. Our results contradict the previous interpretations of the T c ͑x͒ paradox and of the ordering effect: ͑i͒ no general softening of the lattice accompanies the increase of the Rb-vacancy population and ͑ii͒ no general decrease of the electron density of states D EF distinguishes the ordered nonsuperconducting Rb 0.25 WO 3 from its neighboring disordered parents. It appears, therefore, that the electron-electron coupling in this system probably proceeds through well-defined electronic states and phonons. This is a feature these "hexagonal" tungsten bronzes ͑HTB͒ apparently share with several high-T c materials. We discuss what could be the mechanisms responsible for the very selective electron-phonon coupling in the HTB.