No abstract

Composite electrochemical cells incorporating press-contact pellets of yttria-stabilized zirconia (YSZ), electronically blocking electrodes, and YBa2Cu3OT_= (YBCO) were investigated by ac impedance spectroscopy. An additional element, net attributable to either of the individual components, was observed in the impedance spectrum of the composite cell. Although the capacitance associated with the additional element (~10 -~ F) appeared to indicate a bulk response, the capacitance was found to be invariant with dimensions of either the YBCO or YSZ components. A similar additional element was observed when YBCO was replaced by other electronically conducting materials, including gold foil. These results point clearly to an interfacial impedance whose characteristics are governed largely by the YSZ; the extremely low capacitance value arises from a very low effective contact area (10-3%) due to surface roughness. We found no features in the impedance spectrum which could be attributed to oxide ion conduction in YBCO. In the assignment of impedance spectra to the components of a heterogeneous system, it is demonstrated that the use of relaxation frequencies (f,~ = [RC] -~) and their temperature dependences provide a useful comparison, free from geometric factors, between samples with different origins or thermal histories.

No abstract

The new phase Li,Ni2Ta06 has a novel rock salt superstructure, a = 8.4259(3) A, b = 5.9073(3) A, c = 17.7329 A, Fddd. Within the cubic-close-packed oxide array, Ta occupies one set of octahedral sites giving isolated TaO, octahedra which edge-share with Li/NiO, octahedra. Li and Ni are distributed non-randomly over three other sets of octahedral sites with Li : Ni occupancy ratios of 0.73 : 0.27, 0.59 : 0.41 and 0.55 : 0.45, respectively. The higher Li : Ni ratio of the first set, 0.73:0.27 may be associated with cation-cation repulsion effects since this site is closest to Ta. Li,Ni,Ta06 is a very modest semiconductor with conductivity of ca. 4 x l o p 6 R-' cm-' at 300 "C and activation energy 0.77 eV; the disorder in the Li' site occupancy is, therefore, static and does not yield significant levels of Li+ ion conductivity.

The phase K,[SO,]F was found to be dimorphic, undergoing a reversible tetragonal (p) to cubic (R) phase transition at 585 "C. The room-temperature polymorph, P-K,[SO,]F, is isostructural with Ba3Si0,, as confirmed by Rietveld refinement of X-ray powder diffraction data: a = 7.2961(5), c = 10.854(1) A, R = 0.0689 and R,, = 0.0907. The cx form is cubic, a z 5.43 A, with probable space group Pm7m. Partial Rietveld refinement suggested that it has an anti-perovskite structure. Lattice parameter measurements as a function of temperature showed no discontinuity at the a-p transition, which is therefore suggested to be a displacive second-order transition.The compound K,[SO,]F was synthesised as part of an investigation into compounds of the type M,[TO,]X, where T is a tetrahedral cation, M a medium to large cation and X an anion not bonded to the tetrahedra. These structures were described by Eysel and Breuer in terms of close packed layers. The 'basic layer' has the hexagonal plane group p3ml and the approximate lattice parameter a z 7 A with a thickness of c z 3 A. The repeat unit of the basic layer contains one formula unit