The apatite-type phases, La(9.33+x)(Si/Ge)(6)O(26+3x/2), have recently been attracting considerable interest as potential electrolytes for solid oxide fuel cells. In this paper we report results from a range of doping studies in the Si based systems, aimed at determining the key features required for the optimisation of the conductivities. Systems examined have included alkaline earth doping on the rare earth site, and P, B, Ga, V doping on the Si site. By suitable doping strategies, factors such as the level of cation vacancies and oxygen excess have been investigated. The results show that the oxide ion conductivities of these apatite systems are maximised by the incorporation of either oxygen excess or cation vacancies, with the former producing the best oxide ion conductors. In terms of samples containing cation vacancies, conductivities are enhanced by doping lower valent ions, Ga, B, on the Si site. The presence of higher valent ions on these sites, e.g. P, appears to inhibit the incorporation of excess oxygen within the channels, and so limits the maximum conductivity that can be obtained. Overall the results suggest that the tetrahedral sites play a key role in the conduction properties of these materials, supporting recent modelling studies, which have suggested that these tetrahedra aid in the motion of the oxide ions down the conduction channels by co-operative displacements.

Nanoporous ruthenium (np-Ru) comprising reaction active facets is synthesized from a ruthenium–cerium (Ru2Ce) alloy precursor.

Although ionic conductors have been thoroughly investigated, topological features of these materials' nanotextures have been surprisingly overlooked. Here, we report fabrication of a metal-oxide nanocomposite consisting of intertwined phases of platinum (Pt) metal and oxygen-ion conductive cerium oxide (CeO2), i.e., Pt#CeO2. Sectional TEM observations coupled with topological analysis demonstrated that Pt#CeO2 composites having different nanostructures can be classified with a topological measure that corresponds to the phase connectivity of CeO2, namely, the Betti number β0, and another that corresponds to holes of the Pt phase, namely, the Betti number β1. The samples' oxygen ionic conductivity Pt#CeO2 was measured at elevated temperatures in air by alternating current impedance spectroscopy. It was found that the nanostructure changed from a striped appearance to a maze-like appearance as the value of β1/β0 decreased. Both the activation energy E and the pre-exponential factor σ0 for the oxygen ionic conductivity were found to be independent of β1 and exhibited linear, negative correlations with β0. The topological connectivity of the ion-conductive CeO2 phase, which was quantified with the Betti number β0, was suitable as a descriptor to correlate the image data of nanostructures with their ionic transport properties.