1. Could you explain the significance of your article to the non-specialist? 

Recently apatite-type rare earth silicates have been identified as a new class of oxide ion conductor, with potential applications as electrolytes for Solid Oxide Fuel Cells. In contrast to traditional oxide ion conductors, such as fluorite-type materials, the oxide ion conduction mechanism in the apatite systems has been reported to be mediated by oxide ion interstitials rather than vacancies. In addition, there has been considerable debate over the role played by the silicate network in the conduction process. The work presented here provides an experimental link between the observed conductivity and the silicate network. It also identifies a new mechanism for the creation of interstitial oxide ions.

 

2. What has motivated you to conduct this work? 

There has been considerable debate over the mechanism of oxide ion conduction in apatite rare earth silicates. Our previous computational studies suggested that oxide ion conduction proceeded via interstitial oxide ion sites, and was aided by the cooperative displacements of the silicate substructure. If this was indeed the case, then we would expect to see differences in the ²⁹Si NMR spectra between samples that are good conductors and those that show poor conduction. This has indeed been shown to be the case by this work.

 

3. Where do you see this work developing in the future? 

The work presented here provides experimental support for the cooperative conduction mechanism proposed by computational studies. It raises the possibility of similar conduction mechanisms in other silicate and related systems, thus offering the prospect of new series of oxide ion conductors. Additionally, the work suggests that ²⁹Si NMR could potentially be used to screen apatite silicate materials for oxide ion conductivity. In addition to their applications as oxide ion conducting electrolyte, apatite-type materials are also widely researched in a number of other areas, such as biomaterials, where studies of hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂ are widespread. The work here therefore will potentially aid these areas, by providing growing understanding of the complex nature of the apatite structure.

 

4. Are there any particular challenges facing future research in this area? 

For fuel cell applications, there is a need to lower the sintering temperatures to obtain dense membranes of these apatite systems. The preparation of orientated films of these materials is another key challenge. By achieving this aim, further enhancements in the conductivity will result.