An apatite for conduction
Solid oxide fuel cells are energy devices with potential use for combined heat and power applications in homes. However, they are often hampered by their high operating temperatures (about 1,000ºC) owing to their reliance on a zirconia-based oxide-ion conductor as the solid electrolyte.
A collaborative group of scientists at Bath, Birmingham and Warwick universities are looking into new solid electrolytes that work at temperatures of about 500-700ºC. This has spurred growing interest in new apatite-type silicates and germanates with the general formula La10-x(Si/Ge)6O26+y, which are said to show excellent oxide-ion conductivity at these temperatures.
An unusual feature of these electrolytes is that, rather than oxide-ion vacancies being the conducting defects, as for conventional electrolytes, the conduction is mediated by oxide ions located at interstitial sites, which may provide less structural constraints to oxideion conductivity at lower temperatures. But a complete understanding of their local structural and conduction properties on the atomic scale has been lacking, since such defects are difficult to locate precisely.
The team performed an in-depth study of the location of the interstitial oxide-ion defects in La8Y2Ge6O27. They used a combination of 17O magic-angle-spinning nuclear magnetic resonance (MAS-NMR) and advanced computer simulations. The 17O solid-state NMR data is the first of its kind to be acquired on these apatites.
Room-temperature cell parameters and atomic positions were obtained for La8Y2Ge6O27 from the Rietveld refinement of diffraction data to develop an atomistic potential model and provide the starting point for the computer simulations. The location of the interstitial oxideion site was first analysed using density functional theory calculations, suggesting that on introduction of interstitial oxide ions the resulting species is a five-coordinate germanium (GeO5) unit – in line with other studies. To provide experimental confirmation for the interstitial site, the team collected 17O NMR data for 17O-enriched La8Y2Ge6O27.
The team analysed the 17O MAS NMR assignments using DFT calculations to predict the 17O chemical shifts expected for the La8Y2Ge6O27. These corroborated the position of the channel and bulk GeO4 framework species. More importantly, the calculations were sensitive enough to predict the small shift separation between the GeO5 units (interstitial induced) and framework GeO4 tetrahedra.
La8Y2Ge6O27 apatite framework showing GeO4 tetrahedra, La/Y ions and two oxide-ion interstitials. b,c) Local structure and Ge-O distances of GeO4 unit and an O5 interstitial forming a GeO5 unit from DFT calculations: La (grey), Y (cyan), O1-3 tetrahedral (red), O4 channel (green) and O5 interstitial (dark blue)
The modelling was then extended to molecular dynamics simulations to probe the mechanism of oxide-ion diffusion at the atomic scale. The results suggest a range of conduction pathways, including those perpendicular to the channels, which allow interstitial oxide ions to pass between channels. Also, the team says, the simulations indicate that numerous oxide ions are moving between lattice and interstitial sites in a cooperative manner.
Professor Saiful Islam, from the University of Bath, says, ‘These fascinating insights raise the possibility of similar conduction mechanisms in other apatite materials and related structures with tetrahedral networks, offering the prospect of narrowing the search for new families of interstitial oxideion conductors for solid oxide fuel cells.
‘But more research is needed. Challenges include developing low-temperature synthesis routes and optimising compatible electrodes. There are still other avenues to explore to further improve the properties of these apatites, particularly by chemical doping routes.’