Solid state 29Si NMR studies of apatite-type oxide ion conductors

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Abstract
Apatite-type silicates have been attracting considerable interest as a new class of oxide ion conductor, whose conduction is mediated by interstitial oxide ions. We report here the first 29Si solid state NMR studies of these materials with a systematic investigation of thirteen compositions. Our results indicate a correlation between the silicon environment and the observed conductivity. Specifically, samples which show poor conductivity demonstrate a single NMR resonance, whereas fast ion conducting compositions show more complex NMR spectra. For the oxygen excess samples La9M(SiO4)6O2.5 (M = Ca, Sr, Ba) two peaks are observed at chemical shifts of ≈−77.5 and −80.5 ppm, with the second peak correlated with a silicate group adjacent to an interstitial oxygen site. On Ti doping to give La9M(SiO4)6−x(TiO4)xO2.5 (x = 1,2) the second peak disappears, which is consistent with the “trapping” of interstitial oxygens by Ti and the consequent lowering in oxide ion conductivity. The samples La9.33(SiO4)6O2 and La9.67(SiO4)6O2.5 show a further third weak peak at a chemical shift (≈−85.0 ppm) consistent with the presence of some [Si2O7]6− units in these samples, due to condensation of two [SiO4]4− units. The effect of such condensation of [SiO4]4− units will be the creation of additional interstitial oxide ion defects, i.e. 2 [SiO4]4− → [Si2O7]6− + Oint 2−. Overall, the results further highlight the importance of the [SiO4]4− substructure in these materials, and additionally suggest that 29Si NMR could potentially be used to screen apatite silicate materials for oxide ion conductivity