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

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 (29)si 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)(6)O-2.5 (M = Ca, Sr, Ba) two peaks are observed at chemical shifts of approximate to - 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),O-2.5 (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 La-9.33(SiO4)(6)O-2 and La-9.67(SiO4)(6)O-2.5 show a further third weak peak at a chemical shift (approximate to -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-) + O-int(2-). Overall, the results further highlight the importance of the [SiO4](4-) substructure in these materials, and additionally suggest that Si-29 NMR could potentially be used to screen apatite silicate materials for oxide ion conductivity.