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

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)₆O_2.5 (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)_xO_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)₆O₂ and La_9.67(SiO₄)₆O_2.5 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⁻ + O_int²⁻. 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