Effect of oxygen content on the 29Si NMR, Raman spectra and oxide ion conductivity of the apatite series, La8+xSr2−x(SiO4)6O2+x/2

Abstract

²⁹Si NMR data have been recorded for the apatite series La₈₊xSr_2−x(SiO₄)₆O_2+x/2 (0 ≤ x ≤ 1.0). For x = 0, a single NMR peak is observed at a chemical shift of ≈−77 ppm, while as the La : Sr ratio and hence interstitial oxygen content is increased, a second peak at a chemical shift of ≈−80 ppm is observed, which has been attributed to silicate groups neighbouring interstitial oxide ions. An increase in the intensity of this second peak is observed with increasing x, consistent with an increase in interstitial oxide ion content, and the data are used to estimate the level of interstitial oxide ions, and hence Frenkel-type disorder in these materials. The increase in second ²⁹Si NMR peak intensity/interstitial oxide ion content is also shown to correlate with an increase in conductivity. The effect of interstitial oxygen content can also be studied by means of Raman spectroscopy, with a new mode at 360 cm⁻¹ appearing for samples with x > 0 in the symmetric bending mode energy region of the SiO₄group. The intensity of this mode increases with increasing oxygen content, yielding results comparable to those from the NMR studies, showing the complementarities of the two techniques.