Oxygen-17 nuclear magnetic resonance measurements on apatite-type lanthanum silicate (La_9.33(SiO₄)₆O₂)

We used magic angle spinning (MAS), multiquantum (MQ)-MAS, and high-temperature (HT) nuclear magnetic resonance (NMR) measurements to investigate the oxide-ion conduction path of oxygen-17 (O-17) in apatite-type lanthanum silicate La_9.33(SiO₄)₆O₂. A highly crystalline apatite-type lanthanum silicate was specifically synthesized for the measurements. MAS and MQ-MAS NMR confirmed that four kinds of oxide-ion sites are present in the structure and showed a small extra peak (< 1%) possibly due to an interstitial site. The high-temperature measurements showed that the line shape changed and sharpened with increasing temperature from 200 °C, and the peak position shifted at 700 °C. The comparison of the results between MAS NMR (room-temperature) and HT static NMR showed the exchange of oxide ions bonded to Si (O1, O2, and O3) but the apparent exchange between the oxide ions (O1, O2, and O3) and the oxide ion at the isolated site (O4) is not observed. The exchange of the oxide ions that are bonded to Si (O1, O2, and O3) suggests that they are the main diffusion species in oxide-ion conductivity.