Raman and 29Si NMR spectroscopic characterization of lanthanum silicate electrolytes: Emphasis on sintering temperature to enhance the oxide-ion conductivity

Enhancement of oxide-ion conductivity has been investigated with emphasis on the high sintering temperature of apatite-type structure lanthanum silicate (La10Si6O27) as a potential electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The influence of the sintering temperatures 1500, 1550, 1600 and 1650 °C as a function of ionic conductivity of the La10Si6O27 electrolyte synthesized via a diethylamine (DEA) precipitation process has been characterized using impedance spectroscopy. The ionic conductivity of the La10Si6O27 electrolyte sintered at 1650 °C revealed a higher value (1.22 × 10−2 S cm−1 at 700 °C) of one order of magnitude than the pellets sintered at lower temperatures. The sintered La10Si6O27 pellets have been characterized by 29Si NMR and Raman spectroscopy. The 29Si NMR data showed the characteristic secondary peak at ∼81.2 ppm, which confirmed the interstitial oxygen content contributing to high oxide-ion conduction. The Raman spectra revealed the appearance of a new resolved band centered at 861 cm−1 for the pellet sintered at 1650 °C compared with lower temperatures sintered pellets. The results confirmed the possibility of local structural distortion to create additional pathways for interstitial oxide-ion conduction between channels leading to higher conductivity for the pellets sintered above 1600 °C. Thus, the conduction pathway may be determined by the co-operative displacements of the SiO4 substructure units formed at elevated sintering temperatures for high oxide-ion conductivity.