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

Abstract

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 degrees 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 degrees C revealed a higher value(1.22 x 10(-2) S cm(-1) at 700 degrees C) of one order of magnitude than the pellets sintered at lower temperatures. The sintered La10Si6O27 pellets have been characterized by Si-29 NMR and Raman spectroscopy. The Si-29 NMR data showed the characteristic secondary peak at similar to 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 degrees 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 degrees 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. (C) 2009 Elsevier Ltd. All rights reserved.