Structural investigation of the oxide-ion electrolyte with SrMO3 (M = Si/Ge) structure

A neutron-diffraction study of the oxide-ion solid electrolytes Sr1-xNaxSiO3-0.5x (x = 0.2 and 0.4) and Sr0.8K0.2Ge1-ySiyO2.9 (y = 0.0 and 0.5) reveals that there are no interstitial oxygen atoms in the structures; the oxygen vacancies are more concentrated in the planar oxygen sites (O3 and O5) of corner-sharing tetrahedral units of the M3O9 (M = Si/Ge) complexes. From thermogravimetric analysis (TGA), the K-substituted samples lose weight above 100 degrees C and are hygroscopic at room temperature; the oxygen vacancies are concentrated at the in-plane O3 and O5 sites and those in the terminal O2 or O4 are responsible for a 2D oxide-ion conductivity between the M3O9 complexes. The Na-substituted samples lose little weight by 800 degrees C and are not hygroscopic; the oxygen vacancies are located at all oxygen atom positions, being more pronounced at the in-plane O3 and O5; those in the terminal oxygen sites give an excellent oxide-ion conductivity. Moreover the high temperature NPD data for Sr0.6Na0.4SiO2.8 disclose that the vacancies become more randomly dispersed above 400 degrees C to give a smaller activation energy above 550 degrees C for vacancy transfer between Si3O9-0.5x complexes.