The intrinsic origin of the grain-boundary resistance in Sr-doped LaGaO3

In this paper I summarize our recent investigations (Park and Kim, Phys Chem C 111: 14903, 2007; Solid State Ionics 179:1329, 2008) on the origin of the grain-boundary resistance in a doped LaGaO3, a perovskite-structured solid electrolyte. The partial electronic and ionic resistances of the bulk and the grain boundaries, as well as the total resistance, in 1 mol% Sr-doped LaGaO3 were measured separately by means of a dc-polarization method and ac-impedance spectroscopy. Both of the partial resistances at the grain boundaries were greater than the bulk counterparts, indicating that the grain boundaries impede the ionic as well as the electronic transport in this material. The transference number of the partial electronic conductivity at the grain boundary was however greater than that in the bulk. This fact strongly suggests that both electronic and ionic charge carriers deplete at the grain boundaries to form the space-charge zones and that the grain-boundary cores in this material are positively charged. In light of the fact that the effective charge of the oxygen vacancy (+2) is greater than that of the electron hole (+1), the oxygen vacancies deplete more sharply in the space-charge zones compared to the electron holes such that the grain boundaries become more mixed conducting relative to the bulk. These observations verify that the electrical conduction across the grain-boundaries in 1 mol% Sr-doped LaGaO3 is governed by the space charge.