Impedance Spectroscopy Analysis of Field-Assisted Sintered Sr- and Mg-Doped Lanthanum Gallate

In this study, the relationship between the microstructure and electrical conductivity of doped lanthanum gallate was investigated to identify the origin of the relatively high resistivity of the grain boundaries in this ceramic solid electrolyte. LaGaO3 containing acceptor dopants, Sr and Mg, was chemically synthesized and consolidated by field-assisted sintering technology. The relative density achieved 98% upon sintering at 1200 °C, and no intragrain porosity was found. The microstructure consisted of submicron-sized grains and exhibited a predominantly transgranular fracture mode. Structural characterization evidenced that all sintered samples display the characteristic orthorhombic crystal structure. Rietveld analysis revealed a secondary phase content of only ~0.71% in samples sintered at 1200 °C. In addition, Raman spectra revealed only the allowed characteristic vibrational modes expected for doped lanthanum gallate. The electrical conductivity was determined by impedance spectroscopy analysis. The bulk conductivity of sintered samples was found to be independent of the sintering temperature. Analysis of the grain boundary resistivity revealed a dependence on the mean grain size, which constricts the pathway of the charge carriers, leading to the formation of space charge layers. The total activation energy determined for conduction is 0.94 eV.