Processing and electrical conductivity of pure, Fe- and Cr-substituted La0.9Sr0.1GaO3

The material La0.9S0.1GaO3 and four B-site substituted variants with 5 or 20% Fe or Cr for Ga substitution were prepared by solid stare reaction starting from oxide and carbonate precursors. The materials were characterised by scanning electron microscopy (SEM), X-ray diffraction (XRD) and impedance spectroscopy (IS) in air. The sintered samples appeared to have orthorhombic symmetry, had densities higher than 90% of theoretical and had average grain sizes of approx. 1-2.5 mu m. However, second phases were detected by XRD and SEM images showed evidence of a liquid, Sr-rich second phase in all samples. The high grain boundary impedances measured were attributed to concentration of this second phase at the grain boundaries. Generally, conductivity increased and activation energy of conduction decreased with increasing degree of substitution. Increasing p-type electronic conductivity involving CP3+/Cr4+ or Fe3+/Fe4+ couples is suggested. Raising the sintering temperature of these materials was shown to increase both their average grain size and their total conductivity. (C) 1997 Elsevier Science Limited.</p

Ionic and electronic conduction in Fe and Cr doped (La,Sr)GaO3-delta

The ionic and electronic transport properties of La0.9Sr0.1Ga1-xMexO3-delta (with Me = Fe or Cr and x = 0, 0.05, or 0.20) were studied by impedance spectroscopy in air (250 to 1000 degrees C), high temperature (700 to 1000 degrees C) ac conductivity measurements as a function of oxygen partial pressure (P-O2 ranging from air to about 10(-15) Pa), and ion blocking measurements (800 to 1000 degrees C). Combination of data for the p-type and n-type conductivity contributions, obtained with the latter technique with ionic conductivities measured by impedance spectroscopy allowed us to estimate the ionic transport numbers of these materials as a function of PO2. Small Fe and Cr additions (5%) were found effective in increasing the p-type conductivity in air with respect to the base material (x = 0), but large ionic domains could still be identified. For the 20% Fe or Cr doped samples the p-type conductivity in air is about the same or even exceeds the ionic conductivity. Tailoring of mixed ionic-electronic conductivity by compositional effects can be easily achieved in this family of materials.</p