Dielectric and dc measurements have been carried out on La0.8Sr0.2GaO3−δ, which has a high potential for the electrolyte material in a solid oxide fuel cell, so as to elucidate the correlation between O2− diffusion and electric conduction. A dielectric relaxation shows up in loss tangent. The relative relation of activation energies as to dielectric relaxation and dc conduction suggests the ionic conduction due to O2− diffusion assisted by free single oxygen vacancies taking place. The change of dc conductive behavior associated with attenuation of dielectric relaxation around 950 K indicates that another type of O2− diffusion dominates the ionic conduction at high temperature.

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The electric transport properties of La0.95Sr0.05Ga0.9Mg0.1O3−δ (LSGM) crystal with ⟨001⟩p direction grown by the Czochralski method were investigated mainly using ac measurements. The LSGM crystal comprises twin structures composed of different domain states. Only the highest-frequency semicircle was observed in impedance spectroscopy; however, two dielectric relaxation peaks exist in the loss tangent. These results indicate that the equivalent circuit of the twin structures is a parallel R-C circuit because they exhibit two parallel diffusion paths of O2− ions: one path is within the domains and the other is along the domain walls. Based on the migration energies of O2− in the domains and along the domain walls, which are estimated from dielectric relaxation processes, resistances of ionic conductions therein are assessed separately. These assessments explain that low resistance for oxygen ionic conduction along the domain walls considerably reduces the resultant resistance of the LSGM crystal and oxygen vacancies segregate preferentially in domain wall zones. A comparison between these results and those of a polycrystalline LSGM ceramic obtained experimentally suggests that the twin structures within the grains in ceramic lanthanum gallates are one of the main reasons for the high ionic conductivity.

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Four-probe dc conductivities, capacitances, and thermopower have been measured in the temperature range of 80–1123 K for La1−xSrxGa1/2Mn1/2O3−δ, which is a desirable cathode material for lanthanum–gallate electrolytes of solid oxide fuel cells. The dc conductivities in the specimens (0.1⩽x⩽0.3) are insensitive to x but the thermopower is very sensitive to x, although the x=0 specimen exhibits a somewhat different conduction behavior. At T<300 K, a relaxation process has shown in dielectric loss factor with the activation energy higher than that for dc conduction in every specimen. These results at T<300 K have been numerically analyzed within the framework of the multielectronic conduction consisting of the polaronic conduction of Mn 3d eg holes created by Sr doping, the band conduction of O 2p holes and the hopping conduction of Mn 3d eg electrons, where the O 2p holes and Mn 3d eg electrons are created by thermal excitation of electrons from O 2p bands to Mn 3d eg narrow bands. At T>500 K, the band conduction dominates the electronic transports. The ionic conduction due to O2− migration seems difficult to contribute directly to the dc conduction even at high temperature.