Structure and Electrical Properties of Ni-substituted Lanthanum Gallate Perovskites

Abstract

Crystal structure, electrical conductivity and thermal expansion of materials in the system La0.9Sr0.1Ga1–yNiyO3 (LSGN) with 0 < y ≤ 0.5 have been studied as a function of Ni content, temperature and Po 2. The materials have an orthorhombic structure at low dopant content and a hexagonal structure for higher Ni content. The σ(Po 2, T) results show increasing electronic conductivity at high Po 2 with increasing Ni content. At low Po 2 the conductivity fits a model for predominant ionic conductivity. AC impedance spectroscopy on an electron blocking cell of the form M/LSG/LSGN/LSG/M was used to isolate the ionic conductivity in the y=0.1 and 0.2 materials. The materials were found to have appreciable ionic conductivity in air with a similar magnitude and activation energy to the electrolyte materials. An analysis of the low frequency impedance of the blocking cell provided values for D at 800°C of the order of 9 × 10−5 cm2/s. An evaluation of the so called chemical capacitance enabled determination of the electron density and mobility. The calculated electronic mobility of 3.8 × 10−3 cm2/Vs and activation energy of 0.14 eV for the y = 0.2 material are in excellent agreement with expectations of an electronic transport model involving electron hopping within the Ni impurity band. The thermal expansion coefficients of the Ni doped materials were determined as a function of temperature and dopant level. The presence of mixed conductivity suggests that this material may be useful as an electrode for a lanthanum gallate based fuel cell or other electrochemical device.