La0.90Sr0.10Ga0.85Mg0.15O3-delta (LSGM) was prepared by solid state synthesis and mixed with various amounts of Ni and NiO, respectively. The optimum sintering temperature of the material was identified by dilatometric studies to be above 1300 degrees C. The interaction between LSGM and NiO was studied by X-ray diffraction after sintering at 1300 and 1400 degrees C in air as well as after reduction in hydrogen atmosphere at 800 and 1000 degrees C. The LaSrGa3O7 compound was detected after sintering, whereas, LaSrGaO4 was found after reduction treatment of the material. Diffusion of Ni into LSGM was thought to be the reason for the presence of the above mentioned phases. After the treatment, LSGM contained 2.3 at% of Ni determined by energy dispersive X-ray analysis (EDX). The thermal expansion coefficient of LSGM with varying contents of Ni was observed to increase with increasing the Ni content. (c) 2006 Elsevier B.V. All rights reserved.