Generally, La1 - xSrxCoyFe1 - yO3 - delta-phases (LSCF) as mixed ionic electronic conductors (MIEC) are applied as electrode material in solid oxide fuel cells and as oxygen separation membranes in zero CO2 emission power plants. LSCF phases are synthesized using magnetron sputtering from a compound target. The chemical composition could be controlled via synthesis temperature variations as the sticking coefficient is a function of temperature. With decreased synthesis temperature a significant change in chemical composition is observed resulting in an increased A-site deficiency. This leads to an extension of the cubic stability range to temperatures as low as 450 degrees C. Thus, structural transition from the low-temperature rhombohedral modification to the high-temperature cubic structure, accompanied by considerable volume change, will be suppressed. This significantly lowers mechanical stresses on the MIEC-material during application and is expected to increase the lifetime of the cells and membranes. The experimental results are consistent with ab initio density functional theory calculations performed on A-site deficient as well as fully stoichiometric LSCF phases. A-site deficient cubic phases are more stable than the stoichiometric phases. No significant difference in Young's modulus is obtained. (C) 2013 Elsevier B.V. All rights reserved.