Abstract
In this paper we report the results of a detailed investigation of the double perovskite (Ca <sub>2-2x</sub> Sr <sub>2x</sub> )FeMoO <sub>6</sub> system. Chemical size effects on structural, electrical, and magnetic properties caused by the substitution of isovalent, larger Sr ions into the smaller Ca sites, resulting in (Ca <sub>2-2x</sub> Sr <sub>2x</sub> )FeMoO <sub>6</sub> , have been examined. The compounds crystallize in the monoclinic space group P2 <sub>1</sub> /n for 0.0 ≤ x < 0.2, the orthorhombic space group Pbnm for 0.2 ≤ x > 0.4, and the tetragonal space group I4/m and I4/mmm for x ≥ 0.4. Examination of the resistivity of all compounds reveals a metallic behavior which is well described by a T <sup>n</sup> dependence except for x = 1.0. These n values change from 1 to 2 as T decreases lower than T <sub>c</sub> . This is indicative of a variation in the transport mechanism at T <sub>c</sub> . The ferrimagnetic transition temperature T <sub>c</sub> increases with increasing x from 318 (x = 0.0) to 393 K (x = 1.0). For all samples, the saturation magnetization at 82 K obeys M <sub>s</sub> ≤ 3.5 μ <sub>B</sub> /(formation unit), compared to a theoretical spin-only moment of 4 μ <sub>B</sub> /(formula unit) for a perfectly ordered compound.