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The series of the Sr 2 FeMo 1−x Nb x O 6 compounds have been synthesized throughout the range of 0 ≤ x ≤ 1 using conventional solid-state reaction method. The compounds show a tetragonal structure in the range of 0 ≤ x ≤ 0.4 while showing an orthorhombic structure in the range of 0.5 ≤ x ≤ 1. Magnetic and electrical properties of the compounds have been studied in order to investigate the effect of the substitution of non-magnetic Nb 5+ ion for Mo 5+ (or Mo 6+ ) ion in the double perovskite compound. The magnetization (M) values are found to decrease slowly with increasing x in all compounds with 0 ≤ x ≤ 0.75 due to less correlation among the magnetic ions induced by the non-magnetic Nb 5+ ions. The measurements of magnetic susceptibilities at zero-field-cooled and field-cooled conditions show that a transition from a ferromagnetic (FM) to a ferro-cluster glass state gradually transforms from a paramagnetic (PM) to a spin-glass state with increasing Nb 5+ content. With increasing x, the conductivity decreases. Meanwhile, the compounds lose the ferromagnetic component. The compounds have good conductivity from 10 −3 to 10 cm with x in the range of 0 ≤ x ≤ 0.3, though they show insulator behavior. While those with x in the range of 0.6 ≤ x ≤ 0.75 show poor transport property. All samples with x ≥ 0.25 show large negative magnetoresistances (MRs), similar to that observed in Sr 2 FeMoO 6 .

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