Transport and magnetic properties of Sr2FeMoxW1-xO6

We have investigated transport and magnetic properties of a series of doubl e-perovskite alloys in the system Sr2FeMoxW1-xO6. These compounds exhibit a metal-insulator transition as a function of doping, x. The compounds with x in the range 1.0 greater than or equal to x greater than or equal to 0.3 show metallic behaviour in the resistivity while compounds with x in the ra nge 0.2 greater than or equal to x greater than or equal to 0 are insulatin g, All compounds with 1.0 greater than or equal to x greater than or equal to 0.2 are ferrimagnetic, while Sr2FeWO6 (x = 0) is antiferromagnetic. The magnetization (M) is shown to increase slowly with decreasing x in the rang e 1.0 greater than or equal to x greater than or equal to 0.3 due to an enh anced crystallographic ordering; however, M decreases rapidly with decreasi ng x thereafter(x < 0.3), probably due to composition fluctuations near the critical concentration, x(c). Our data suggest primarily Fe3+-(Mo, W)(5+) ordering for x <greater than or equal to> 0.3, while compositions with smal ler x possibly contain both Fe3+ and Fe2+ species inhomogeneously due to th e presence of the W6+ state. All samples with x greater than or equal to 0. 3 show a significant amount of negative magnetoresistance, as has been obse rved earlier for Sr2FeMoO6 (x = 1.0).