Large Negative Magnetoresistance in Ba_3−xEu_xNb₅O₁₅

We found a large negative magnetoresistance in Ba_3-xEu_xNb₅O₁₅ with a tetragonal tungsten bronze structure, which changes from a metallic state (x = 0) to an insulating state (x = 3) under variation of x. The magnitude of the magnetoresistance ρ_c(0)=ρ_c(H), where ρ_c(H) is the resistivity along the c axis with the applied magnetic field of H, increases with x and reaches ∼5000 near the metal–insulator phase boundary. Such a large negative magnetoresistance is likely to be due to the coupling between the itinerant 4d electrons in Nb and the localized 4 f spins in Eu, and the disorder arising from Eu ions in this series of compounds.