Electronic Tuning of Two Metals and Colossal Magnetoresistances in EuWO1+xN2−x Perovskites

摘要

A remarkable electronic flexibility and colossal magnetoresistance effects have been discoverednin the perovskite oxynitrides EuWO1+xN2-x. Ammonolysis of Eu2W2O9 yields scheelite-type intermediatesnEuWO4-yNy with a very small degree of nitride substitution (y ) 0.04) and then EuWO1+xN2-x perovskitesnthat show a wide range of compositions -0.16 e x e 0.46. The cubic lattice parameter varies linearly withnx, but electron microscopy reveals a tetragonal superstructure. The previously unobserved x < 0 regimencorresponds to oxidation of Eu (hole doping of the Eu:4f band), whereas x > 0 materials have chemicalnreduction of W (electron doping of the W:5d band). Hence, both the Eu and W oxidation states and thenhole/electron doping are tuned by varying the O/N ratio. EuWO1+xN2-x phases order ferromagnetically atn12 K, and colossal magnetoresistances (CMR) are observed in the least doped (x)-0.04) sample. Distinctnmechanisms for the hole and electron magnetotransport regimes are identified.