Synthesis, structure, and ferromagnetism of the oxygen defect pyrochlore system Lu2V2O7-x (x=0.40-0.65)

A fcc oxygen defect pyrochlore structure system Lu2V2O7-x with x=0.40-0.65 was synthesized from the known fcc ferromagnetic semiconductor pyrochlore compound Lu2V2O7 which can be written as Lu2V2O6O' with two inequivalent oxygen sites O and O-'. Rietveld x-ray diffraction refinements show significant Lu-V antisite disorder for x greater than or similar to 0.5. The lattice parameter versus x (including x=0) shows a distinct maximum at x similar to 0.4. We propose that these observations can be explained if the oxygen defects are on the O-' sublattice of the structure. The magnetic susceptibility versus temperature exhibits Curie-Weiss behavior above 150 K for all x, with a Curie constant C that increases with x as expected in an ionic model. However, the magnetization measurements also show that the (ferromagnetic) Weiss temperature theta and the ferromagnetic ordering temperature T-C both strongly decrease with increasing x instead of increasing as expected from C(x). The T-C decreases from 73 K for x=0 to 21 K for x=0.65. Furthermore, the saturation moment at a field of 5.5 T at 5 K is nearly independent of x, with the value expected for a fixed spin 1/2 per V. The latter three observations suggest that Lu2V2O7-x may contain localized spin 1/2 vanadium moments in a metallic background that is induced by oxygen defect doping, instead of being a semiconductor as suggested by the C(x) dependence. We present evidence of magnetic granularity in Lu2V2O7-x with increasing x, due to the random distribution of oxygen vacancies and associated Lu-V antisite mixing. For x=0.65, isothermal magnetization versus magnetic field M(H) and low-field M(T) measurements indicate the formation of interacting nanoscopic ferromagnetic domains below T-C.