Synthesis, Structure, and Ferromagnetism of a New Oxygen Defect Pyrochlore System Lu2V2O_{7-x} (x = 0.40-0.65)

TL;DR

This study synthesizes a new oxygen-deficient pyrochlore Lu2V2O_{7-x} system, revealing how oxygen vacancies affect its structure and magnetic properties, indicating potential metallic behavior induced by doping.

Contribution

It introduces a new oxygen defect pyrochlore system and provides detailed structural and magnetic analysis, highlighting the effects of oxygen vacancies on ferromagnetism and suggesting metallic behavior.

Findings

Lattice parameter peaks at x ~ 0.4 indicating oxygen defect location.

Curie-Weiss behavior observed with increasing Curie constant C.

Ferromagnetic transition temperature T_C decreases with x.

Abstract

A new fcc oxygen defect pyrochlore structure system Lu2V2O_{7-x} with x = 0.40 to 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 showed significant Lu-V antisite disorder for x >= 0.5. The lattice parameter versus x (including x = 0) shows a distinct maximum at x ~ 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 Lu2V2O_{7-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.