Refinement of the critical V–V separation for spontaneous magnetism in oxides

Abstract

Cubic Liy[V2]O4 (0.7 ≤ y ≤ 2.0) samples with the [V2] O4 spinel framework have been prepared by room-temperature chemical delithiation–lithiation of Li[V2]O4. The Li+-ion distribution in the interstitial sites is distinguishable in different compositional regions: all-tetrahedral lithium in Li1−x[V2]O4 and in Li1+x[V2]O4 tetrahedral–octahedral pairs in association with tetrahedral lithium for 0 < x ≤ 0.4 transforming to all-octahedral lithium as x → 1.0. Identification of localized V3+:d2 configurations in Li2[V2]O4 narrows the range for a critical V–V separation to 2.89 Å < Rc < 2.93 Å for spontaneous magnetism in oxides containing a periodic network of energetically equivalent V3+ ions. The cubic spinel Li[V2]O4 has been prepared from stoichiometric LiVO2 by room-temperature lithium extraction with bromine to Li0.5VO2 followed by heating to T > 570 K. The extraction product Li1−xVO2 (0 ≤ x ≤ 0.3) has the low-temperature LiVO2 phase; Li0.3VO2 is nearly cubic with V displaced to the Li-deficient layers, and the compositional range 0.3 < x < 0.7 is two-phase. LiVO2 is air sensitive; surface-adsorbed oxidants extract lithium from the bulk up to the phase limit Li0.7VO2. Heating Li1−xVO2 to above T1, which increases with x, causes it to disproportionate into LiVO2 and Li[V2]O4, which explains the anomalies reported by Hewston and Chamberland on thermal cycling acid-leached LiVO2.