Charge Ordering in the Spinels AlV₂O₄ and LiV₂O₄

Abstract

We develop a microscopic theory for the charge ordering (CO) transitions in the spinels AlV₂O₄ and LiV₂O₄ (under pressure). The high degeneracy of CO states is lifted by a coupling to the rhombohedral lattice deformations which favors a transition to a CO state with inequivalent V(1) and V(2) sites forming Kagomé and trigonal planes, respectively. We construct an extended Hubbard-type model including a deformation potential which is treated in an unrestricted Hartree–Fock approximation and describes correctly the observed first-order CO transition. We also discuss the influence of associated orbital order. Furthermore, we suggest that due to different band fillings AlV₂O₄ should remain metallic while LiV₂O₄ under pressure should become a semiconductor when charge disproportionation sets in.