Locating redox couples in the layered sulfides with application to Cu[Cr2]S4

Use of LiPF₆ in EC:DEC as electrolyte has allowed electrochemical extraction of Li from LiV_1−yM_yS₂ and LiTi_1−yM_yS₂ (M=Cr or Fe). The data show access not only to the Ti(IV)/Ti(III) and V(IV)/V(III) redox couples, but also to the V(V)/V(IV) and Fe(III)/Fe(II) couples in these layered sulfides. However, the Cr(IV)/Cr(III) couple could not be accessed. The concept of redox-couple pinning is outlined and applied to the V(V)/V(IV) and Fe(III)/Fe(II) couples, which are pinned at the top of the S-3p bands. Holes associated with the “pinned” couples occupy orbitals of dominant S-3p character, but they have sufficient cation-3d character to prevent condensation of the holes into p-p antibonding states of disulfide bonds. Strong covalent bonding in the pinned couples creates itinerant-electron states in the partially occupied couples. Application to the metallic, ferromagnetic thiospinel CuCr₂]S₄ favors location of the itinerant holes in states of a pinned Cu(II)/Cu(I) couple having primarily S-3p character.