Transferred hyperfine interaction between a tetrahedral transition metal and tetrahedral lithium: Li6CoO4
Résumé
Li6CoO4 presents an antifluorite-type structure, with both the Co and Li ions in tetrahedral oxygen coordination. 7Li MAS NMR shows remarkably different shifts (+885 and −232 ppm) for the two different crystallographic types of Li. In order to assign the signals and to understand the mechanisms whereby the electron spins on the e orbitals of Co2+ ions (e4 t23 electronic configuration) are transferred toward the two different types of Li with opposite polarization, we have carried out GGA and GGA+U calculations of the electronic structure using the VASP code. Spin density maps in selected planes of the structure reveal (as expected) that lobes of the t2 orbitals point toward the faces of the CoO4 tetrahedra and can thus overlap with the neighboring Li(2) through empty square pyramidal sites. As concerns Li(1), a mechanism is evidenced where the (filled) e orbitals of Co2+ are polarized by the electron spins in the t2 ones. These polarized e orbitals overlap with Li(1) through the common edge of the tetrahedra. The relative magnitude of the experimental shifts for the two types of Li are however not fully reproduced by the calculations, and the influence of the U parameter as well as of the pseudopotential method used is discussed.