Bulk defects in Co 3O 4, pure and slightly doped with lithium, revealed by EPR of the tetrahedral Co 2+ ions

Abstract

The influence of the preparation temperature of Co 3O 4, pure and slightly doped with lithium (up to 2 mol%), on the EPR signal of tetrahedral Co 2+ ions was studied. The decrease in the preparation temperature of pure Co 3O 4 has a similar effect on the corresponding Lorentzian singlet as the increase in the lithium content. This effect is explained in terms of a Gaussian distribution of the Co 2+ coordination tetrahedra with respect to the tetrahedral angle arising out of bulk defects such as cationic vacancies or doped lithium ions and Co 4+ octahedral ions compensating the additional negative charges. The cationic vacancies are due to small deviations from stoichiometry decreasing with increasing preparation temperature of the pure Co 3O 4. The zero-field splitting energy of the Co 2+ ion groundstate, mostly determining the linewidth, is rather high even at small deviations of the local symmetry from the purely cubic one, due to the relatively high value of the single electron spin-orbit coupling constant of Co 2+ ions and the small cubic ligand field parameter.