Optical investigation of charge-transfer transitions in spinel Co3O4

Optical transitions in normal-spinel Co3O4 have been identified by investigating the variation of its optical absorption spectrum with the replacement of Co by Zn. Three optical-transition structures were located at about 1.65, 2.4, and 2.8 eV from the measured dielectric function of Co3O4 by spectroscopic ellipsometry. The variation of the absorption structures with the Zn substitution (ZnxCo3−xO4) can be explained in terms of charge-transfer transitions involving d states of Co ions. The 1.65 eV structure is assigned to a d–d charge-transfer transition between the t2g states of octahedral Co3+ ion and t2 states of tetrahedral Co2+ ion, t2g(Co3+)→t2(Co2+). The 2.4 and 2.8 eV structures are interpreted as due to charge-transfer transitions involving the p states of O2− ion: p(O2−)→t2(Co2+) for the 2.4 eV absorption and p(O2−)→eg(Co3+) for the 2.8 eV absorption. The observed gradual reduction of the 1.65 and 2.4 eV absorption strength with the increase of the Zn composition for ZnxCo3−xO4 can be explained in terms of the substitution of the tetrahedral Co2+ sites by Zn2+ ions. The crystal-field splitting ΔOh between the eg and the t2g states of the octahedral Co3+ ion is estimated to be 2 eV.