Correlation between thermal expansion and Seebeck coefficient in polycrystalline Co3O4

Characteristics of the cobalt-oxide spinel Co₃O₄ are described. Spinel is the name for a certain crystal structure that is built up out of three sublattices; one sublattice contains, in this case, only oxygen ions, and the other two sublattices, tetrahedral and octahedral, contain the metal cobalt (Co). Co₃O₄ is shown to exhibit an abnormally high thermal expansion above 750 K. This expansion is explained by an electronic transition of the nonmagnetic smaller trivalent Co ion to the magnetic larger trivalent Co ion. Seebeck effect measurements reveal that the number of charge-carriers also changes with temperature, which is explained by a charge-disproportionation reaction on octahedral sites. Both electronic processes are linked by the decrease of the bandgap, which is drastically reduced by the high expansion