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The optical study of trivalent 3d transition-metal-oxide compounds (RMO₃) with the perovskitelike structure has revealed the variation of their electronic structure with the 3d element (M) as well as the A-site rare-earth element (R). The crossover of the gap nature from the Mott type to charge-transfer (CT) type with increasing atomic number of M is observed to occur around M=Cr. The variation of Mott and CT gaps with M species is quantitatively consistent with the tendency expected from an ionic model. However, for the low-energy electronic structures for the narrow-gap (or metallic) compounds (M=Ti, Co, Ni), the effects of the M-3d--O-2p hybridization must be included.
Arima et al. (Wed,) studied this question.