BANDWIDTH DEPENDENCE OF INSULATOR-METAL TRANSITIONS IN PEROVSKITE COBALT OXIDES

Insulator-metal transitions and related electronic structures have bee n investigated for single crystals of perovskite-type cobalt oxides RC oO(3) (R=La, Pr, Nd, Sm, Eu, and Gd) with R-dependent variation of one -electron bandwidth (W). All the compounds exhibit the insulating grou nd state but undergo gradual insulator-metal (I-M) transitions at 500- 700 K. Systematic variation of the I-M transition temperatures (T-IM) as well as the thermal and optical charge gaps in the insulating phase have been observed with change of R species (or equivalently W). Anom alously low T-IM as compared with the charge gap in each RCoO(3) sugge sts that the I-Ari transition should be viewed as a thermally induced Mott transition.