著書

The electronic nature of delafossite-type AgMO_2_ (M=Fe, Co, and Ni) and PdCoO_2_ have been investigated by the (MO_6_)^9-^ octahedral cluster model analysis and by the band calculation using the local spin density pproximation plus on-site Coulomb interaction (LSDA+U) method. The cluster analysis of M 2p x-ray photoemission spectra reveals that edge-shared MO_6_ octahedra layers, which are conected by O-Ag-O dumbbells, belong to the strongly correlated electron system. The O2p-M3d charge transfer (CT) energy (Δ) and on-site Coulomb energy (U) decrease in going from Fe→Co→Ni, and that AgMO_2_has highly covalent and strongly correlated MO_6_ octahedral layers. According to the consequent LSDA+U band caluculation, the insulating behaviour of AgFeO_2_ and AgCoO_2_ originate from the nonmetallic nature of strongly correlated octahedral layers with large U and Ag layers, while the metallic behaviour of AgNiO_2_ comes from the metallic character of Ag layers and correlated metallic NiO_6_ octahedral layers. The sign of the thermopower of single-crystalline PdCoO_2_ is negative, while that of poly-crystalline PdCoO_2_ is positive. Our analysis indicates that Pd_x_CoO_2_ has the heterogenous electronic character; the $n$-type conduction with small thermopower takes place in the Pd layers, while the p-type conduction with large thermopower (S〜100μV/K at 300K) takes place in the strongly correlated CoO_6_ octahedral layers. These compounds can be considered as key materials which opens the possibility of using PdCoO_2_-type heterogenous electronic structure to control the performance of thermoelectric materials.