Abstract

Fundamental electronic structure of transition-metal compounds is characterized by the Coulomb interaction U between d electrons and the charge transfer (CT) energy D from the ligand p to transition-metal d orbitals [1,2]. In case of CT type systems with D < U, the ligand p orbitals become relevant in addition to the transition-metal d orbitals. For example, YNiO3 falls in the charge-transfer regime [3], and the effect of O 2p hole plays important roles in the metal-insulator transition. The effect of ligand holes can be investigated by means of x-ray absorption spectroscopy (XAS) and angle-resolved photoemission spectroscopy (ARPES). In the present talk, we focus on LixCoO2 [4,5] and Ta2NiSe5 [6,7]. The CT energy D of Co 3+(d6 ) in LiCoO2 is ~ 1 eV indicating that Co4+ species in delithiated LixCoO2 have the electronic configuration of d6L rather than d5 (Here, L represents a ligand hole). The O 2p states are probed by polarization dependent O K-edge XAS [4] and ARPES [5], and are found to play effective roles to screen the Li ion charge. As for Ta2NiSe5, the negative D of Ni2+ is indicated by XAS and ARPES [6,7]. The attractive interaction between the Se 4p hole and the Ta 5d electron provides a unique electronic state in which the electron-hole pairs condensate in a BEC manner and induce a semimetal-insulator transition. In addition to the above two examples, we discuss other negative CT energy systems including Mn4+ , Fe4+, and Au 2+ which are expected to show interesting physical and chemical properties.