Jahn-Teller effects in transition-metal compounds with small charge-transfer energy

We have studied Jahn-Teller effects in Cs2Au2Br6, ACu3Co4O12(A=Ca or Y), and IrTe2 in which the ligand p-to-transition-metal d charge-transfer energy is small or negative. The Au⁺/Au³⁺ charge disproportionation of Cs2Au2Br6 manifests in Au 4f photoemission spectra. In Cs2Au2Br6 with negative Δ and intermediate U, the charge disproportionation can be described using effective d orbitals constructed from the Au 5d and Br 4p orbitals and is stabilized by the Jahn-Teller distortion of the Au³⁺ site with low-spin d⁸ configuration. In ACu3Co4O12, Δs for Cu³⁺ and Co⁴⁺ are negative and Us are very large. The Zhang-Rice picture is valid to describe the electronic state, and the valence change from Cu²⁺/Co⁴⁺ to Cu³⁺/Co³⁺ can be viewed as the O 2p hole transfer from Co to Cu or d⁹ + d ⁶L → d⁹L + d⁶. In IrTe2, both Δ and U are small and the Ir 5d and Te 5p electrons are itinerant to form the multi-band Fermi surfaces. The ideas of band Jahn-Teller transition and Peierls transition are useful to describe the structural instabilities.