Valence and spin states in delafossite AgNiO2 and the frustrated Jahn-Teller system ANiO2 (A=Li,Na)

Electronic structures of delafossite oxides AgNi1-xCoxO2 and the frustrated Jahn-Teller (JT) system ANiO(2) (A=Li,Na) have been investigated by employing soft x-ray absorption spectroscopy and photoemission spectroscopy (PES). It is found that Ni ions are in the Ni2+-Ni3+ mixed-valent states and that the low-spin (LS) Ni3+ component increases from LiNiO2 to AgNiO2 and NaNiO2, in agreement with the presence of the JT transition in NaNiO2 and the absence of the JT transition in LiNiO2 and AgNiO2. In AgNi1-xCoxO2, the Ni3+ component increases with x, while Co ions are in the LS Co3+ states for all x, which is consistent with the metallic nature for low values of x. A good agreement is found between the measured PES spectra and the calculated local spin density approximation (LSDA) electronic structures of AgNiO2 and AgCoO2, but the pseudogap feature in PES of AgNiO2 is not described by the LSDA.