Mechanism of intermetallic charge transfer and bond disproportionation in BiNiO3 and PbNiO3 revealed by hard x-ray photoemission spectroscopy

Perovskites with Bi or Pb on the A site host a number of interesting and yet to be understood phenomena such as negative thermal expansion in BiNiO₃. We employ hard x-ray photoemission spectroscopy of Ni $2p$ core level as well as valence band to probe the electronic structure of BiNiO₃ and PbNiO₃. The experimental results supported by theoretical calculations using dynamical mean-field theory reveal essentially identical electronic structure of the Ni--O subsystem typical of Ni²⁺ charge-transfer insulators. The two materials are distinguished by filling of the Bi(Pb)--O antibonding states in the vicinity of the Fermi level, which is responsible for the Bi disproportionation in BiNiO₃ at ambient pressure and absence of similar behavior in PbNiO₃. The present experiments provide evidence for this conclusion by revealing the presence/absence of Bi/Pb $6s$ states at the top of the valence band in the two materials.