Electronic structure ofPrNiO3studied by photoemission and x-ray-absorption spectroscopy: Band gap and orbital ordering

The electronic structure of ${\mathrm{PrNiO}}_3$ has been studied by photoemission and x-ray-absorption spectroscopy. By analyzing the spectra using configuration-interaction calculations on a ${\mathrm{NiO}}_6$ cluster model, it has been found that the charge-transfer energy Δ is ∼1 eV and the Ni 3d and O 2p orbitals are strongly hybridized in the ground state. From the cluster-model calculation, the magnetic moment of Ni 3d is estimated to be ∼0.9${\mathrm{\mu }}_{\mathit{B}}$, which is close to the ionic value of ${\mathrm{Ni}}^{3+}$ and in good agreement with that obtained from neutron-diffraction experiments. Using the electronic-structure parameters deduced from the cluster-model analysis, we have performed unrestricted Hartree-Fock calculations on a Ni 3d–O 2p perovskite-type lattice model in order to study the effect of ${\mathrm{GdFeO}}_3$-type distortion on the orbital polarization and band gap.