The electronic structure of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{NiO}}_4$ is studied by use of photoemission spectroscopy, bremsstrahlung-isochromat spectroscopy (BIS), and electron-energy-loss spectroscopy. Quantitative analyses are made on the valence-band and Ni 2p core-level photoemission spectra through configuration-interaction calculations on a ${\mathrm{NiO}}_6$ cluster model. On the basis of these analyses, it is concluded that ${\mathrm{La}}_2$${\mathrm{NiO}}_4$ is a charge-transfer (CT) insulator and the magnitude of the band gap is about 4 eV, nearly the same as that of NiO. The BIS spectra show that unoccupied states induced by hole doping are spread over the CT gap, which is incompatible with a rigid-band picture for the hole doping. We discuss the origin of the different insulator-to-metal transition behavior between this system and ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$.

• Received 12 December 1991

DOI:https://doi.org/10.1103/PhysRevB.45.12513