Electronic structure anisotropy andd-configuration in Ni-based materials

Ni K-edge x-ray-absorption spectroscopy (XAS) measurements on a series of perovskite-layer-based nickelate compounds, with varying formal Ni valence, are presented. Spectral features that vary systematically with the Ni configuration are identified for these Ruddlesden-Popper phase and Sr substituted Ln_2-x{Sr}ₓNiO₄ (Ln=La and Nd) materials. Selected polarized XAS results on the La_2-x{Sr}ₓNiO₄ system emphasize that a collapse of the Ni 4p_σ-4p_π energy separation accompanies the substantial contraction of the apical Ni-O bond length in this series. In view of this, it is suggested that a displacement of the apical oxygen (Oab) could play an important role in the small polarons previously proposed to form in this system. The polarized XAS results also provide evidence for in-plane character for the Ni-site component of the doped holes in this system. The XAS evidence supporting an increasing Ni d⁷ ground-state admixture in these materials, with increasing formal Ni valence, is discussed. This discussion is couched in terms of simplified Ni d configuration modeling, which emphasizes the close energetic proximity of the Ni d⁸ state to O p states in these systems. {} 1996 The American Physical Society.