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 (Formula presented)(Formula presented)(Formula presented) (Ln=La and Nd) materials. Selected polarized XAS results on the (Formula presented)(Formula presented)(Formula presented) system emphasize that a collapse of the Ni 4(Formula presented)-4(Formula presented) 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 ((Formula presented)) 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 (Formula presented) 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 (Formula presented) state to O p states in these systems.
|Original language||English (US)|
|Number of pages||8|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - 1996|
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics