Ground-state oxygen holes and the metal–insulator transition in the negative charge-transfer rare-earth nickelates

TLDR

The metal–insulator transition and related properties of rare‑earth perovskite nickelates have attracted attention, yet a complete understanding remains elusive. The study combines X‑ray absorption and resonant inelastic X‑ray scattering to resolve key aspects of the electronic structure of NdNiO₃ thin films. The authors employ X‑ray absorption, RIXS, cluster calculations, and Anderson impurity modeling to probe the electronic structure of NdNiO₃. RIXS shows bound and continuum excitations that evidence abundant oxygen holes, and cluster calculations confirm a negative charge‑transfer energy, supporting a bond‑disproportionation model of the metal–insulator transition.

Abstract

Abstract The metal–insulator transition and the intriguing physical properties of rare-earth perovskite nickelates have attracted considerable attention in recent years. Nonetheless, a complete understanding of these materials remains elusive. Here we combine X-ray absorption and resonant inelastic X-ray scattering (RIXS) spectroscopies to resolve important aspects of the complex electronic structure of rare-earth nickelates, taking NdNiO 3 thin film as representative example. The unusual coexistence of bound and continuum excitations observed in the RIXS spectra provides strong evidence for abundant oxygen holes in the ground state of these materials. Using cluster calculations and Anderson impurity model interpretation, we show that distinct spectral signatures arise from a Ni 3 d 8 configuration along with holes in the oxygen 2 p valence band, confirming suggestions that these materials do not obey a conventional positive charge-transfer picture, but instead exhibit a negative charge-transfer energy in line with recent models interpreting the metal–insulator transition in terms of bond disproportionation.