RIXS Study of the NiO₆ charge excitations in bulk samples of RENiO₃

The unique metal to insulator transition (MIT) in the rare-earth niceklate series, RENiO₃, remains a well debated topic. This is due to the complexities in the multiple electron-electron interactions that give rise to a negative charge transfer bond-disproportionate low-temperature ground state[1]. We have conducted intensive Resonant Inelastic X-ray Spectroscopy (RIXS) studies on nine compounds (RE = La, Pr, Nd, Sm, Gd, Dy, Y, Ho, Er) within this nickelate series[2]. Our results show a systematic evolution of the NiO₆ multiplet excitations and an increase of the in-gap spectral weight as the rare earth radii increases[3]. We use Density Functional Theory and exact diagonalization calculations to model the RIXS spectra and extract energy values for the charge transfer (∆), Coulomb Potential (U ), and Hund’s coupling (J_H )[4] . We will discuss the implications of our results regarding the Mott vs charge transfer vs negative charge transfer description of the RENiO₃ series.