Local electronic properties of La3Ni2O7 under pressure

Abstract

The recent discovery of superconductivity in $\mathrm{L}{\mathrm{a}}_3\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$ has attracted significant attention due to its high critical temperature and analogy to cuprate oxides. The oxidation and spin states of Ni ions are among the most important local properties in this compound, extensively discussed in the context of its superconductivity. Despite their direct link to the electron filling configurations of the relevant $3{\mathrm{d}}_{{\mathrm{x}}^2-{\mathrm{y}}^2}$ and $3{\mathrm{d}}_{{\mathrm{z}}^2}$ orbitals, these local electronic properties of $\mathrm{L}{\mathrm{a}}_3\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$ yet to be systematically investigated. In this work, we address this issue using x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) measurements under pressure. Comparison of Ni \textit{K}-edge XAS and $\mathrm{K}{\hat{\mathrm{I}}}^2$ XES with the reference spectra of $\mathrm{NiO}$ and $\mathrm{LaNi}{\mathrm{O}}_3$ shows that Ni ions, with an average valence of $\sim 2.53+$, are in a low-spin ($\mathrm{S}=1/2$) ground state under ambient conditions. High pressure XAS and XES data clearly show that the oxidation ($\sim 2.5+$) and spin ($\mathrm{S}=1/2$) states of Ni ions remain stable across the investigated pressure (up to 30 GPa) and temperature (down to 10 K) ranges, ruling out previously proposed spin transition scenarios.