First-principles multielectron calculations of NiL2,3NEXAFS and ELNES forLiNiO2and related compounds

Abstract

A technique to compute $3d$ transition-metal ${L}_{2,3}$-edge near-edge x-ray-absorption fine structure (NEXAFS) and electron energy-loss near-edge structure (ELNES) is adopted to predict and interpret spectra from NiO, $\mathrm{Li}\mathrm{Ni}{\mathrm{O}}_{2}$, and $\mathrm{Ni}{\mathrm{O}}_{2}$. Multielectron wave functions were obtained as a linear combination of Slater determinants. The Slater determinants were constructed using all $\mathrm{Ni}\text{\ensuremath{-}}2p$- and $\mathrm{Ni}\text{\ensuremath{-}}3d$-based molecular orbitals, instead of atomic orbitals, obtained by solving the Dirac equation within the local density approximation for $\mathrm{Ni}\mathrm{O}_{6}{}^{m\ensuremath{-}}$ clusters ($m=10$, 9, and 8). The $\mathrm{O}\text{\ensuremath{-}}2p$ contributions through covalency can therefore be included automatically without any other scheme. Experimental ELNES for the three compounds were satisfactory reproduced by the present calculations, which unambiguously shows that Ni atoms are Ni(III) with a low-spin state in $\mathrm{Li}\mathrm{Ni}{\mathrm{O}}_{2}$, and Ni(IV) with a low-spin state in $\mathrm{Ni}{\mathrm{O}}_{2}$. The origin of satellite peaks in the spectra is also clarified. The present method is based on the robust multielectron quantum theory, which could be applied to the analysis and prediction of ${L}_{2,3}$-edge spectra of other transition-metal compounds.