Paraorbital ground state of trivalent Ni ion in LiNiO$_2$ from DFT+DMFT calculations

TL;DR

This study uses DFT+DMFT calculations to reveal that the Ni$^{3+}$ ion in LiNiO$_2$ has an orbitally degenerate ground state, explaining the absence of cooperative Jahn-Teller distortion and matching experimental magnetic susceptibility data.

Contribution

The paper demonstrates that the ground state of Ni$^{3+}$ in LiNiO$_2$ is a thermodynamical mixture of ionic states with preserved orbital degeneracy, unlike previous symmetry-breaking results.

Findings

Ground state is a mixture of $d^7$ and $d^8L$ states.

Orbital degeneracy remains unlifted, preventing cooperative Jahn-Teller distortion.

Calculated magnetic susceptibility agrees with experimental data.

Abstract

In LiNiO$_2$ Ni$^{3+}$ ion has $d^7$ configuration in cubic crystal field with one electron on double degenerate $e_g$ orbitals, and such ion is considered to be Jahn-Teller (JT) active. However despite the fact, that this compound is an insulator, and hence $d$-electrons are localized, a cooperative JT lattice distortion was not observed. This problem was usually supposed to be resolved by the presence of local JT-distortions that do not order in cooperative JT distorted crystal structure. In the present work DFT+DMFT approach, combining Density Functional Theory with Dynamical Mean-Field Theory, was applied to study electronic and magnetic properties of LiNiO$_2$. In the result, insulating solution with a small energy gap value was obtained in agreement with experimental data. However, in contrast to previous calculations by other methods, the symmetry was not broken and the calculated ground state is a thermodynamical mixture of $\alpha d^7 + \beta d^8L $ ($\alpha \approx 60\%, \beta \approx 40\%$) ionic states. The $d^8L $ state is JT inactive and we have found that for the nickel $d^7$ state two configurations with an electron on the Ni $d_{x^2-y^2}$ or $d_{3z^2-r^2}$ orbital have equal statistical weights. So the orbital degeneracy of Ni$^{3+}$ ion is not lifted and that explains the absence of the cooperative JT lattice distortion in this compound. Also, the temperature dependence of inverse magnetic susceptibility of LiNiO$_2$ has been calculated and a good agreement with experimental data was obtained.