Paraorbital ground state of the trivalent Ni ion in LiNiO2 from DFT+DMFT calculations

In LiNiO₂, the Ni^{3 +} ion has a d⁷ configuration in a cubic crystal field with one electron on doubly degenerate e_g orbitals, and such an 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 a cooperative JT distorted crystal structure. In the present work, the DFT+DMFT approach, combining density functional theory with dynamical mean-field theory, was applied to study the electronic and magnetic properties of LiNiO₂. In the result, an 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 α d⁷+β d⁸L (α ≈60 %,β ≈40 % ) ionic states. The d⁸L state is JT inactive, and we have found that for the nickel d⁷ state two configurations with an electron on the Ni d_x²-y² or d_{3 z²-r²} orbital have equal statistical weights. So the orbital degeneracy of the 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 LiNiO₂ has been calculated, and a good agreement with experimental data was obtained.