First-principles Hartree-Fock description of the electronic structure of monoclinic C2/m LixMnO₂ (1 ≥ x ≥ 0)

Abstract

First-principles periodic spin-unrestricted Hartree-Fock calculations of C2/m and Pnmn LiMnO₂ are reported which indicate that the ground-state valence configuration is essentially d⁵ with a local moment close to ⁴μB in an antiferromagnetic (AF³) spin arrangement. Calculations for unrelaxed 25% periodic Li-Mn antisite disordered C2/m and Pnmm structures are entirely consistent with a value of about 10% disorder reported for the C2/m phase. For the system Li_xMnO₂ in the region 1 ≤ x ≤ ½, where the C2/m structure is apparently retained and the discharge voltage associated with Li removal is constant, Mulliken analyses and spin-density distributions indicate that the valence configuration of Mn is essentially unaltered with the formation of localized holes on the 0 sites with strong spin polaron character as in MnO. Both the Fermi and the LI removal energies are calculated to change by only a few per cent. In this region, new empty O(p) states are created below the conduction-band edge of LiMnO₂, from which a bandgap of ∼1.9eV can be deduced, as in the case of NiO and MnO. No significant changes are found in the empty Mn(d) states. It is suggested that the changes in the densities of unoccupied states, which are an accurate indicator of the nature of the valence electron configurations, should be directly verifiable by O K-edge spectroscopy and indirectly from the absence of any significant changes in the Mn K- and L-edge spectra.