First-principles Hartree-Fock description of the electronic structure of monoclinic C2/m Li_xMnO₂ (1 >= x >= 0)

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 d(5) with a local moment close to 4 mu(B) in an antiferromagnetic (AF(3)) spin arrangement. Calculations for unrelaxed 25% periodic Li-Mn antisite disordered C2/m and Pnmn structures are entirely consistent with a value of about 10% disorder reported for the C2/m phase. Far the system LixMnO2 in the region 1 less than or equal to x less than or equal to 1/2, 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 O 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 similar to 1.9 eV 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.