Singlet and triplet doped-hole configurations in La₂Cu₀.5Li₀.5O₄

The ordered alloy La_2Li_{0.5}Cu_{0.5}O_4 is found to be a band insulator in local density approximation (LDA) calculations with the unoccupied conduction band having predominantly d_{x^2-y^2}-symmetry and substantial weight in O 2p-orbitals. This is equivalent to a predominant local singlet configuration d^9L or a low spin Cu^{3+}-ion with both holes in orbits having d_{x^2-y^2}-symmetry, i.e. Zhang-Rice singlets. A fairly modest reduction of the apical Cu-O bondlength is sufficient to stabilize a high spin triplet Cu^{3+}-ionic configuration with holes in both d_{x^2-y^2} and d_{3z^2-r^2} orbits in LDA + U calculations. This leads us to identify the low energy triplet excitation found in NQR studies by Yoshinari et al. as a local high spin Cu^{3+} -ionic configuration accompanied by a substantial reduction of the apical Cu-O separation, i.e. a anti-Jahn-Teller triplet polaron.