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

The ordered alloy La2Li0.5CU0.5O4 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(9)L or a low-spin CU3+ 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 bond length is sufficient to stabilize a high spin tripler Cu3+ ionic configuration with hales 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 tripler excitation found in nuclear quadrupole resonance studies by Yoshinari et al. as a local high-spin Cu3+ ionic configuration accompanied by a substantial reduction of the apical Cu-O separation, i.e., an anti-Jahn-Teller triplet polaron.