On the electronic structure of Cu(III) and Ni(III) in La2Li1/2Cu1/2O4, Nd2Li1/2Ni1/2O4, and Cs2KCuF6

The electronic structure of copper and nickel in the oxidation state fm was studied for La2Li1/2Cu1/2O4, Cs2KCuF6, and Nd2Li1/2Ni1/2O4 by X-ray absorption near-edge structure (XANES) spectroscopy at the L-2.3-thresholds of Cu and Ni as well as the K-thresholds of O and F. The observed multiplet structures are well described by cluster configuration-interaction calculations, which result in a high-spin ground state for Cs2KCuF6 and low-spin ground states for La2Li1/2CU1/2O4 and Nd2Li1/2Ni1/2O4. The simulations yielded 30% and 40% 3d(8) contributions for La2Li1/2Cu1/2O4 and Cs2KCuF6, respectively, and 57% 3d(7) for Nd2Li1/2Ni1/2O4 in the ground state. The O-K and F-K XANES spectra exhibit pronounced preedge peaks that monitor ligand-2p holes induced by covalency, These preedge peaks move to lower energies with increasing oxidation state of Cu and Ni as well as when going from nickelates to cuprates. (C) 1998 Elsevier Science B.V. All rights reserved.