S. A. Warda

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

Abstract The electronic structure of copper and nickel in the oxidation state +III was studied for La2Li1/2Cu1/2O4, Cs2KCuF6, and Nd2Li1/2Ni1/2O4 by X-ray absorption near-edge structure (XANES) spectroscopy at the L2,3-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% 3d8 contributions for La2Li1/2Cu1/2O4 and Cs2KCuF6, respectively, and 57% 3d7 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.

Difference in spin state and covalence between La1−xSrxCoO3 and La2−xSrxLi0.5Co0.5O4

We present a comparative study of the spin states and electronic properties of La1-xSrxCoO3 and La2-xSrxLi0.5Co0.5O4 using X-ray absorption near-edge structure spectroscopy at both the O-K and Co-L-2.3 thresholds. In the La2-xSrxLi0.5Co0.5O4 system the CoO6 octahedra are isolated, the holes induced by Sr doping are trapped in the isolated Co(IV)O-6 octahedra, and a low-spin state is found for the Co ions, which does not change upon Sr doping. In the La1-xSrxCoO3 system, the interconnected CoO6 octahedra, with a 180degrees Co-O-Co bond angle, give rise to a transition from low-spin to intermediate-spin state with a ferromagnetic alignment of the Co spins. The double-exchange, ferromagnetic coupling between Co ions mediated by the 180degrees bond angle is responsible for suppressing the low spin-state. We find that the branching ratio of spectral intensities at the L-2 and L-3 thresholds in the Co-L-2.3 X-ray absorption spectra is sensitive to the spin state of the Co ions allowing its direct spectroscopic determination