NONLOCAL ELECTRONIC EFFECTS IN CORE-LEVEL PHOTOEMISSION, UV AND OPTICAL ELECTRONIC ABSORPTION-SPECTRA OF NICKEL OXIDES

The analysis of core-level and valence band photoemission spectra of N iO and CuO, yields that the final states of the former should be assig ned to cd(9) (L) under bar (main peak, lower binding energy) and cd(8) (satellite) states, both configurations being considerably intermixed by covalency. On the basis of cluster calculations it is shown that n on-local screening effects with contributions from d(n+1)d(n-1) charge transfer states and multiplet structure have a distinct influence on the energies of the core-level photoemission lines and should be discu ssed on an equal footing when considering photoemission data of solids built up of transition metals and bridging ligands. Ab initio calcula tions based on isolated MO6 units yield rather high values of the liga nd-to-metal charge transfer energy Delta, compared to the experimental value of solids such as NiO. This is most probably due to the neglect of electronic delocalization as the consequence of the charge transfe r to neighbouring metal and more distant ligand atoms and to the addit ional relaxation for excited charge transfer states with contributions from the remote surrounding. A correlation between non-local screenin g and magnetic coupling is pointed out, and their effect on core-level photoemission line shapes is explored theoretically and supported exp erimentally by available temperature and concentration dependent 2p XP S spectra of NixMg1-xO mixed crystals, UV absorption spectra of the Ni xMg1-xO mixed crystals series are reported and show large red shifts w ith increasing x, favouring a charge transfer mechanism with non-local contributions from neighbouring metals and more distant ligands. The energies of spin-allowed and intensities of spin-forbidden transitions in the d-d spectra reflect, in an attenuated way, the large energetic effects in the excited states when going from isolated to interconnec ted polyhedra, as manifested in a lowering of the Racah parameter B wi th increasing x. A further red shift of the charge transfer bands is o bserved when hole doping in NiO occurs (Ni1-deltaO, delta > 0) This re sult can be accounted for by extending the presented model to oxidic s ystems with hole doping. EPR experiments show that Ni(III) in oxide ce ramics possesses a low-spin d(7) ground state in isolated NiO6 centres (large Delta value), while a d(8) (L) under bar ground state may be p resent in the case of interconnected polyhedra with extended electron delocalization (smaller Delta value). (C) 1997 Elsevier Science B.V.