LOCAL AND COOPERATIVE BONDING EFFECTS IN K2NIF4-TYPE COPPER SUPERCONDUCTORS - STRUCTURAL, SPECTROSCOPIC AND MAGNETIC INVESTIGATIONS ON MIXED-CRYSTALS LA1+XSR1-XGA1-XCUXO4

Mixed crystals between LaSrGaO4 and La2CuO4 (K2NiF4 structure) are prepared and the change in the nature of the equatorial Cu-O bonds with increasing Cu2+ concentration is analysed by structural, spectroscopic (optical, IR, ESR) and magnetic methods. The results give evidence that in La2CuO4 broad overlapping bonding bands, connected with the (001) plane of the tetragonal structure, extend nearly up to the higher occupied band b1g* (dx2-y2), which is rather narrow (< 1.8 eV). The b1g* electrons are mainly localized on copper, as deduced from the EPR spectra. The doping of La2CuO4 with Sr2+ (Ba2+) seems to induce an overlap of the bonding bands with the narrow band in the Fermi region such that holes are created in the bonding (''Cu2+-O-'') rather than in the b1g* band (''Cu3+-O2-''). A vibronic superconductivity mechanism is suggested. Already a few mole% of Ga3+ on the Cu2+ sites suppress the superconductivity above 5 K. Finally results are presented, which demonstrate the diversity of oxygen and Cu2+ with respect to their bonding behaviour - depending on the structure and the chemical constitution of the host compound.