GOLD EFFECT ON CHEMICAL BONDING IN YBA2(CU1-Y, AUY)3O6+X

Single crystals of YBa2(Cu1-y, Au(y))3O6+x were synthesized during the growth of 123 crystals in Au crucibles. The structure of YBa2(Cu0.967Au0.033)3O6.50 has been determined by X-ray diffraction data taken with an automatic diffractometer equipped with MoK-alpha-radiation. The P/4mmm space group was used in the final refinement. The lattice parameters determined by the Gandolfi technique were: a = 3.842(1)-angstrom, b = 3.878(1)-angstrom, c = 11.765(2)-angstrom. Electron diffraction patterns did not exhibit any superstructure reflections or diffuse scattering. Our results confirm those published earlier by Wong-Ng et al. who showed that the Au cations substitute for Cu(1). The interatomic distances compared to those calculated from the Shannon radii show that the Au cations are in the 3+ valence state and have a square coordination. Since the Au cations are isolated and 3-coordinated Cu cations are unusual, the Au incorporation pins down the chain formation along one axis and the orthorhombicity of the samples occurs at lower oxygen stoichiometry. The most important consequence of the Au-doping at a given oxygen content is the shortening of the Cu(2)-O(1) bond which explains why Au-doping has a small effect on T(c). The Au(1)-O(1) distance being larger than Cu(1)-O(1) one induces a pressure effect on Cu(2)-O(1) bond.