Incorporation of gold into YBa2Cu3O7: Structure and Tc enhancement

The structural and superconducting properties of gold-doped YBa2Cu3O7 compounds have been studied with x-ray and neutron diffraction, optical microscopy, weight-loss analysis, magnetization, and resistivity measurements. The solubility of Au in YBa2Cu3O7 is close to 10 at. %, below which all samples are thermally stable up to the processing temperature (950 degree C). Metallic Au grains precipitate out when the Au content exceeds 10 at. %. Analysis of the x-ray and neutron-scattering intensities shows that Au substitutes for the Cu(1) chain site exclusively and has a probable valence state of Au3+. The main effect of Au doping is a substantial uniaxial lattice expansion, while the orthorhombic structure is preserved. We present a detailed set of structural data including the Cu-O bond lengths and interplanar distances in a Au-doped sample. Unlike all other known dopants on Cu sites, the Au dopant is not detrimental to the superconducting properties. On the contrary, Tc is slightly enhanced over that of YBa2Cu3O7. Such an enhancement, observed for the first time for a Cu site dopant, is intrinsic and reproducible. We discuss the possible causes of the Tc increase and the features of Au dopant useful in the investigation of high-Tc superconductivity