Incorporation of gold into ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$: Structure and ${\mathit{T}}_{\mathit{c}}$ enhancement

The structural and superconducting properties of gold-doped ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ compounds have been studied with x-ray and neutron diffraction, optical microscopy, weight-loss analysis, magnetization, and resistivity measurements. The solubility of Au in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ is close to 10 at. %, below which all samples are thermally stable up to the processing temperature (950 °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 ${\mathrm{Au}}^{3+}$. 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, ${\mathit{T}}_{\mathit{c}}$ is slightly enhanced over that of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. Such an enhancement, observed for the first time for a Cu site dopant, is intrinsic and reproducible. We discuss the possible causes of the ${\mathit{T}}_{\mathit{c}}$ increase and the features of Au dopant useful in the investigation of high-${\mathit{T}}_{\mathit{c}}$ superconductivity.