Superconducting properties and structure of Nb3Ge samples prepared by high pressure D.C. sputtering and by solid state diffusion

Properties of Nb-Ge A15 samples prepared by solid state diffusion and high pressure D.C. sputtering are reported and discussed in term of correlations between critical temperature, crystal structure and conditions of elaboration. In samples prepared by diffusion, it is found that the A15 phase may be obtained. So prepared compounds are in thermodynamical equilibrium whatever the annealing temperature up to 1 750°C and the quenching rate up to 1.6 10⁴K/s. When the annealing temperature is very closed to the melting point of the compound, Tconset may reach 16 K. In samples prepared by sputtering, it is found that the Nb/Ge ratio is not an important parameter while the substrate temperature is a very critical one. The highest Tc´s, close to 22 K are correlated with the elimination of a defect which is the hexagonal Nb5Ge3phase. From both kinds of experiment it is suggested that, in our samples, oxygen almost has no effect on superconducting properties in the low Tcrange but seems to be harmful to obtain critical temperatures in the 22 K range.