High TCphases in the Nb-Si, Nb-Ge, and the Nb-Ge-Si ternary systems

In the course of a systematic investigation of the Nb-Ge-Si binary and ternary systems we have found, apart from the A15-phase, two new high Tcphases. The first one, observed in the Nb-Si system, in the range of 8-13 at% Si, is cubic with a lattice parameter a0falling from 5.195 to 5.186 Å, with Tcvalues of 9.5 to 17.7 K, respectively. Our analysis of a sample prepared by Dew-Hughes and Linse shows the existence of the Nb7Si phase with a Tcof 17.7 K. The primitive cubic x-ray line pattern and the lattice parameter indicate some relation with the A15 phase; however, the line intensities and residual resistance ratios are distinctly different. The second high Tcphase, which we first found around the Nb2Ge composition, is a tetragonal phase with lattice parameters of about a0=10.175 and CO=5.151 Å. It is similar to the D8mtype, but with line intensities distinctly different. Its resistive Tcis about 18 K. The highest resitive Tc's of up to 19.5 K were found for compositions near Nb2Ge.7Si.3. These samples showed no detectable presence of the A15-phase. Either the above tetragonal phase or a small superconducting fraction of the hexagonal C40 phase could be the cause of the observed Tc.