High T<inf>C</inf>phases in the Nb-Si, Nb-Ge, and the Nb-Ge-Si ternary systems

Abstract

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 T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> phases. The first one, observed in the Nb-Si system, in the range of 8-13 at% Si, is cubic with a lattice parameter a <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> falling from 5.195 to 5.186 Å, with T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> values of 9.5 to 17.7 K, respectively. Our analysis of a sample prepared by Dew-Hughes and Linse shows the existence of the Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</inf> Si phase with a T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> of 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 T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> phase, which we first found around the Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Ge composition, is a tetragonal phase with lattice parameters of about a <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> =10.175 and C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</inf> =5.151 Å. It is similar to the D8 <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> type, but with line intensities distinctly different. Its resistive T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> is about 18 K. The highest resitive T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> 's of up to 19.5 K were found for compositions near Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Ge <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">.7</inf> Si <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">.3</inf> . 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 T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> .