Superconductivity in Nb5Si3

Abstract

A sample of the high-temperature (β) phase of Nb5Si3, possessing the tetragonal W5Si3 structure, had a superconducting transition temperature T0 of 0.7 K and a transition width of about 0.25 K. The value of −67 mT/K (670 Oe/K) for the initial slope of the curve of the critical magnetic field versus temperature indicates that β-Nb5Si3 is a type-II superconductor. The dominant structural feature of β-Nb5Si3 is the parallel linear chains of closely spaced Nb atoms which occur along the tetragonal c axis. The Nb-Nb intrachain spacing is 0.254 nm, very close to the Nb-Nb spacing of the mutually orthogonal linear chain atoms in a hypothetical cubic A15-Nb3Si structure. The near-neighbor environment of the Nb chain atoms is virtually identical in A15-Nb3Si and β-Nb5Si3. However, the T0 of A15-Nb3Si is expected to be greater than 20 K, much higher than the T0 of β-Nb5Si3. The presence of closely spaced Nb atoms in linear chains therefore does not appear to be a sufficient condition for a high T0. The low-temperature (α) phase of Nb5Si3 was found to be normal down to 0.01 K.