Metastable A15 phase Nb₃Si synthesized by high dynamic pressure

Nb3Si in the majority Ti3P-type tetragonal phase was subjected to high dynamic pressures of 0.82, 0.96, and 1.04 Mbar. Dynamic pressures were generated by the Lawrence Livermore National Laboratory's two-stage light-gas gun. This technique produces dynamic pressures of up to a Mbar and temperatures of up to a few thousand degrees Kelvin with high quench rates. High dynamic pressures induced a partial phase change to the cubic A15-type phase in our specimens. The synthesis of this phase is supported by X-ray diffraction, optical microscopy, electrical resistivity, specific heat, and ac magnetic susceptibility data. The midpoint of the resistive transition to the superconducting state occurs at 17.5 K in the specimen shocked to 0.96 Mbar. Upper critical magnetic field and the dependence of Tc on pressure have also been measured, yielding slopes of −1.20 tesla/K and — 3 × 10−5, respectively. The specific heat data indicate that about 67% of the specimen shocked to 1.04 Mbar was converted to the A15 superconducting phase.