A15 Nb₃Si: a 'high' T_c superconductor synthesized at a pressure of one megabar and metastable at ambient conditions

A15 Nb₃Si is, until now, the only 'high' temperature superconductor produced at high pressure (∼110 GPa) that has been successfully brought back to room pressure conditions in a metastable condition. Based on the current great interest in trying to create metastable-at-room-pressure high temperature superconductors produced at high pressure, we have restudied explosively compressed A15 Nb₃Si and its production from tetragonal Nb₃Si. First, diamond anvil cell pressure measurements up to 88 GPa were performed on explosively compressed A15 Nb₃Si material to trace T _c as a function of pressure. T _c is suppressed to ∼5.2 K at 88 GPa. Then, using these T _c (P) data for A15 Nb₃Si, pressures up to 92 GPa were applied at room temperature (which increased to 120 GPa at 5 K) on tetragonal Nb₃Si. Measurements of the resistivity gave no indication of any A15 structure production, i.e. no indications of the superconductivity characteristic of A15 Nb₃Si. This is in contrast to the explosive compression (up to P ∼ 110 GPa) of tetragonal Nb₃Si, which produced 50%–70% A15 material, T _c = 18 K at ambient pressure, in a 1981 Los Alamos National Laboratory experiment. This implies that the accompanying high temperature (1000 °C) caused by explosive compression is necessary to successfully drive the reaction kinetics of the tetragonal → A15 Nb₃Si structural transformation. Our theoretical calculations show that A15 Nb₃Si has an enthalpy vs the tetragonal structure that is 70 meV atom⁻¹ smaller at 100 GPa, while at ambient pressure the tetragonal phase enthalpy is lower than that of the A15 phase by 90 meV atom⁻¹. The fact that 'annealing' the A15 explosively compressed material at room temperature for 39 years has no effect shows that slow kinetics can stabilize high pressure metastable phases at ambient conditions over long times even for large driving forces of 90 meV atom⁻¹. (paper)