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A15 Nb$_3$Si is, until now, the only high temperature superconductor producedat high pressure (~110 GPa) that has been successfully brought back to roompressure conditions in a metastable condition. Based on the current greatinterest in trying to create metastable-at-room-pressure high temperaturesuperconductors produced at high pressure, we have restudied explosivelycompressed A15 Nb$_3$Si and its production from tetragonal Nb$_3$Si. First,diamond anvil cell pressure measurements up to 88 GPa were performed onexplosively compressed A15 Nb$_3$Si material to trace Tc as a function ofpressure. Tc is suppressed to ~ 5.2 K at 88 GPa. Then, using these Tc (P) datafor A15 Nb$_3$Si, pressures up to 92 GPa were applied at room temperature(which increased to 120 GPa at 5 K) on tetragonal Nb$_3$Si. Measurements of theresistivity gave no indication of any A15 structure production, i.e., noindications of the superconductivity characteristic of A15 Nb$_3$Si. This is incontrast to the explosive compression (up to P~110 GPa) of tetragonal Nb$_3$Si,which produced 50-70% A15 material, Tc = 18 K at ambient pressure, in a 1981Los Alamos National Laboratory experiment. Our theoretical calculations showthat A15 Nb$_3$Si has an enthalpy vs the tetragonal structure that is 0.07eV/atom smaller at 100 GPa, implying that the accompanying high temperature(1000 deg C) caused by explosive compression is necessary to successfully drivethe reaction kinetics of the tetragonal -> A15 Nb$_3$Si structuraltransformation. Annealing experiments on the A15 explosively compressedmaterial reaching time scales of 39 years are consistent with this viewpoint.

A15 Nb$_3$Si -- A "high" Tc superconductor synthesized at a pressure of one megabar and metastable at ambient conditions