Influence of pressure on the superconducting critical temperature of explosively precompressed metastable A15 Nb3Si*

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. Using these T_c (P) data for A15 Nb₃Si, applying pressures up to 120 GPa at room temperature on tetragonal Nb₃Si and measuring superconductivity present via resistivity gave no indication of any transition to the A15 structure. This is in contrast to the explosive compression (up to P ~ 110 GPa) of tetragonal Nb₃Si, which produced 50-70% A15 material and T_c = 18 K at ambient pressure in a 1981 Los Alamos National Laboratory experiment. We believe that, despite theoretical calculations that A15 Nb₃Si has an enthalpy vs the tetragonal structure that is 0.07 eV/atom smaller at 100 GPa, it is the accompanying high temperature (1000 ^oC) caused by explosive compression that is necessary to successfully drive the reaction kinetics of the tetragonal to A15 Nb₃Si structural transformation. We performed annealing experiments on the A15 explosively compressed material that are consistent with this viewpoint.