Pressure-induced symmetry lowering in Nb₃Sn_1-superconductor

CubicPm-3nNb₃Sn_0.92superconductor (T_c∼ 16 K) was found to exhibit tetragonal instabilities at the superconducting state (T= 10 K). These instabilities are manifested through the appearance of reflections which are forbidden in thePm-3nsymmetry but are compatible with theP4₂/mmcstructure which is observed in the Nb₃Sn_1-system for higher Sn content at temperatures lower than ∼43 K. Nevertheless, the low-temperature structure of Nb₃Sn_0.92remains metrically fully cubic, as concluded from single crystal synchrotron radiation diffraction experiments. Subsequent application of external pressure amplifies the observed instabilities with a resulting pseudo-cubic-tetragonal transformation atP= 3 GPa at 10 K and this transition is energy driven, as concluded fromab initiocalculations. The electronic structures of the corresponding phases are virtually identical and, therefore, the pseudo-cubic-tetragonal transformation does not influence significantly the underlying electronic interactions. Consequently, no anomalies in the behavior of the critical temperature,T_c, are expected at this pressure. However, anomalies in the upper critical field are anticipated during this transition, in analogy to the corresponding behavior observed during the cubic-tetragonal transformation in Nb₃Sn_1-induced by increase in Sn content. Therefore targeted changes in composition could be used to enhance upper critical field of Nb₃Sn_1-for specific extreme conditions of temperature and pressure.