Pressure-induced symmetry lowering in Nb3Sn1-xsuperconductor.

CubicPm-3nNb3Sn0.92superconductor (Tc∼ 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 Nb3Sn1-xsystem for higher Sn content at temperatures lower than ∼43 K. Nevertheless, the low-temperature structure of Nb3Sn0.92remains 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,Tc, 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 Nb3Sn1-xinduced by increase in Sn content. Therefore targeted changes in composition could be used to enhance upper critical field of Nb3Sn1-xfor specific extreme conditions of temperature and pressure.