Crystal structure and superconductivity of NaAlSi1–xGex single crystals

NaAlSi and NaAlGe are nodal-line semimetals with an anti-PbFCl type structure. The former is a superconductor with a transition temperature Tc of 7 K, while the latter does not exhibit superconductivity. We prepared single crystals of solid solution NaAlSi1–xGex by the Na–Ga flux method and investigated their crystal structures and superconductivity by X-ray structural analysis and magnetization measurements, respectively. The crystal structure parameters vary continuously with x, while the Tc initially decreases gradually from 6.9 K for x = 0 to 5.6 K for x = 0.33 and then decreases rapidly to below 1.8 K for x = 0.45. This sudden variation in Tc at x ~ 0.4 may not be explained by the decrease in phonon frequency due to the Ge substitution, but suggests an unusual change in the electronic structure, in spite that the calculated band structures of the end members are almost identical with nearly equal density of states.