Soft-mode enhanced type-I superconductivity in LiPd2Ge

The synthesis, crystal structure, and physical properties (magnetization, resistivity, heat capacity) in combination with theoretical calculations of the electronic structure and phonon properties are reported for intermetallic compounds LiPd2X (X=Si, Ge, and Sn). LeBail refinement of powder x-ray diffraction data confirms that all compounds belong to the Heusler family (space group Fm-3m, No. 225). The lattice parameter increases with atomic size of X, and its value varies from a=5.9059(4)Å for LiPd2Si and a=6.0082(3)Å for LiPd2Ge, to a=6.2644(1)Å for LiPd2Sn. The first compound, LiPd2Si, has apparently not been previously reported. All measured quantities demonstrate that LiPd2Ge exhibits superconductivity below Tc=1.96K and the normal- and superconducting-state data indicate that it is a weak-strength type-I superconductor (C/γTc=1.38) with electron-phonon coupling constant λe-p=(0.53-0.56). LiPd2Si and LiPd2Sn are not superconducting above 1.68 K. The experimental observations are supported by theoretical calculations which show that LiPd2Ge has the highest computed λe-p and Tc of the group. A strong softening of the acoustic phonon mode is calculated, and in the case of X=Ge and Sn, imaginary phonon frequencies were computed. The soft mode is most pronounced in the case of LiPd2Ge, which suggests its correlation with superconductivity.