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 $\mathrm{Li}{\mathrm{Pd}}_{2}X$ ($X=\mathrm{Si}$, Ge, and Sn). LeBail refinement of powder x-ray diffraction data confirms that all compounds belong to the Heusler family (space group $Fm\text{−}3m$, No. 225). The lattice parameter increases with atomic size of X, and its value varies from $a=5.9059\left(4\right)\AA$ for $\mathrm{Li}{\mathrm{Pd}}_2\mathrm{Si}$ and $a=6.0082\left(3\right)\AA$ for $\mathrm{Li}{\mathrm{Pd}}_2\mathrm{Ge}$, to $a=6.2644\left(1\right)\AA$ for $\mathrm{Li}{\mathrm{Pd}}_2\mathrm{Sn}$. The first compound, $\mathrm{Li}{\mathrm{Pd}}_2\mathrm{Si}$, has apparently not been previously reported. All measured quantities demonstrate that $\mathrm{Li}{\mathrm{Pd}}_2\mathrm{Ge}$ exhibits superconductivity below $T_c=1.96\mathrm{K}$ and the normal- and superconducting-state data indicate that it is a weak-strength type-I superconductor ($C/\gamma T_c=1.38$) with electron-phonon coupling constant ${\lambda }_{e-p}=\left(0.53-0.56\right)$. $\mathrm{Li}{\mathrm{Pd}}_2\mathrm{Si}$ and $\mathrm{Li}{\mathrm{Pd}}_2\mathrm{Sn}$ are not superconducting above 1.68 K. The experimental observations are supported by theoretical calculations which show that $\mathrm{Li}{\mathrm{Pd}}_2\mathrm{Ge}$ has the highest computed ${\lambda }_{e-p}$ and $T_c$ of the group. A strong softening of the acoustic phonon mode is calculated, and in the case of $X=\mathrm{Ge}$ and Sn, imaginary phonon frequencies were computed. The soft mode is most pronounced in the case of $\mathrm{Li}{\mathrm{Pd}}_2\mathrm{Ge}$, which suggests its correlation with superconductivity.

• Received 20 April 2020
• Revised 11 June 2020
• Accepted 16 June 2020

DOI:https://doi.org/10.1103/PhysRevB.102.024507