Superconductivity-Electron Count Relationship in Heusler Phases─the Case of LiPd₂Si

We report superconductivity in the full Heusler compound LiPd₂Si (space group Fm3̅m, No. 225) at a critical temperature of T_c = 1.3 K and a normalized heat capacity jump at T_c, ΔC/γT_c = 1.1. The low-temperature isothermal magnetization curves imply type-I superconductivity, as previously observed in LiPd₂Ge. We show, based on density functional theory calculations and using the molecular orbital theory approach, that while LiPd₂Si and LiPd₂Ge share the Pd cubic cage motif that is found in most of the reported Heusler superconductors, they show distinctive features in the electronic structure. This is due to the fact that Li occupies the site which, in other compounds, is filled with an early transition metal or a rare-earth metal. Thus, while a simple valence electron count-property relationship is useful in predicting and tuning Heusler materials, inclusion of the symmetry of interacting frontier orbitals is also necessary for the best understanding.