Superconductivity-Electron Count Relationship in Heusler Phases-the Case of LiPd 2 Si.

We report superconductivity in the full Heusler compound LiPd 2 Si (space group Fm3̅m , No. 225) at a critical temperature of T c = 1.3 K and a normalized heat capacity jump at T c , Δ C /γ T c = 1.1. The low-temperature isothermal magnetization curves imply type-I superconductivity, as previously observed in LiPd 2 Ge. We show, based on density functional theory calculations and using the molecular orbital theory approach, that while LiPd 2 Si and LiPd 2 Ge 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.