Endohedral Ga cluster compounds feature nontrivial superconducting states including the two-gap superconductivity similar in nature to MgB 2 . We use the joint flux synthetic technique to introduce Sn into the Ga matrix and tune the valence electron count in the two new endohedral cluster superconductors Mo 8 Ga 41−x Sn x and Mo 4 Ga 21−x−δ Sn x with critical temperatures of T c = 8.7 and 5.85 K, respectively. While the former compound is a derivative of the previously known Mo 8 Ga 41 superconductor, where Sn atoms are enclosed inside the Sn@Ga 6 octahedral clusters, the latter is a new architecture built upon Mo@Ga 9 Sn clusters, Ga@Ga 12 cuboctahedra, and Sn 4 squares. We show that this novel Mo 4 Ga 21−x−δ Sn x superconductor features strong electron−phonon coupling with the large ratio of 2Δ(0)/(k B T c ) = 4.1 similar to that of the Mo 8 Ga 41 superconductor with the closely related crystal structure. ■ INTRODUCTIONGa-rich 4d or 5d transition-metal-containing intermetallic compounds have recently attracted interest as a playground for discovering new superconductors. 1 Among this class of compounds

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In this paper, the potential existence of two-gap superconductivity in Mo8Ga41 is addressed in detail by means of thermodynamic and spectroscopic measurements. A combination of highly sensitive bulk and surface probes, specifically ac-calorimetry and scanning tunneling spectroscopy (STS), are utilized on the same piece of crystal and reveal the presence of only one intrinsic gap in the system featuring strong electron-phonon coupling. Minute traces of additional superconducting phases detected by STS and also in the heat capacity measured in high magnetic fields on a high-quality and seemingly single-phase crystal might mimic the multigap superconductivity of Mo8Ga41 suggested recently in several studies.