Endohedral Cluster Superconductors in the Mo–Ga–Sn System Explored by the Joint Flux Technique

Endohedral Ga cluster compounds feature nontrivial superconducting states including the two-gap superconductivity similar in nature to MgB2. 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 Mo8Ga41–x Sn x and Mo4Ga21–x–δSn 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 Mo8Ga41 superconductor, where Sn atoms are enclosed inside the Sn@Ga6 octahedral clusters, the latter is a new architecture built upon Mo@Ga9Sn clusters, Ga@Ga12 cuboctahedra, and Sn4 squares. We show that this novel Mo4Ga21–x–δSn 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 Mo8Ga41 superconductor with the closely related crystal structure.