From endohedral cluster superconductors to approximant phases: synthesis, crystal and electronic structure, and physical properties of Mo8Ga41-xZnx and Mo7Ga52-xZnx.

Using the crystal-growth joint flux technique based on the combination of two aliovalent low-melt metals, gallium and zinc, we adjust the gross valence electron count in the Mo-Ga-Zn system and produce the Mo8Ga41-xZnx and Mo7Ga52-xZnx intermetallic compounds. Gradual reduction in the valence electron count first leads to the Zn for Ga substitution in the Mo8Ga41 endohedral cluster superconductor, accompanied by the formation of Zn-containing clusters in the crystal structure and by the gradual suppression of superconductivity. Mo8Ga41-xZnx with x = 7.2(2) exhibits superconducting properties below TC = 4 K, whereas there is no superconducting transition at temperatures above 2 K for the limiting composition of x = 11.3(2). Further, the Mo7Ga52-xZnx phase is formed from the flux with a higher content of Zn. Mo7Ga52-xZnx crystallizes in the Mo7Sn12Zn40 structure type with a narrow homogeneity range and exhibits metallic behavior with no sign of superconductivity down to at least 1.8 K. Its experimental valence electron count of 2.9 e per atom is below that of endohedral gallium cluster superconductors. Electronic structures of Mo8Ga41-xZnx and Mo7Ga52-xZnx feature the opening of a pseudogap slightly below the Fermi level indicating the specific stability of these structure types at the valence electron count of 3.2 e per atom and 2.7 e per atom, respectively.