Superconductivity in Y4RuGe8 with a Vacancy-Ordered CeNiSi2‑Type Superstructure

We report a new compound, Y4RuGe8, with a transition metal vacancy-ordered CeNiSi2-type superstructure, which has a superconducting transition at 1.3 K. Y4RuGe8 crystals were grown by indium flux at relatively low temperatures (below 1273 K), which makes it possible to stabilize such a vacancy-ordered phase. The crystal structure of Y4RuGe8 was solved by single-crystal X-ray diffraction and confirmed by transmission electron microscopy. The as-grown Y4RuGe8 crystals are always twinned, crystallizing in the space group P1̅ (no. 2) with the lattice parameters a = 5.7680(1) Å, b = 8.2042(2) Å, c = 11.5093(3) Å, α = 79.696(1)°, β = 88.491(1)°, and γ = 79.637(2)°; this structure is a superstructure deriving from the higher symmetry CeNiSi2-type structure (Cmcm, no. 63) due to the ordering of Ru vacancies. The ordering of Ru sites breaks slightly distorted Ge planes in the CeNiSi2 prototype into infinite cis–trans Ge chains in Y4RuGe8. The presence of bulk superconductivity in Y4RuGe8 is well supported by zero resistance and a jump in specific heat at the critical transition temperature. The Sommerfeld coefficient (19 mJ K–2 mol–1) of the specific heat is greater than that (11 mJ K–2 mol–1) estimated using the bare density of states (4.7 states/eV/f.u.) from first-principles calculations. The ab initio calculations indicate that 4d electrons of both Y and Ru and 4p electrons of Ge are the main contributors to the total density of states at the Fermi level in Y4RuGe8.