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.