Emergence of heavy-fermion behavior and distorted square nets in partially vacancy-ordered Y4FexGe8 (1.0 ≤ x ≤ 1.5)†

Disorders in intermetallic systems belonging to the CeNiSi₂-family are frequently overlooked. Even compounds presumed to be stoichiometric, such as YFeGe₂, can be misidentified. Here, we report a series of Y₄Fe_xGe₈ (1.0 ≤ x ≤ 1.5) compounds and show, using high-resolution synchrotron X-ray diffraction, that they feature asymmetrical structural distortions in the Fe and Ge sites that lead to a superstructure with partially ordered Fe vacancies and distorted Ge square-net in the triclinic crystal system, space group P with a = 11.4441(3) Å, b = 32.7356(7) Å, c = 11.4456(3) Å, α = 79.6330(10)°, β = 88.3300(10)°, and γ = 79.6350 (10)°. The unit cell is 16 times the conventional orthorhombic cell with the space group Cmcm. We identified the lower and upper limits for Fe in Y₄Fe_xGe₈ (1.0 ≤ x ≤ 1.5). Our physical property measurements yielded a Sommerfeld coefficient γ = 39.8 mJ mole⁻¹ K⁻², a Kadowaki–Woods ratio of 1.2 × 10⁻⁵ μΩ cm mole² K² mJ⁻², and a Wilson ratio of 1.83, suggesting heavy fermion behavior in the absence of f electrons, a rather rare case. Furthermore, we observed strong spin frustration and noted findings indicating possible superconductivity associated with the Fe content.