We present a study of the crystal structure and physical properties of single crystals of a new
Fe-based ternary compound, Zr2−xFe4Si16−y (x = 0.81, y = 6.06). Zr1.19Fe4Si9.94 is a layered
compound, where stoichiometric β-FeSi2-derived slabs are separated by Zr-Si planes with
substantial numbers of vacancies. High resolution transmission electron microscopy
(HRTEM) experiments show that these Zr-Si layers consist of 3.5 nm domains where the Zr
and Si vacancies are ordered within a supercell sixteen times the volume of the stoichiometric
cell. Within these domains, the occupancies of the Zr and Si sites obey symmetry rules that
permit only certain compositions, none of which by themselves reproduce the average
composition found in x-ray diffraction experiments. Magnetic susceptibility and
magnetization measurements reveal a small but appreciable number of magnetic moments that
remain freely fluctuating to 1.8 K, while neutron diffraction confirms the absence of bulk
magnetic order with a moment of 0.2μB or larger down to 1.5 K. Electrical resistivity
measurements find that Zr1.19Fe4Si9.94 is metallic, and the modest value of the Sommerfeld
coefficient of the specific heat γ = C/T suggests that quasi-particle masses are not
particularly strongly enhanced. The onset of superconductivity at Tc 6 K results in a partial
resistive transition and a small Meissner signal, although a bulk-like transition is found in the
specific heat. Sharp peaks in the ac susceptibility signal the interplay of the normal skin depth
and the London penetration depth, typical of a system in which nano-sized superconducting
grains are separated by a non-superconducting host. Ultra low field differential magnetic
susceptibility measurements reveal the presence of a surprisingly large number of trace
magnetic and superconducting phases, suggesting that the Zr-Fe-Si ternary system could be a
potentially rich source of new bulk superconductors.