Intrinsic nanostructure in Zr_2-xFe₄Si _16-y(x=0.81, y=6.06)

We present a study of the crystal structure and physical properties of single crystals of a new Fe-based ternary compound, Zr_2-xFe ₄Si_16-y(x=0.81, y=6.06). Zr_1.19Fe ₄Si_9.94 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 3.5nm 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 1.8K, while neutron diffraction confirms the absence of bulk magnetic order with a moment of 0.2μ_B or larger down to 1.5K. Electrical resistivity measurements find that Zr _1.19Fe₄Si_9.94 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 T_c≃6K 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.