Synthesis, Crystal Structure, and Magnetism of the New UIr$_{4}$Al$_{15}$ Compound

We report on single-crystals of a new compound grown from Al flux, UIr$_{4}$Al$_{15}$, crystallizing in the NdRh$_{4}$Al$_{15.4}$ type structure in the tetragonal space group $P$4$_{2}$/\textit{nmc} (No. 137) with unit cell parameters $a$ = 9.0239(6) {\AA} and $c$ = 15.513(2) {\AA}. The crystal structure of the compound was established from single-crystal X-ray diffraction and was found to be void of any crystallographic disorder. The U atoms center a polyhedron with the coordination number of each U atom being 20. The physical properties of UIr$_{4}$Al$_{15 }$were measured which included magnetic susceptibility, specific heat, and electrical resistivity. The compound was found to undergo long-range antiferromagnetic order at T$_{N}$ = 20 K with a Sommerfeld coefficient extrapolated in the antiferromagnetically ordered state of \textit{ca}. 42 mJ/mol-K$^{2}$. The physical properties suggest that UIr$_{4}$Al$_{15 }$may be an itinerant antiferromagnet. The compound belongs to a series of isostructural compounds, and preliminary results indicate that Th, La, Ce, Pr, Sm, Yb, and Lu analogues can be synthesized under identical conditions.