Anisotropic Structural and Magnetic Response in Sm$_2$Ir$_8$Al$_{31}$: Al-Sublattice Reconstruction upon Sm Substitution

Abstract

Chemical pressure plays a critical role in determining the crystal structures and physical properties of intermetallic compounds. Here, we report the synthesis, crystal structure, and physical properties of Sm$_2$Ir$_8$Al$_{31}$, a structural analog of UIr$_4$Al$_{15}$, in which the smaller 5f uranium atoms are replaced by larger 4f samarium atoms. Single-crystal X-ray diffraction reveals a tetragonal $\sqrt{2}\times\sqrt{2}\times1$ superstructure with space group $P\bar{4}2_1m$, derived from a UIr$_4$Al$_{15}$-like average structure with space group $P4_2/nmc$. Comparison with UIr$_4$Al$_{15}$ shows that chemical pressure from U-to-Sm substitution produces a strongly anisotropic structural response rather than a uniform expansion of the Ir-Al framework. While one Ir coordination environment undergoes only modest changes, the other exhibits substantial local rearrangement. The Al sublattice primarily accommodates these structural distortions, with selected Al sites showing positional splitting, anisotropic atomic displacement ellipsoids, and partial occupancies, resulting in an Al-rich composition of approximately Sm$_2$Ir$_8$Al$_{31}$. Magnetic susceptibility measurements reveal a direction dependence, including a broad anomaly near 50 K for selected magnetic-field orientations. However, the absence of corresponding anomalies in heat capacity, electrical resistivity, and lattice parameters suggests that this feature is more consistent with an anisotropic magnetic crossover than a bulk thermodynamic phase transition. These results highlight how chemical substitution at the f-element site can induce Al-sublattice reconstruction and modify the magnetic response within a closely related Ir-Al structural framework.