We investigated the structural and magnetic properties of single-crystalline ${\mathrm{SmCuAs}}_2$ using high-resolution synchrotron x-ray diffraction and x-ray resonant magnetic scattering (XRMS) at the Sm $L_2$ and $L_3$ edges. Temperature-dependent diffraction measurements confirm that ${\mathrm{SmCuAs}}_2$ maintains its tetragonal symmetry from room temperature down to 8 K, with lattice parameters showing anomalous behavior below the resistivity minimum ($T\approx 30\mathrm{K}$). Notably, the c-axis lattice parameter exhibits a plateau and subsequent increase near the Néel temperature, indicating magnetoelastic coupling. XRMS measurements reveal a commensurate antiferromagnetic structure with a propagation vector $\mathit{q}=(0,0,0.5)$. Our measurement shows that the Sm moments are aligned within the $\mathit{ab}$ plane and arranged in a $++--$ stacking along the $\mathit{c}$ axis. Comparison with related $RE{\mathrm{CuAs}}_2$ compounds (RE = Pr, Nd, and Gd) suggests that in-plane moment orientation and associated magnetic frustration play a key role in the emergence of the resistivity minimum. Differences in spin-orbit and magnetoelastic coupling across the series highlight their importance in governing low-temperature transport behavior.

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• Magnetic order
• Magnetism
• Metals
• Rare-earth alloys
• X-ray resonant magnetic scattering