The 112-type ternary pnictides often exhibit intriguing physical properties arising from their two-dimensional layered structures. Here, we report two new members of this family, ${\mathrm{CeMg}}_{0.66}\mathrm{S}{\mathrm{b}}_2$ and ${\mathrm{PrMg}}_{0.73}\mathrm{S}{\mathrm{b}}_2$, which possess the highest $c/a$ ratio and the lowest magnetic ordering temperature among known compounds in this series. The small ${\mathrm{Mg}}^{2+}$ results in such a high lattice anisotropy and plays a role of chemical pressure, which helps to drive the Ce/Pr magnetic ordering close to a quantum critical point. Single-crystal x-ray diffraction confirms that both compounds have the two-dimensional square nets of Ce/Pr and Sb atoms and large $c/a$ ratio. Magnetic measurements reveal anisotropic magnetic ground states: ${\mathrm{CeMg}}_{0.66}\mathrm{S}{\mathrm{b}}_2$ shows in-plane ferromagnetic ordering at $T_{\mathrm{c}}=2.44$ K, and out-of-plane antiferromagnetic ordering at $T_{\mathrm{N}}=2.47$ K; ${\mathrm{PrMg}}_{0.73}\mathrm{S}{\mathrm{b}}_2$ exhibits in-plane antiferromagnetic order at $T_{\mathrm{N}}=2.36$ K, while no magnetic ordering is observed out of plane. These findings expand the 112-type pnictide family and offer a valuable platform for studying low-temperature magnetism in layered square-net systems. Furthermore, by comparing isostructural analogs, we establish an empirical correlation between the $c/a$ ratio and the magnetic transition temperature, providing a practical guideline for the targeted design of new 112-type materials.

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• Crystal structures
• Pnictides
• Electrical transport techniques
• Magnetization measurements
• X-ray diffraction