The transition-metal chalcogenides ${\mathrm{Fe}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ and ${\mathrm{Ni}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ have been widely studied in recent years due to their remarkable properties, such as strong electron correlations and the coexistence of antiferromagnetism and Mott physics, which make these materials attractive for exploring fundamental and applied 2D physics. One powerful approach to tailor the magnetic order in these 2D van der Waals compounds is to exploit the strong magnetoelastic coupling, which is essential for multifunctional applications. In this work, the magnetoelastic coupling and the uniaxial pressure dependence of the antiferromagnetic ordering temperature $T_{\text{N}}$ are studied and quantified by high-resolution dilatometry. Using the Clausius-Clapeyron and Ehrenfest relations, we find that uniaxial pressure applied along the stacking axis ($c^{*}$) enhances the $T_{\text{N}}$ in both compounds, which is verified via direct measurements for ${\mathrm{Fe}}_2{\mathrm{P}}_2{\mathrm{S}}_6$, i.e., magnetic susceptibility under applied uniaxial pressure. Remarkably, despite the orbitally degenerate ${\mathrm{Fe}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ exhibiting a much stronger magnetoelastic coupling compared to the nondegenerate ${\mathrm{Ni}}_2{\mathrm{P}}_2{\mathrm{S}}_6$, their uniaxial pressure dependence remains of the same order of magnitude, suggesting that it primarily arises from the van der Waals interactions between adjacent sulfur layers.

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• Magnetic order
• Magnetism
• Magnetoelastic effect
• Specific heat
• Thermal expansion
• Antiferromagnets
• van der Waals systems
• Pressure techniques