We report a comparative neutron single crystal diffraction study of the structural and magnetic properties of layered halides ${\mathrm{CrCl}}_3$ and $\alpha \text{−}{\mathrm{RuCl}}_3$. They host a honeycomb arrangement of transition metal ions with distinct electronic configurations and undergo a first-order structural transition between high-temperature $C2/m$ and low-temperature $R\overline{3}$. Both compounds show a step-like change in the $c$-lattice parameter across the structure transition. In contrast, the in-plane lattice response is quite different: $\alpha \text{−}{\mathrm{RuCl}}_3$ exhibits an abrupt hysteretic change across the transition accompanied by progressive crystalline degradation upon thermal cycling, whereas ${\mathrm{CrCl}}_3$ shows a smooth in-plane lattice evolution and remains structurally robust. Magnetically, ${\mathrm{CrCl}}_3$ orders into an A-type antiferromagnetic structure at $T_N=14$ K and exhibits pronounced diffuse magnetic scattering extending up to about 40 K. $\alpha \text{−}{\mathrm{RuCl}}_3$ shows no observable magnetic diffuse scattering above its zigzag antiferromagnetic ordering temperature $T_N=7.6$ K. These results suggest that the contrasting structural responses arise from an interplay between interlayer sliding energetics, stacking-strain coupling, and elastic accommodation of the stacking transition. The distinct chemical bonding and electronic configurations of the two compounds provide a microscopic basis for their different lattice responses to the structure transition and magnetic correlations.

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• Magnetic order
• Phase transitions
• Layered crystals
• Single crystal materials
• Neutron scattering