Contrasting structural reversibility and magnetic correlations in isostructural honeycomb magnets CrCl${}_3$ and $\alpha$-RuCl${}_3$

We report a comparative neutron single crystal diffraction study of the structural and magnetic properties of layered halides CrCl${}_3$ and $\alpha$-RuCl${}_3$, which host a honeycomb arrangement of transition metal ions with distinct electronic configurations and undergo a first-order structural transition between high-temperature $\mathit{\text{C}}$2/$\mathit{\text{m}}$ and low-temperature $\mathit{\text{R}}$$\bar{3}$. Both compounds show a step-like change in the $c$-lattice, consistent with an expected stacking rearrangement. In contrast, the in-plane lattice response is quite different: $\alpha$-RuCl${}_3$ exhibits an abrupt hysteretic change across the transition accompanied by progressive crystalline degradation upon thermal cycling, whereas CrCl${}_3$ shows a smooth in-plane lattice evolution and remains structurally robust. Magnetically, 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. $\alpha$-RuCl${}_3$ shows no observable magnetic diffuse scattering above its zig-zag antiferromagnetic ordering temperature T${}_N$=7.6,K. These results suggest that the contrasting structural and magnetic behaviors arise from an interplay between interlayer sliding energetics and the fundamentally different electronic configurations of the two compounds.