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

Abstract

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 \textit{C}2/\textit{m} and low-temperature \textit{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.