We report x-ray diffraction and magnetic susceptibility studies of the structural phase transition in $\alpha \text{−}{\mathrm{RuCl}}_3$. By utilizing a single crystal sample with a predominantly single-twin domain, we show that $\alpha \text{−}{\mathrm{RuCl}}_3$ goes from a high-temperature $C2/m$ structure to a rhombohedral structure with $R\overline{3}$ symmetry at low temperature. While the defining feature of the structural transition is changing the stacking direction from the monoclinic $a$ axis to the $b$ axis, bond-length anisotropy disappears when the structural change occurs, indicating that the local $C_3$ symmetry is restored within the honeycomb layer. The symmetry change is corroborated by the vanishing magnetic anisotropy in the low-temperature structure. Our study demonstrates that the magnetic interaction is extremely sensitive to the structural details in $\alpha \text{−}{\mathrm{RuCl}}_3$, which could explain the sample dependence found in this material.

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• Crystal structure
• Magnetic interactions
• Magnetic susceptibility
• Structural phase transition