Structural transition and magnetic anisotropy in α-RuCl3

We report x-ray diffraction and magnetic susceptibility studies of the structural phase transition in alpha -RuCl 3 . By utilizing a single crystal sample with a predominantly single -twin domain, we show that alpha -RuCl 3 goes from a high -temperature C 2 / m structure to a rhombohedral structure with R 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 -RuCl 3 , which could explain the sample dependence found in this material.