Low-temperature crystal symmetry of Nb3Cl8

Determine the low-temperature crystal symmetry of Nb3Cl8, distinguishing among the proposed R3, R-3m, and C2/m space groups.

Background

Nb3Cl8 undergoes a coupled structural and magnetic transition into a low-temperature singlet state accompanied by a rearrangement of the stacking sequence between van der Waals layers. The precise symmetry of this low-temperature phase is important for identifying the structural order associated with the transition and for interpreting spectroscopic signatures such as phonon modes and polarization responses.

The paper notes that several space groups have been proposed for the low-temperature phase of Nb3Cl8. Although the reported THz polarimetry measurements support the R-3m assignment and disfavor R3 and C2/m, the broader crystallographic issue is presented as unresolved in the literature.

References

In contrast, the low-temperature crystal symmetry of Nb$_3$Cl$_8$ remains under debate, with several space groups having been proposed, including $R3$, $R\bar{3}m$, and $C2/m$.

— Hysteresis without coexistence: disorder-rounded first-order transitions in a van der Waals magnet  (2608.19352 - Guo et al., 19 Aug 2026) in Section 1, Introduction