Raman scattering has been employed to investigate lattice and magnetic
excitations of the honeycomb Kitaev material α-RuCl3​ and its
Heisenberg counterpart CrCl3​. Our phonon Raman spectra give evidence for a
first-order structural transition from a monoclinic to a rhombohedral structure
for both compounds. Significantly, only α-RuCl3​ features a large
thermal hysteresis, consistent with the formation of a wide phase of
coexistence. In the related temperature interval of 70−170 K, we observe a
hysteretic behavior of magnetic excitations as well. The stronger magnetic
response in the rhombohedral compared to the monoclinic phase evidences a
coupling between the crystallographic structure and low-energy magnetic
response. Our results demonstrate that the Kitaev magnetism concomitant with
fractionalized excitations is susceptible to small variations of bonding
geometry.Comment: 9 pages, 8 figures, To appear in PR