Pressure induced structural phase transition and dimer formation in $\alpha$-RuCl${}_3$

Currently $\alpha$-RuCl${}_3$ is the most promising candidate for the long sought realization of the Kitaev model, which is believed to be very close to a gapless spin-liquid. Under ambient conditions, however, $\alpha$-RuCl${}_3$ exhibits antiferromagnetic ordering below 7 K, but several experimental studies indicate the proximity to the Kitaev limit nonetheless. In accordance very recent magnetization measurements as a function of pressure revealed a significant decrease in the magnetization. We therefore performed single crystal XRD as a function of pressure in the range $<$ 2 GPa at 4 - 300 K to determine the structural changes related to the magnetic transition. Our study shows that the monoclinic phase under ambient conditions transforms into a metastabile hexagonal phase followed by a trigonal phase featuring strong Ru-Ru dimer bonds. Specifically the latter transition coincides with the observation of a significant decrease in the magnetization. One possible explanation for this effect is the formation of a S=0 singlet state on the Ru-Ru dimers.