Pressure-tuning of $\alpha$-RuCl$_3$ towards the ideal Kitaev-limit

We report the discovery of an intriguing pressure-driven phase transformation in the layered Kitaev-material $\alpha$-RuCl$_3$. By analyzing both the Bragg scattering as well as the diffuse scattering of high-quality single crystals, we reveal a collective reorganization of the layer stacking throughout the crystal. Importantly, this transformation also effects the structure of the RuCl$_3$ honeycomb layers, which acquire a high trigonal symmetry with a single Ru--Ru distance of 3.41\r{A} and a single Ru--Cl--Ru bond angle of 92.8{\deg}. Hydrostatic pressure therefore allows to tune the structure of $\alpha$-RuCl$_3$ much closer to the ideal Kitaev-limit. The high-symmetry phase can also be stabilized by biaxial stress, which can explain conflicting results reported earlier and, more importantly, makes the high-symmetry phase accessible to a variety of experiments.