Raman spectroscopy evidence for dimerization and Mott collapse in α-RuCl₃ under pressures

We perform Raman spectroscopy studies on α-RuCl₃ at room temperature to investigate its phase transitions of magnetism and structure under pressures. The Raman measurements resolve two critical pressures at p₁=1.1 GPa and p₂=1.7 GPa with very different structural and magnetic behaviors. With increasing pressure, a stacking order phase transition of α-RuCl₃ occurs at p₁, indicated by the inversion symmetry breaking and the appearance of new Raman modes. The second transition at p₂ is signaled by the in-plane Ru-Ru bond dimerization accompanied by the Mott collapse, and the system becomes a correlated band insulator. Our studies demonstrate the competition among spin-orbit coupling, magnetism, and chemical bondings in Kitaev compounds.