Raman spectroscopy evidence for dimerization and Mott collapse in α − RuCl 3 under pressures

We perform Raman spectroscopy studies on $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ at room temperature to investigate its phase transitions of magnetism and structure under pressure. The Raman measurements resolve two critical pressures at ${p}_{1}=1.1$ GPa and ${p}_{2}=1.7$ GPa with very different structural and magnetic behaviors. With increasing pressure, a stacking order phase transition of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ occurs at ${p}_{1}$, indicated by the new Raman modes. The appearance of infrared active modes in Raman spectrum usually results from the inversion symmetry breaking, which is confirmed by the second harmonic generation measurement. The second transition at ${p}_{2}$ 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.