Modulated structure and molecular dissociation of solid chlorine at high pressures

Among diatomic molecular halogen solids, high pressure structures of solid chlorine (Cl₂) remain elusive and least studied. We here report first-principles structural search on solid Cl₂ at high pressures through our developed particle-swarm optimization algorithm. We successfully reproduced the known molecular Cmca phase (phase I) at low pressure and found that it remains stable up to a high pressure 142 GPa. At 150 GPa, our structural searches identified several energetically competitive, structurally similar, and modulated structures. Analysis of the structural results and their similarity with those in solid Br₂ and I₂, it was suggested that solid Cl₂ adopts an incommensurate modulated structure with a modulation wave close to 2/7 in a narrow pressure range 142–157 GPa. Eventually, our simulations at >157 GPa were able to predict the molecular dissociation of solid Cl₂ into monatomic phases having body centered orthorhombic (bco) and face-centered cubic (fcc) structures, respectively. One unique monatomic structural feature of solid Cl₂ is the absence of intermediate body centered tetragonal (bct) structure during the bco → fcc transition, which however has been observed or theoretically predicted in solid Br₂ and I₂. Electron-phonon coupling calculations revealed that solid Cl₂ becomes superconductors within bco and fcc phases possessing a highest superconducting temperature of 13.03 K at 380 GPa. We further probed the molecular Cmca → incommensurate phase transition mechanism and found that the softening of the A_g vibrational (rotational) Raman mode in the Cmca phase might be the driving force to initiate the transition.