Novel Superconducting Phases of HCl and HBr under High Pressure: An Ab Initio Study

The novel superconducting C2/m phases of HCl and HBr were predicted using the developed particle swarm optimization algorithm for crystal structure prediction. The calculated results showed that the phase transition from P-1 to C2/m phases occurred at 250 and 120 GPa for HCl and HBr, respectively. Hydrogen bond symmetrization was observed in the newly predicted C2/m phase. No imaginary phonon frequencies in the whole Brillouin zone indicated that the C2/m phases of HCl and HBr were dynamically stable. In addition, the C2/m phase of HBr was the most stable structure, which rejected the earlier hypotheses of elemental decomposition into bromine and hydrogen. Perturbative linear response calculations predicted that the critical temperature Tc of the C2/m phase of HCl could reach 20 K at 250 GPa.