  The high pressure phases of HBr are explored with an ab initio crystal
structure search. By taking into account the contribution of zero-point energy
(ZPE), we find that the P4/nmm phase of HBr is thermodynamically stable in the
pressure range from 150 to 200 GPa. The superconducting critical temperature
(Tc) of P4/nmm HBr is evaluated to be around 73 K at 170 GPa, which is the
highest record so far among binary halogen hydrides. Its Tc can be further
raised to around 95K under 170 GPa if half of the bromine atoms in the P4/nmm
HBr are substituted by the lighter chlorine atoms. Our study shows that, in
addition to lower mass, higher coordination number, shorter bonds, and more
highly symmetric environment for the hydrogen atoms are important factors to
enhance the superconductivity in hydrides.
