  The stability of different stoichiometric H$_n$Br ($n$=1-7) compounds under
pressure are extensively studied using density functional theory calculations.
Five new energetically stable stoichiometries of H$_2$Br, H$_3$Br, H$_4$Br,
H$_5$Br, and H$_7$Br were uncovered at high pressure. The results show that HBr
is stable below 64 GPa, then decomposes into new compound H$_2$Br and Br$_2$
molecular crystal. For H$_2$Br and H$_3$Br compounds, they were found to become
stable above 30 GPa and 8 GPa, respectively. In addition, we accidentally
discovered the triangular H$_3^+$ species in H$_5$Br compounds at 100 GPa.
Further electron-phonon coupling calculations predicted that hydrogen-rich
H$_2$Br and H$_4$Br compounds are superconductors with critical temperature of
superconductivity $T_c$ of 12.1 K and 2.4 K at 240 GPa, respectively.
