text: PBr is prepared by treating red phosphorus with bromine.  An excess of phosphorus is used in order to prevent formation of PBr:
  :P +  6 Br  →  4 PBr
  Because the reaction is highly exothermic, it is often conducted in the presence of a diluent such as PBr. Phosphorus tribromide is also generated in situ from red phosphorus and bromine.