Crystal structure and thermodynamic characteristics of monobromides of zirconium and hafnium

In reduction of hafnium tetrabromide vapours by metallic hafnium (pressure 30-40 atm) at temperature 480-580 deg C followed by keeping of the reduction product at a temperature of 500 deg C under 1 atm, pure hafnium monobromide is obtained. The rhombohedral unit cell parameters are a=9.620+-0.003 A; α=20.80+-0.01 A; z=2; rhosub(calc)=8.8 g/cm3. The dependence of vapour pressure for zirconium and hafnium tetrabromides on temperature in disproportioning appropriate monobromides is established by the tensimetric method. The enthalpies and entropies of disproportioning processes are determined for zirconium and hafnium monobromides. The values of ΔHsub(T)sup(0), ΔSsub(T)sup(O), ΔH2980 and ΔS2980 have been found for crystalline ZrBr and HfBr