Compounds of hafnium have physical and chemical properties very similar to the corresponding compounds of zirconium, except for the much higher density of hafnium compounds. The melting and subliming temperatures of some hafnium compounds are compared with corre­sponding zirconium compounds in Table 7.8. As the vapor pressures of corresponding hafnium and zirconium compounds are so nearly equal, separation by fractional distillation is imprac­tical. As corresponding compounds, such as HfCU and ZrCU, form solid solutions miscible over the entire composition range and have nearly the same melting point, separation by fractional crystallization is also difficult.

The free energies of formation of corresponding hafnium and zirconium compounds are also nearly equal, so that separation by preferential reaction of one species is difficult, too. Table 7.9 compares the free energies of formation of hafnium and zirconium tetrahalides at 1000 K.

Successful processes for separating hafnium from zirconium take advantage of rather rare occurrences of substantial differences in solubilities of corresponding hafnium and zirconium compounds in water, organic solvents, fused salts, or liquid metals.