Why are zirconium and hafnium so difficult to separate?

Zirconium and hafnium are notoriously difficult to separate because they exhibit the most similar chemical behavior of any known pair of elements. This extreme similarity is primarily caused by the lanthanide contraction, a phenomenon where the steady decrease in atomic and ionic radii across the lanthanide series results in the heavier hafnium having an atomic and ionic radius nearly identical to that of the lighter zirconium.123

Because their atomic and ionic sizes are so similar, their chemical properties—such as the solubilities and volatilities of their compounds—are nearly indistinguishable. Consequently, they are invariably found together in nature, typically within zirconium minerals.12

Separating these two transition metals requires sophisticated methods, such as liquid-liquid countercurrent extraction, to exploit the very small quantitative differences in their chemical behavior. This separation is critically important for nuclear applications, as zirconium must be free of hafnium to maintain its low neutron-absorption cross section, whereas hafnium’s high neutron-capture capability makes it useful for nuclear-control rods.12