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The difficulty in separating hafnium from zirconium arose from the virtual identical atomic sizes of the two elements, leading to very similar solubilities of their analogous compounds. Hence, the need was to discoverer a system that allowed an efficient method of fractional crystallization. Hevesy discovered that potassium hexafluorohafnate (K2HfF6) is more soluble than potassium hexafluorozirconate (K2ZrF6), shown here. Repeated recrystallizations slowly led to an enrichment of the hafnium compound in the soluble portion.