Heavy rare-earth (HRE) metals function very efficiently in advanced materials for magnetooptical-information storage, giant magnetostrictive energy conversion, and high-strength permanent magnets. [1] A 7 % annual growth in global demand for rare-earth (RE) metals was recently predicted. [2] Although the separation of mixed RE compounds has been greatly refined, [3] extraction of pure RE metals, especially the heavy ones, remains a historical challenge. The difficulty is multifold but particularly related to the high oxygen affinity of HRE metals. Current industrial methods convert HRE oxides into fluorides (e.g. by reaction with HF or NH 4 F), from which the metal is extracted by calciothermic reduction at % 1500 8C in argon. [4] The obtained HRE metal needs further separation from excess calcium under vacuum and deoxygenation by special methods. [5] HRE metals have high melting points (> 1350 8C), and hence cannot be extracted as a liquid in the same way as aluminum and light RE metals. [6] Electrodeposited solid reactive metals from molten salts are always dendritic and absorb oxygen easily when exposed to air. Dendrite formation may be avoided by deposition onto a suitable transition-metal substrate to form a liquid alloy, [7] but the alloy composition is difficult to control precisely.
Recently, solid TiO 2 , [8] SiO 2 , [9] and some mixed oxides [10] were electrochemically reduced to the respective solid metals or alloys in molten salts. The new process requires that 1) the metal oxides to be reduced are thermodynamically less stable than the oxide of the metal element of the molten salt used, [8][9][10][11][12][13] and 2) electrodeposition of the metal of the molten salt should be avoided. [8] CaO is extremely stable, and hence molten CaCl 2 is the preferred choice for the new process. However, HRE oxides are almost as stable as CaO, and their reduction may not occur at potentials more positive than that
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