General chemistry properties of rare earth elements and compounds

Many of the factors which control the properties and chemistry of rare earth elements and compounds have been considered in Section 1.2 - rare earth elements and their place in the periodic system. These factors are electronic configurations, ionic radii, lanthanoid contraction and redox potentials/oxidation states (Table 1.1). Some general properties are listed in Table 1.1. For a more detailed account see refs 32-34. 

This has always been a challenge owing to similar properties in the usual +III oxidation state. Initial separations based on fractional crystallization of salts were time-consuming and tedious with many false dawns (Section 1.3.1). Current separation methods are based on (i) less usual oxidation states and (ii) variations in complex stability resulting from ionic radii differences. 

Before the separation of the lanthanoids, thorium and uranium have to be removed. From an acidic solution of mixed Ln ions, cerium is first removed based on the existence of the Ce oxidation state and the higher basicity of Ln(OH)3 than CeO.  

With careful control of pH, treatment of Ln with base and air leads to insoluble CeO whilst the remaining Ln stay in solution. Bastnaesite may be roasted directly to give Ce02+Ln203, the latter being extracted by acid. Europium is next removed taking advantage of the relative stability of Eu° and the insolubility of europium(II) sulfate. 

Europium is one of the most valuable of the rare earth elements, hence the cost of zinc as a reductant can be borne. 

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