Neodymium tetravalent

Total tetravalent uranium may be precipitated as an insoluble oxalate or fluoride (Grinberg et at., 1957) or in trace amounts co-precipitated for example with neodymium fluoride (cf. Pu ). Such a procedure can be used to show whether uranium is present in the lower oxidation state, for example, in anoxic waters because hexavalent uranium does not co-precipitate in this way. 

Despite Brauner s belief in the validity of the Mendeleev methodology, he also had to admit that he had not yet succeeded in resolving the rare-earth crisis. Thus Brauner wrote in 1901 with reference to praseodymium that its maximum valency was tetravalent, like that of cerium but that no place had been found in the periodic table for an element possessing the physical and chemical properties of praseodymium and its compounds (Brauner, 1901b). He also admitted that the difficulties of finding a place for neodymium in the periodic table were even greater than in the case of praseodymium. 

Few lanthanide elements may be oxidized to the tetravalent state and stabilized, almost exclusively, in fluorides and oxides. These are the elements cerium, terbium, praseodymium, dysprosium, neodymium, holmium, for example all in the ternary fluorides CS3RF7 (Hoppe and Roedder 1961). There are also hints at pentavalent praseodymium, CsPrFe would be the example (Hoppe 1980). 

The most common raw materials for the REM molten salt electrolysis are in the RE " state, such as RE2O3, RECI3. But RE " still exists to a certain extent in the molten salts, especially in the chloride melts, some rare earth metal elements have presented a higher level of divalent oxidation states, such as neodymium, samarium, europium, dysprosium, thulium, and ytterbium, which result in a lower current efficiency. For Sm and Eu molten salt electrolysis processes, even no metals can be obtained at the cathodes due to a cyclic transformation of Sm VSm (Eu /Eu ) and Sm /Sm (Eu /Eu ) on the electrodes during electrolysis. And some of the rare earth metal elements show tetravalent oxidation states at the chlorine pressure far in excess of atmospheric pressure, such as Ce. Most of the rare earth metal elements in oxidation state of -1-4 are not stable in chloride melts, because the reaction occurs according to the following equation RE " -I- Cl = RE -" -I- I/2CI2. 

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