Compound metallothermic reduction

Metallothermic reduction of compounds. Metallothermic reduction of halides (fluorides), a method used for lanthanide metal preparation, was among the first methods to be successfully applied to actinides ... 

Metal-rich compounds are readily obtained when a metallothermic reduction is included into the metathesis reaction by using an electropositive metal (10). In addition, metal-rich and nitrogen-rich compounds are obtained when a metal and a metal nitride are employed in reactions (11, 12) ... 

The metallothermic reduction of oxides is essentially a reaction involving only condensed phases. It follows therefore, that the entropy changes in these reactions are small and that the differences in the heats of formation of the pertinent compounds determine the feasibility of a given reaction. Among the metallic reductants, calcium forms the oxide whose heat of formation is the most negative. As a first approximation, calcium may be considered to be the most effective reducing agent for metal oxides. 

Preparation Methods. Actinide metal preparation is based on methods known or developed to yield high purity material by metallothermic reduction or thermal dissociation of prepurified compounds. Electrolytic reduction is possible from molten salts, but not from aqueous solutions. Further purification of the metals can be achieved by electrorefining, selective evaporation or chemical vapour transport. 

The synthesis of compounds of the lanthanides containing cluster complexes follows in general the same routes as described in The Divalent State in Solid Rare Earth Metal Halides, the conproportionation route and the metallothermic reduction route, for example... 

All of the actinide elements are metals with physical and chemical properties changing along the series from those typical of transition elements to those of the lanthanides. Several separation, purification, and preparation techniques have been developed considering the different properties of the actinide elements, their availability, and application. Powerful reducing agents are necessary to produce the metals from the actinide compounds. Actinide metals are produced by metallothermic reduction of halides, oxides, or carbides, followed by the evaporation in vacuum or the thermal dissociation of iodides to refine the metals. 

The separation of the products is not easy, although not impossible. For the exploration of the respective systems and their phase contents, it is, however, often not necessary to obtain pure products because modem, fast X-ray crystallography is an easy means to analyze even multiproduct reactions. Afterward, when the respective compounds are known, care has to be taken to produce the new compounds as pure phase by whatever route is desirable. One major advantage of the metallothermic reduction route is the fairly low reaction temperatures as these allow for the synthesis and crystal growth of compounds that decompose in the solid state, melt incongmently, or even form and decompose in the solid state at fairly low temperatures. Also, low-temperature modifications may be grown as single crystals below the transition temperature. 

For the above reasons we will discuss the metallothermic methods first, followed by purification processes and then the electrolytic reduction process. This section will close by reviewing the preparation of single crystals and intermetallic compounds. 

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