Metallothermic reduction route

In the following sections, only the comproportionation route and the metallothermic reduction route are discussed as the two most coimnonly used synthetic methods. 

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... 

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. 

Single crystals are also frequently obtained in the course of the investigation of special routes. For example, LiGdCU and Na3GdCl6 came across by metallothermic reduction of GdCl3 with liffiium and sodium, respectively (Meyer 1984a,b). 

All these processes were reviewed in 1974 by the National Materials Advisory Board committee (NMAB), and most were considered by the authoring NMAB panel to be unlikely to progress to production in the near future except electrowinning, which seemed to be the most promising alternative route at that time. However, despite the numerous attempts made to date, there are still no current electrolytic processes for producing titanium metal industrially. Actually, to reach industrial success the new electrolytic method should solve the major issues of metallothermic reduction, which is still expensive and labor intensive. 

The reduction of rare earth metal trihalides, RX3, is in principle possible with all kinds of reducing agents as long as they have standard electrode potentials E° that can overcome that of the respective potentials of E° (R + R +). This is discussed below in more detail. Therefore, the classical reducing agents, nonmetals such as hydrogen or carbon, or like metals (comproportionation route) and unlike metals (metallothermic reduction) are all possible but (may) lead to different products. Cathodic reduction of appropriate melts is also an option. 

Figure 25 shows the general process steps for the melting route. All components of the alloys must he in metallic form, either as elements, or as master alloys which may he available more economically. Examples of the latter are RE-TM eutectic alloys (except with Sm) prepared by electrowinning, and Fe-Zr or Fe-Ti which are standard products for the steel industry. The RE metals used are made either metallothermically by reducing RE-oxides with calcium (and in the case of Sm with La or mischmetal) as the reductant, or by molten-chloride electrolysis. Electrolytic methods do not work with samarium because of its stable divalent state. Samarium is usually further refined by vacuum distillation, which is easy because of the low boiling point. 

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