Metal Reduction of the Halides

Although hydrogen is the most common reductant, there are other elements which are more powerful, such as zinc, cadmium, magnesium, sodium, and potassium, as shown in Table 3.1. 

In this table, the free energy of formation, AGf of the chloride of these metals is listed for four different temperatures. As can be seen, the values are more negative than that of hydrogen chloride. These metals can be used to reduce the halides of titanium, zirconium, or hafnium, whereas hydrogen, as mentioned above, cannot do so readily. In order to be useful in CVD, the by-product chloride must be volatile at the deposition temperature. This may rule out the use of sodium or potassium, which evaporate above 1400°C. 

Of these reductant metals, the most commonly used is zinc. The reason is that the zinc halides are more volatile than the parent metal and the chances of codeposition of the halides are minimized. Either chloride or iodide is used, although the iodide, being the most volatile, is usually preferred. The volatility of these halides decreases as one goes from the iodides to the chlorides to the fluorides. The reaction is as follows  

Another reductant, magnesium, is used in the industrial production of titanium metal as follows  

The alkali metals, sodium and potassium, shown in Table 3.1 are not generally used as reductant because their reductive power is so high that it tends to cause premature and detrimental gas-phase precipitation and, as mentioned above, high temperature is necessary to volatilize these metals. 

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