Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.4 Acid-base equilibria in ionic solvents 635
gen-substituted compounds with a relatively low halogen content, carbon (carbon black)
accumulates in the melt and is displaced by the crystallization front when single crystals are
grown. 112
The presence of a suspension of carbon in alkali metal halide melts is not always desirable
and this method cannot therefore be recognized as universal.
The carbohalogenation of melts results in formation of CO 2 as one of products of
[10.4.58]-[10.4.60] processes, carbon dioxide reacts with remaining oxide ions and induces
the accumulation of fairly stable carbonate-ions. The latter is the main form in which oxygen-containing
impurities exist in melts purified in this way. The purification threshold of
carbohalogenation is estimated as 1-2×10 -4 mole/kg of O 2- . 24
The use of halo-derivatives of silicon for the purification of iodide melts 120 is based on
the reaction:
2−
−
SiX ↑+ 2O= 4X+
SiO ↓
4
2
[10.4.61]
the product of [10.4.61] is silicon dioxide, which is separated from the melt as a consequence
of the difference between the densities. Thermodynamic analysis showed 121 that the
effectiveness of the purification of iodide melts diminishes in the sequence SiI4>HI>I2. The use of silicon halides for the purification of the melts used in the growing single
crystals does not lead to the appearance of additional impurities, because the processes are
carried out in quartz (i.e., SiO2) containers.
Although the methods of purification described are fairly effective in most cases,
some kind of oxygen-containing and cationic impurities, characteristic of each method, always
remains in the melt. The method of purification must therefore be selected depending
on the aims of the subsequent application of the melt.
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