Crystal Chemistry of Boron and of Some Boron-Rich Phases; Preparation of Boron Modifications

Abstract

For many years, most physico-chemical studies of boron and its compounds were made with elemental boron which was of low purity and usually amorphous or at least very poorly crystallized. Such a type of boron was generally obtained either by reduction of a boron compound using a highly electropositive metal (reduction of B2O3by magnesium or of KBF4 by sodium for example), or by electrolysis of molten salts containing a borate or a fluoborate as a boron source. In all cases, the boron resulting from the reduction was separated from byproducts by prolonged washing in aqueous solutions, so that its purity generally was not higher than 80 to 95%. WEINTRAUB and KAHLENBERG even made the hypothesis that in certain cases (for example MOISSAN boron obtained by reduction of B2O3 by magnesium or alkali metals) the reaction product was a boron sub-oxide, with a formula close to B6O, rather than elemental boron. Although these materials have low purity, they are easily prepared, and their purity can be greatly improved by outgasing in high vacuum and at a temperature near to the melting point of boron.