Boron Silicides, B₂Si and B₆Si

Boron Silicides, B₂Si and B₆Si are produced when a mixture of crystalline silicon (5 pts.) and amorphous boron (1 pt.), packed in a refractory earthenware tube, is heated for 40 to 60 seconds by means of an electric current of 600 amperes at 45 volts. The outer portions of the product and those portions which were in proximity to the carbon electrodes are discarded, and the remainder treated with a cold mixture of nitric and hydrofluoric acids to eliminate free silicon. The residue is purified by heating for half an hour with commercial potassium hydroxide, which is just melted but not dehydrated. After washing with water, dilute nitric acid, and boiling water, and drying at 130°, a mixture of the two silicides is obtained.

The silicide B₃Si is left when the mixture is heated with an excess of boiling nitric acid, the other compound being completely decomposed. It forms black, rhombic plates which are transparent and yellow or browji in very thin layers. The density is 2.52. It easily scratches quartz and ruby, but is softer than boron carbide. Fluorine, chlorine, and bromine attack it in increasing order of difficulty, and it is only superficially oxidised by air or oxygen at a red heat. It is rapidly oxidised by fused anhydrous potassium hydroxide, less rapidly by fused alkali carbonates, and not at all by fused alkali nitrates. Boiling concentrated sulphuric acid slowly oxidises it.

The silicide B₆Si is isolated from a mixture of the silicides, in which it is more abundant than the other, by heating with fused anhydrous potassium hydroxide. It forms thick, opaque crystals of density 2.47. In chemical properties it resembles the other silicide, except in its behaviour towards fused potassium hydroxide and boiling nitric acid.