Borides of Silicon

Two borides of silicon are known: SiB₃ and SiB₆. They are formed together when boron is heated with excess of crystallised silicon in an electric furnace. Uncombined silicon is removed from the cooled mass by treatment with a cold mixture of nitric and hydrofluoric acids, and the crystals are further purified by heating them with moist potash for half an hour, washing with nitric acid and hot water, and drying at 130° C.

Each compound may be separated from the mixture by suitable treatment. Boiling nitric acid oxidises the hexaboride, leaving the triboride in black, rhombic plates, of density 2.52; whilst fused anhydrous potash decomposes the triboride and leaves the hexaboride in black, opaque crystals of density 2.47, which are intermediate in hardness between ruby and diamond.

Both borides are good conductors of electricity; fluorine acts vigorously upon them, and chlorine at a red heat; they are stable when heated in the air, but must not be heated in platinum, with which the silicon would combine; concentrated sulphuric acid attacks them slowly at 330° C.