Tetrachromium Dicarbide, Cr₄C₂

By melting chromium at temperatures above 1840° C. for fifteen minutes in a crucible of pure carbon, frequently stirring with a carbon rod, alloys- saturated with carbon are obtained. These contain no free chromium, but only carbide and graphite. On washing with hot 24 per cent, hydrochloric acid part of the alloy dissolves, and the ratio of chromium to carbon dissolving indicates that the soluble carbide has the composition Cr₄C₂, but the existence of this has not been confirmed by isolation.

The residue, insoluble in the hot acid, contains graphite and trichromium dicarbide, Cr₃C₂, which, by extraction of graphite, can be obtained from 99 to 99.5 per cent. pure. Moissan 3 obtained this carbide during the reduction of chromic oxide by carbon in the electric furnace; also by heating metallic chromium with a large excess of carbon in the crucible of an electric furnace for ten to fifteen minutes, using a current of 350 amperes under 70 volts; and by heating a mixture of equal parts of chromic oxide and calcium carbide in the electric arc for five minutes, using 900 amperes under 45 volts. It is obtained as lustrous lamellae, unctuous, darker in colour than the pentachromium carbide. It is resistant to all acids; fused potassium hydroxide has little action upon it, but it is easily destroyed by fused potassium nitrate. Its density at 21.3° C. is 6.683, and it is hard Miough to scratch quartz and topaz. It does not attack water either at ordinary temperatures or at the boiling-point. It is stable at high temperatures, but it appears to undergo partial decomposition into the tetrachromium dicarbide, Cr₄C₂, and carbon on melting at about 1890±10° C., the molten carbide readily reducing magnesia, alumina and zirconia. It is decomposed by chlorine at a red heat, forming chromic chloride and amorphous carbon.