Chromium Aluminium Silicide, Cr₂AlSi₃

Chromium Aluminium Silicide, Cr₂AlSi₃, has been obtained by fusing together silicon, aluminium, and chromium in an atmosphere of hydrogen in a reverberatory furnace; by the reduction of the powdered silicate of aluminium and chromium, or of a mixture of the oxide with silica by the thermit process; and by heating chromium, chromium sesquioxide, chromium sesquisulphide, potassium dichromate or potassium chromifluoride, with a large excess of potassium silicofluoride and aluminium in a Perrot's furnace, and dissolving away the regulus by alternate treatment with dilute hydrochloric acid and sodium hydroxide solution. The double silicide is obtained in the form of greyish-white, brittle, hexagonal crystals, which have a metallic lustre and conduct electricity. Its hardness is 4.7; density 5. It is insoluble in acids, except hydrofluoric acid, and in aqueous alkalies, but dissolves readily in molten sodium hydroxide. It is attacked by the halogens at high temperatures.

If in the method of preparation last described the proportions are so chosen that the regulus contains 35 to 50 per cent, of free silicon, then a second silicide, Cr₂AlSi₄, is formed, yielding small crystals of hardness rather less than 5, and of density 4.8.

Manchot and Kieser have studied the reaction of these silicides with hydrofluoric acid quantitatively, with the object of arriving at their constitution. They suggest the double formulae:



and