Crystalline Silicon

Silicon assumes the crystalline form under favourable conditions. Thus it may crystallise from a state of solution in metals, from a state of fusion, or from the condition of vapour.

1. Crystallised silicon was first obtained by Deville in 1854, during the production of aluminium by the electrolysis of fused sodium aluminium chloride containing silica as an impurity. The silicon crystallised from the aluminium, and remained behind when the metal was dissolved in hydrochloric acid, just as graphite remains when cast iron is similarly treated.
2. Wohler obtained crystallised silicon in a similar way, and also devised the following method for the preparation of this substance in quantity: 1 part of aluminium was fused with 20 parts of sodium silicifluoride for fifteen minutes in a Hessian crucible; and the product, when cold, was treated with hydrochloric and hydrofluoric acids, which dissolved the aluminium and sodium fluorides, leaving the silicon behind. Yigouroux modified this method by heating a mixture of 40 grams of aluminium and 125 grams of potassium silicifluoride for half an hour in an iron crucible to a bright red heat. In this way about 50 grams of silicon were obtained after treatment with acids:
   
   3K₂SiF₆ + 4Al = 6KF + 4AlF₃ + 3Si.
3. Deville and Caron adopted a modification of Wohler's method. They heated together 3 parts of potassium silicifluoride, 1 part of sodium, and 1 part of granulated zinc to a temperature just below the boiling-point of zinc. The silicon remained after treatment with acid, having crystallised from the molten zinc.
4. A further method of Vigouroux consists in heating powdered quartz with excess of aluminium in an electric furnace. Some of the aluminium reduces the silica, and the remainder dissolves the silicon.
5. Silicon is obtained crystallised from aluminium, when the vapour of silicon tetrachloride is passed over the molten metal. No combination takes place between aluminium and silicon.
   
   All the above methods for obtaining crystallised silicon depend on the crystallisation of this substance from molten aluminium or zinc. Further methods are as follow:
6. Crystallised silicon is formed by fusing the amorphous variety, and allowing the mass to cool. The allotropic change from amorphous to crystalline silicon is accompanied by the evolution of heat in accordance with the equation:
   
   Si_amorph. = Si_cryst + 8059 calories.
   
   According to von Wartenberg, however, the heat evolved is less than two thousand calories.
7. Crystalline silicon is also formed by sublimation, and was obtained by Moissan during the distillation of silica contained in a graphite crucible heated in an electric furnace.
   
   It is formed in the same way when the vapour of silicon tetrachloride is passed through a heated porcelain tube containing amorphous silicon. In this case the sublimation appears to be due to the formation and decomposition of a lower chloride.
8. Silicon is also obtained in small, hard crystals when the tetrachloride in presence of hydrogen is decomposed by a carbon rod heated electrically to a high temperature.
9. Crystallised silicon is prepared according to the method of Kuhne, when a mixture of powdered quartz, aluminium turnings, and sulphur is heated. Water hydrolyses the resulting aluminium sulphide, and sets free the silicon.
10. Finally, crystalline silicon is produced on a large scale when silica and carbon are heated together in an electric furnace, the carbon being present in quantity insufficient to form carborundum.

When silicon crystallises from molten silver, some of it is in a form which differs from the rest in density (2.42) and in being soluble in hydrofluoric acid. It is regarded by the discoverers as a new allotropic form.

Crystalline silicon is light orange in colour and transparent when the crystals are small, but generally appears either in black, shining, and metallic-looking six-sided plates which resemble graphite, and are called graphitoidal silicon, or in long needles, known as adamantine silicon. Both kinds of crystals are composed of regular octahedra. Crystallised silicon has a hardness of 7 on Moh's scale and scratches glass; its density is 2.49. The specific heat of crystalline silicon increases with temperature. At -184° C. it is only 0.0876, at -40° C. it is 0.136, and at 22° C. 0.1697. The gradient of rise becomes less at higher temperature, and above 200° C. almost disappears, so that at about 232° C. the specific heat becomes constant and equal to 0.203, which gives a value for the atomic heat = 5.75. Crystalline silicon resembles graphite in its power to conduct electricity. The spectrum of silicon contains a large number of lines, the chief of which occur in the ultra-violet.

The most intense lines in the spectrum of silicon are as follow:

Arc: 2507.01, 2516.20, 2524.22, 2528.60, 2881.70, 3905.70.

Spark: 2516.26,* 2528.60,* 2881.73.

The lines asterisked are also the most persistent, i.e. the ultimate lines in the spark spectrum of silicon.

In chemical properties crystalline differs from amorphous silicon in being less reactive; otherwise it is. capable of similar reactions. With most of the metals except those of the alkalis silicides are formed; zinc, aluminium, tin, lead, cadmium, gold, silver, and mercury dissolve silicon in quantities which increase with temperature, so that they form crystallising media for this element. The solubility of silicon in zinc, lead, and silver has been studied by Moissan and Siemens.