Allotropy of Selenium

The analogy of selenium with sulphur is observable in the occurrence of allotropy, although no rigid relationship can be traced between the various forms of the two elements.

As with sulphur, determinations of the vapour density of selenium give indications of alteration in the molecular condition, the values up to 900° C. suggesting a mixture mainly of Se₆ and Se₂ molecules; below 550° C. there may be a small proportion of Se₈ molecules present, whilst at temperatures above 900° C. it is possible that a few monatomic molecules are formed, but the vapour is then almost entirely composed of Se₂ molecules.

This also is variable, depending on temperature and concentration as well as on the nature of the solvent. There is, however, a much greater tendency for the larger molecule to dissociate than is the case with sulphur. When dissolved in molten iodine, with which element selenium does not combine, cryoscopic measurements indicate the presence of diatomic molecules, Se₂, if the concentration of the solution is over 5 per cent., but in more dilute solutions these molecules undergo partial dissociation into the monatomic condition. In molten mercuric chloride selenium ranges from the octa-atomic condition to the tetra-atomic condition in dilute solution; sulphur under similar conditions appears uniformly octa-atomic. In sulphur chloride selenium is apparently monatomic. In pyrosulphuric acid, metalloidal selenium dissolves as Se₂, but the "metallic" form (see the following) dissolves as Se.

In organic solvents dissolved selenium is generally in the form of more complex molecules. In concentrated solutions using, for example, diphenyl or anthraquinone as solvent, the molecular weight of selenium is represented by Se₈ (yellow phosphorus as solvent gives a similar result), whilst in methylene iodide the molecular condition is stated to be represented by Se₁₀. On dilution of these solutions the selenium undergoes disruption into smaller molecules.

Both amorphous and crystalline varieties of selenium occur. Amorphous selenium is best known as the " vitreous " and the finely divided brick-red forms, which are frequently described as two distinct allotropes; they are, however, identical. The crystalline allotropes include several monoclinic varieties, red to brown in colour, as well as the so-called " metallic " selenium.

Amorphous Selenium

Vitreous Selenium

When molten selenium is cooled in not too protracted a manner, no definite solidification or crystallisation ensues, but the mass gradually hardens and the product really represents a strongly under cooled liquid like glass. Vitreous selenium is a brittle reddish-brown substance, exhibiting a conchoidal fracture. When finely powdered and viewed in thin layers it has a deep red colour. This form has an average density of 4.28; the value varies slightly, possibly owing to the presence of other allotropic modifications of the element.

On heating, a gradual softening commences at about 50° C. and the mass slowly changes into a viscous liquid at 150° C., which becomes distinctly mobile at 250° C. When allowed to cool these changes occur in the reverse order until the solid amorphous variety is again reached. If the cooling is very slow some of the "metallic" modification may be formed. The relation of the vitreous form to the "metallic" form is one of monotropy, as at all temperatures up to its melting-point the latter is the more stable form; this would be expected from the fact that the amorphous form merely represents the molten substance in which crystallisation has not occurred although the normal freezing-point has been passed. At the ordinary temperature the rate of transformation is so slow that vitreous selenium may be preserved indefinitely.

Brick-red Amorphous Selenium

When selenium is produced by the reduction of selenious acid with sulphurous acid, glucose or other reducing agent, it is obtained as a brick-red powder. This product is identical with the vitreous form when the latter is in a fine state of division. Its behaviour on heating is the same, and if the pasty mass obtained at 50° C. is cooled, the more massive vitreous modification results. It has been suggested that the name "liquid selenium" should be used to denote these forms of amorphous selenium having, no definite melting-point. Transformation to the vitreous form may also be effected by subjecting the red precipitated element to high compression, for example to 10,000 kgms./cm². On account of the smallness of its particles the brick-red form of amorphous selenium is more reactive than the more massive form, although for full use to be made of this advantage it is obvious that the temperature should not be raised sufficiently either to cause caking or conversion into the "metallic" form.

Both of the foregoing varieties of amorphous selenium are somewhat soluble in carbon disulphide, but selenium chloride, carbon diselenide and methylene iodide are better solvents. On account of its finer state of division the red form appears more soluble. Discordant results are easily obtained with such solutions because of the tendency to change, especially on warming, into the less soluble crystalline variety - generally the monoclinic form.

The brick-red variety dissolves in concentrated sulphuric acid to give a green solution containing a polymeric form of selenium sulphoxide, SeSO₃. This slowly decomposes according to the equation

SeSO₃ + H₂O = Se + H₂SO₄,

yielding a brown allotrope which is only very sparingly soluble in carbon disulphide. On exposure to sunlight this is slowly converted into an amorphous black variety. This black selenium is also produced in small quantity when the reduction of selenious acid with sulphurous acid is effected at 100° C. The brick-red amorphous form, when exposed to light during seven months, undergoes change and forms slaty amorphous leaflets, devoid of lustre.

Crystalline Varieties of Selenium

Monoclinic Selenium

Monoclinic Selenium is obtained when a carbon disulphide solution of amorphous selenium is allowed to crystallise, for example by evaporation or by keeping red amorphous or vitreous selenium in contact with carbon disulphide. In the latter case the relatively unstable amorphous form is the more soluble, so that a gradual separation of the more stable crystalline form ensues.

Several varieties of monoclinic selenium are obtainable in this way: reddish-brown leaflets, more deeply coloured granules, or stout prisms of still deeper colouring.

Monoclinic selenium has a density of 4.44 at 0° C. It is only sparingly soluble in carbon disulphide, giving a red solution. When heated rapidly it melts at 170° to 180° C., but transformation into "metallic" selenium commences to occur slowly near 120° C. The liquid obtained by rapid heating passes into the vitreous condition when cooled.

Grey Crystalline Selenium, "Metallic" Selenium

Grey Crystalline Selenium, or, "Metallic" Selenium, being the most stable modification of selenium, is obtained by transformation of the preceding forms, but at ordinary temperatures the rate of change is negligibly slow. Thus amorphous selenium exhibits a very slow transformation at temperatures up to 90° C., but between this and 217° C. the change is much more rapid and the temperature may rise very considerably on account of the heat liberated. The red crystalline form also gradually undergoes conversion into-the "metallic" form at temperatures just over 120° C. From the foregoing considerations it will be clear that molten selenium below the melting-point of the "metallic" form will slowly tend to change into this form and at 180° C. the molten element soon changes to a semi-crystalline mass.

This form of selenium can also be produced from amorphous selenium by heating with quinoline, pyridine, aniline or other basic organic solvent, and also by sublimation. In this latter case the sublimate also contains the amorphous form. Atmospheric oxidation of solutions of alkali selenides gives this variety of selenium as a granular deposit. When produced by these last three methods, leafy crystals may be obtained, and Muthmann (1890) discovered that crystals which he obtained by the sublimation method were of the trigonal (rhombohedral) system and isomorphous with those of tellurium.

The product from slowly cooled molten selenium is almost black, with a leaden grey surface. It exhibits a fine-grained fracture. It is, however, identical with the foregoing crystals except in its state of aggregation. Both varieties have a density of 4.78 at 0° C. This leaden modification of the element gives a black powder, although when in a very fine state of division a red tint is perceptible. It melts at 217° C. If rapidly cooled from temperatures above its melting-point, vitreous selenium is obtained. The black form has a specific heat of 0.084 and is only sparingly soluble in carbon disulphide. It is soluble in selenium monochloride, yielding the grey crystalline variety on crystallisation.

A violet-grey crystalline variety is obtained by melting vitreous selenium at 200° C. and cooling rapidly under pressure. The slender crystals are very sensitive photo-electrically, but are very unstable.