Physical Properties of Selenium

Selenium boils at about 690° C., forming a vapour the colour of which is intermediate between that of chlorine and that of sulphur. The element can be sublimed and distilled at a much lower temperature under very low pressure. When selenium is heated on charcoal the vapour has an odour resembling that of rotten radishes; this has been attributed to the formation of a small quantity of selenium suboxide, but it is more probably due to the formation of carbon diselenide. The thermal conductivity of selenium depends upon the allotropic form, its age, and the temperature. As might be expected the value for the grey metallic form is greater than that for the vitreous modification; thus at 25° C. the thermal conductivity of the former varies between 0.00070 and 0.00183, whilst that of the latter lies between 0.000293 and 0.000328. The values generally increase with rise in the temperature of preparation of the sample, and with exposure of the sample to light; on the other hand, the conductivity diminishes with age.

The mean specific heats of the various forms are given in the following table:

Allotropic Form.	Temperature Range, ° C.	Mean Specific Heat.
Vitreous	15-100	0.106
Red crystalline	15- 75	0.082
Metallic	15- 75	0.078
Metallic	15-217	0.084
Liquid	217-300	0.118

The heat of transformation of vitreous selenium to the "metallic" form at 130° C. is 13.5 calories per gram; with rise in temperature this value increases until at the melting-point, 217° C., it is identical with the latent heat of fusion, namely, 16.4 calories per gram. The heat of transformation of the red crystalline variety to the "metallic" form at 150° C. is 2.2 calories per gram. The heat of dissociation of diatomic selenium has been found by the optical method to be 84,000 gram-calories. The element is diamagnetic.

The electrical conductivity of selenium is exceedingly small at ordinary temperatures and in the dark, but with rise -in temperature or on exposure to light the resistance diminishes in a remarkable and unique manner. The conductivity also depends on the allotropic form and its previous history; the grey metallic form is the most sensitive to these changes, and on heating any other form the rate of transformation into the "metallic" variety considerably influences the actual value of the conductivity. In general the electrical conductivity of selenium increases first rapidly and then more slowly with rise in temperature up to the boiling-point, 690° C. On cooling, it decreases again at the same rate, but if the cooling is protracted, the conductivity of the resulting grey selenium is not constant for a given temperature, but can be diminished at will by slight heating and re-cooling.

On exposure to light, the resistance of selenium immediately sinks to a value which is only a few thousandths of the value in the dark; even exposure for 0.001 sec. will produce a considerable effect. This phenomenon was first observed by Willoughby Smith in 1873. When the light is shut off the resistance increases, somewhat slowly, becoming normal in a short time, however. Very feeble rays, such as the light from a star, can produce an appreciable effect on the resistance. All visible rays are effective, but the influence is most intense in the case of the red, of wave-length about 700 μμ. Ultra-violet rays, Rontgen rays, cathode rays and rays from radioactive substances act in the same way. In the case of the last-named, the effect of the y-rays is small compared with that of the β-rays. Temperature has little effect on the influence of light, the sensitivity of the selenium only being reduced by 10 to 25 per cent, at the temperature of liquid air. It is upon this action towards light that the extremely sensitive photo-electric selenium cell depends. Such cells are employed in the construction of the photophone, by means of which speech may be transmitted by a beam of light. The first transmitter of this kind was made in 1880 by Graham Bell, the inventor of ordinary telephony. Another instrument, the optophone, invented by Fournier d'Albe, makes it possible for the blind to read ordinary books and newspapers by sound.

In order to explain this remarkable property of selenium many theories have been put forward, most of which are now untenable, and only two need be considered.

It was first suggested by Siemens in 1875, and the hypothesis was strongly supported by later investigations, that crystalline selenium exists in two forms "A" and "B," "A" being a non-conductor and "B" a good conductor of electricity. In the dark the equilibrium mixture consists almost entirely of "A"; the equilibrium is displaced in the direction of "B" both by the action of heat and by exposure to light:

"A" ⇔ "B"

The isolation of the two modifications was described by Pelabon, but was not confirmed by later investigation. The theory is discounted by the fact that at exceedingly low temperatures the action of light is only slightly diminished, whereas it would be expected that such a transformation would no longer proceed.

However, Briegleb, from X-ray investigations, maintains that in all the allotropic modifications two such pseudo-components do exist, and that these may be separated in some degree by taking advantage of the fact that although their absolute solubilities in carbon disulphide are almost identical, the rates at which they dissolve are different. By spectroscopic methods evidence has been obtained that the two forms exist in equilibrium in this solution and that the equilibrium varies with the temperature.

What appears to be a more satisfactory explanation and one largely favoured by physicists is that the phenomenon is an effect of purely electronic character. The actual mechanism of the action is not yet completely understood, but the light appears to cause ionisation at the surface of the selenium, with immediate increase in conductivity. The splitting off of electrons may be not only from the selenium atoms but also from the incident stream. It has also been suggested that the interatomic space occupied by the conducting electrons may be increased. The theory explains why the recovery of the selenium is not immediate when the light is removed, and why after exposure to the more deeply penetrating rays, such as the X-rays, the recovery is even slower.

The emission, absorption and fluorescence spectra have been investigated. According to de Gramont, selenium gives neither raies ultimes nor raies de grande sensibilite. The most persistent lines of the emission spectrum are (in Å), 1960.2, 2039.7, 2062.6, 4730.9, 4739.1 and 4742.3, and Kimura has observed that non-luminous selenium vapour absorbs the lines 1960 and 2040 Å. The vapour, when excited by intense illumination from a quartz-mercury arc, exhibits at 325° C. a fluorescence spectrum extending from 5079 to 2229 Å, apparently analogous to the ultra-violet fluorescence spectrum of iodine; with rise in temperature this spectrum disappears and at 430° C. is replaced by one containing nine faint broad bands extending from 4178 to 4829 Å.