Black Phosphorus

This form must not be confused with the black substance which appears on the outside of sticks of white phosphorus which have been exposed to light, and which is neither a definite form nor a pure substance. The preparation of a dark substance by the sudden cooling of phosphorus could not be repeated, and this result probably was also due to impurities.

Black phosphorus was discovered by Bridgman, who prepared it by subjecting white phosphorus at 200° C. to pressures of 12,000 to 13,000 kilograms per square centimetre.

The density, 2.69, is much higher than that of violet phosphorus. The vapour pressures were found to be lower than those of the violet element at the same temperatures:—

t° C.	Black Phosphorus, p (mm.).
357.1	2.3
445.2	585

In spite of this relation between the vapour pressures the violet form could not be converted into the black at ordinary temperatures. Later investigations showed that the black form probably is only in a state of false equilibrium or suspended transformation at ordinary temperatures and pressures.

The black phosphorus was with difficulty freed from the kerosene, through which pressure had been applied, by heating in evacuated tubes to 550° C. The vapour pressures of the product (which might also of course have been altered by this treatment) were now almost, if not quite, equal to, and above 560° C. greater than, those of the violet form. The conversion of the black into the violet was effected by heating in the presence of about 1 per cent, of iodine at 480° C. or without the iodine at about 575° C.

It has already been mentioned that black phosphorus has a greater electrical conductivity than other forms, and is therefore to be considered as the most " metallic " variety. The conductivity, however, like that of graphite, increases with rise of temperature, and this form is therefore only pseudo-metallic.

X-ray Data

The structure of black phosphorus has been calculated from the X-ray reflection spectrum, using the powder method of Debye and Scherrer. It is a rhombohedral space-lattice having a characteristic angle of 60° 47', and a side of 5.96 Å. The unit cell contains 8 atoms, and therefore the volume of the unit molecular aggregate is

8M×1.65×10^-24/2.699 = 151.6×10^-24 c.c.

Where 2.699 is the density, Δ, of black phosphorus and 1.65×10^-24 is the mean value of the constant in which d is the distance between the diffracting planes.

Black phosphorus and combined phosphorus give characteristic absorption spectra for X-rays, the limits of wavelength being different in the respective cases. Red phosphorus shows two limits, which are those characteristic of black and of combined phosphorus. The absorption coefficient for X-rays is 5.68, as compared with 569 in the case of lead.

In its chemical properties black phosphorus is on the whole similar to red phosphorus. It ignites in air at about 400° C., and is insoluble in carbon disulphide.