Phosphorus, black Hittorf

Figure 4 The structures of P4, white phosphorus (1) Hittorf s violet allotrope (2) Orthorhombic black allotrope (3) Rhombohedral black allotrope (4) showing the hexagonal arrangement and the distortion from planar the phosphides [Pis] (5), [P2i] (6) and...

A review of the alleged allotropes of phosphorus reduces their number to four, namely, the a- and/3-forms of yellow phosphorus, red or violet phosphorus, and black phosphorus. Most of the work of various investigators has been directed towards elucidating the nature of red phosphorus, and of the transformation of yellow to red phosphorus and conversely. Red phosphorus was formerly considered to be amorphous, and it was often called amorphous phosphorus. The term amorphous, however, here referred more to the general appearance of the powder rather than to its minute structure. J. W. Retgers 5 showed that the particles of ordinary red phosphorus are rhombohedral crystals, which are well developed in those of W. Hittorf s violet phosphorus. All four varieties are therefore crystalline. J. W. Terwen has reviewed this subject in a general way and M. Copisarow discussed the theory of allotropy,... 

Fig. 14.3 (a) The tetrahedral P4 molecule found in white phosphorus, (b) Part of one of the chain-like arrays of atoms present in the infinite lattice of Hittorf s phosphorus the repeat unit contains 21 atoms, and atoms P and P" are equivalent atoms in adjacent chains, with chains connected through P -P" bonds, (c) Part of one layer of puckered six-membered rings present in black phosphorus and in the rhombohedral allotropes of arsenic, antimony and bismuth. 

The substitution of As for P in any compound with P-P linkages is possible, at least in principle. Amongst the simplest known examples are white phosphorus derivatives P4. As , the black phosphorus derivative AsP, AS2P and the mixed sulphides P4. tAS tS3 (Chapter 4.3). Grey arsenic and black phosphorus have been reported as forming limited solid solutions, but the composition P20AS is believed to consist of ordered chains of P atoms as in Hittorf s phosphorus. The As atoms occupy special positions which allow them to link these adjacent chains. 

Whereas single-crystalline flakes of black phosphorus can be grown from a bismuth solution [138], crystallization from liquid lead leads to red platy crystals of another modification, called Hittorf s phosphorus. Its monoclinic structure [137] (Cih—P2c, Z = 84, all atoms in 4g) consists of cage-like Pg and P9 groups which are linked alternately by pairs of P atoms to form tubes of pentagonal cross section. Parallel tubes form double layers in which tubes of different layers are approximately perpendicular (at 89.6°) to each other. In one layer, the tubes are parallel with [110], in the crossed layer, parallel with [iTO]. Each tube is /... 

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