Structure types black phosphorus

Fig. 51. The SnS-type structure of GeSe (left) and the structure of black phosphorus (right).

There is sometimes ambiguity as to which nearest neighbours to assign to the local coordination polyhedron. This is illustrated by the two different puckered layer structure types which are displayed by the most stable polymorphs of phosphorus and arsenic respectively. They may both be thought of as resulting from the breaking of three bonds about each atom on a simple cubic lattice as shown in Fig. 1.5. The layers then distort Black... 

Fig. 1.6 The breaking of three-bonds about each simple cubic site that leads to the black phosphorus and arsenic structure types. After Burdett and Lee (1985).

H-bonded sheets analogous to those of black phosphorus are possible when two thirds of the trans-fusion between the super-cyclohexane chairs is replaced by effusions. Such a super-black phosphorus type sheet structure may be obtained by cutting through a cubic diamond lattice (Figure 6) rather than a hexagonal one [28],... 

The monosulfides of Ge, Sn and Pb are all obtained by precipitation from aqueous media. Both GeS and SnS crystaUize with layer structures similar to that of black phosphorus (see Section 15.4). Lead(II) sulfide occurs naturally as galena and adopts an NaCl-type structure. Its formation as a black precipitate (Aijp 10 ) is observed in the qualitative test for H2S (equation 14.78). The colour and very low solubility of PbS suggest that it is not a purely ionic compound. 

Phosphorus also exists in several allotropic modifications. White phosphorus in the crystalline state consists of discrete P4-tetrahedral molecules and is soluble in carbon disulfide. However, on the addition of catalysts and the application of pressure, it changes first to red, then to violet, and finally to black phosphorus. For this it is necessary to have 35,000 bar at 20°C, and 12,000 bar at 200 C. Thus, the ceiling temperature in this equilibrium polymerization must lie below the melting point of white phosphorus at 1 bar. Black phosphorus possesses a complicated parquet structure similar to graphite and is a type 2 polymer that is no longer soluble in CS2. Both red and violet phosphorus possess a lower degree of polymerization. 

Just as the half-filled one-dimensional chains of the previous section underwent a Peierls distortion, so we expect that the simple cubic lattice with three p electrons would be similarly unstable. A distortion of this type has indeed occurred when we look at the observed structures of elemental black phosphorus (13.46) and arsenic (13.47). Alternate linkages have been broken along all three directions leaving each atom pyramidaUy three coordinate. The result is in accord with their atomic conjuration. At the simple cubic structure Px,Py and p bands would each be... 

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