Phosphorus polymorphic forms

Four allotropes of phosphorus are known, the hexagonal /(-white, stable only below —77°C, the cubic a-white trap 44.1°C), the violet, and the black (which is thermodynamically the most stable). The a-white form is usually taken as the standard state. The violet is obtained by continued heating at 500°C of a solution of phosphorus in lead. When a-white phosphorus is heated to 250 C in the absence of air, a red variety (rap 590CC) is obtained which is believed to consist of a mixture of the a-white and violet allotropes, although the studies of the violet component in the mixture have shown that at least four polymorphic forms of red (violet) phosphorus exist. 

S. TJ. Pickering and A. E. H. Tutton could not produce the rhombic form. P. Jolibois obtained what he called a polymorphous form of red phosphorus stable below 450°, but there is nothing to show that this is essentially different from ordinary red phosphorus. 

Phosphorus crystallizes in at least five polymorphic forms. The white form is metastable and is prepared by condensing the vapour. There are apparently two closely related modifications of white P, with a transition point at —77°C. The... 

In the case of phosphorus, therefore, it is more than possible that we are dealing, not simply with two polymorphic forms of the same substance, but with polymeric forms, and that there is no transition point at temperatures above the absolute zero, unless we assume the molecular complexity of the two forms to become the same. The curve for violet phosphorus would therefore lie below that of white phosphorus, for the vapour pressure of the polymeric form, if produced from the simpler form with evolution of heat, must be lower than that of the latter. As the vapour-pressure curve of molten white phosphorus is continuous with that of molten violet phosphorus, we must assume that in molten phosphorus wc have an equilibrium between associated and non-associated molecules varying with the temperature. ... 

Black Phosphorus. Polymorphic. Orthorhombic crystalline form stable in air resembles graphite in texture produced from the white modification under high pressures Bridgman, J. Am. Chem. Soc. 36, 1344 (1914) Jacobs, J. Chem. Phys. 5, 945 (1937) Krebe, Inorg. Syn. 7, 60 (1963), d 2.691. Does not catch fire spontaneously. Insol in organic solvents. Amorphous form prepd at lower pressures Ja-cobe, loc. cit At higher pressure the orthorhombic form undergoes reversible transition to a rhombohedral structure, d 3.56, and a cnbic structure, d 3.83 Jamieson, Science 139, 1291 (1963). 

White and red phosphorus are used commercially for several purposes. Polymorphic black phosphorus is formed from white phosphorus under high pressure (Merck 1996). It is stable in air and the fire hazard is low. It does not catch fire spontaneously. On the other hand, white and red phosphorus are flammable and highly toxic. A detailed discussion on these is given below. 

Fosinopril sodium, an angiotensin-converting enzyme inhibitor, has been found to exist in two anhydrous polymorphic forms. No single crystal of sufficiently high crystallographic quality was obtained for the polymorphic phases and polymorphism was characterized by solid state and P NMR. The analysis of chemical shifts in C spectra suggested that polymorphism originates from conformational differences in the acetal side chains and/or cis-trans isomerization about the peptide C-N bond. The difference in chemical shift between P resonances in the two spectra is 2.2 ppm and demonstrates the existence of various envirorunents of phosphorus nuclei in each polymorph [43]. 

Details of the structure of vitreous phosphoric oxide are dependent upon the source of P2O5 used to produce the melt. Crystalline phosphoric oxide exists in three polymorphic forms hexagonal, orthorhombic, and tetragonal. These crystals all contain phosphorus-oxygen tetrahedra, but contain different intermediate range units in the form of... 

Phosphorus nanotubes (or nanorods), although not yet properly characterised, will constitute yet another polymorphic form of the element. Several types are present in the structures of complexes such as (CuI)3Pi2 and (Cul)3P]2, which may serve as starting points for their synthesis [39]. 

Phosphorus pentoxide exists in at least four polymorphic forms as well as a glass. The common variety of laboratory and commerce already discussed (mp = 420 C, p = 2.30 g/cc) consists of large cage molecules (4.41a) arranged in a hexagonal crystal lattice with only weak van der Waals-type forces between the cages. This was sometimes known as the H form or Form T of the oxide. 

Phosphorus (like C and S) exists in many allotropic modifications which reflect the variety of ways of achieving catenation. At least five crystalline polymorphs are known and there are also several amorphous or vitreous forms (see Fig. 12.3). All forms, however, melt to give the same liquid which consists of symmetrical P4 tetrahedral molecules, P-P 225 pm. The same molecular form exists in the gas phase (P-P 221pm), but at high temperatures (above 800°C) and low pressures P4 is in equilibrium with the diatomic form P=P (189.5 pm). At atmospheric pressure, dissociation of P4 into 2P2 reaches 50% at 1800°C and dissociation of P2 into 2P reaches 50% at 2800°. 

P2vi2P and P4 2P2. R. Wegscheider and F. Kaufler argued that yellow and red phosphorus are not merely polymorphous, but that they are chemically different, because, if polymorphous, the liquid forms should be identical, and if violet phosphorus is the yellow phosphorus, or a sat. soln. of yellow phosphorus... 

As a ligand, elemental phosphorus not only appears in the oligomeric forms present in its polymorphs (like P4), but as P with n covering a large range from n = 1 to at least n = 6, and as the corresponding cations and anions (the polyphosphides). 

The easiest and usually most precise quantification of polymorphic composition is achieved when a sensitive heteronucleus (such as fluorine or phosphorus) is present in the molecule, and its spectrum shows chemical shift differences between the polymorphs. Direct polarization experiments of these nuclei are sensitive, and therefore the polymorphs can be quantified quickly directly from the peak ratios. The limit of quantification can be well below 1% of one form in the presence of other forms. As an added advantage, the spectra of these nuclei are usually very simple to analyze, since only a very limited number of resonances are typically present in the spectrum. 

Polymorphism occurs when different structures can occur for the same chemical formula. The atoms are the same but their atomic arrangement differs between the structures. Although ultimately thermodynamics (the minimum Gibbs free energy) dictates which of the probable structures is formed at a particular temperature and pressure, other factors such as electrostatic interaction mean that a variety of different structures is possible. Polymorphism applies not only to elements, e.g. black and red phosphorus, but also to compounds such as calcium carbonate, which can exist in a number of forms including calcite, aragonite and vaterite. 

Red phosphorus. Red to violet powder polymorphism Roth et al, J. Am. Chem. Soe. 69, 2881 (1947) Corbridge, loc. cit. Crystal structure of one form, Htttmfs phosphorus Thum, K-rehs, Acta Cryst 25B, 125 (1969). The proparties of red phosphorus are intermediate between those of the white and black Forms. Sublimes at 416", triple point 589-5" under 43.1 atm. d 2.34. Inso] in organic solvents. So] in phosphorus tribromide. Less active than the white form reacts only at high temp. Yields the white modification when distilled at 290". Catches fire when heated in air to about 260 and burns with formation of the pantoxide. Bums when heated in an atmosphere of chlorine. 

---