Ultraphosphates

Condensed phosphates in which at least some of the PO4 tetrahedra share three comer O atoms are called ultraphosphates. In contrast to structures which involve only doubly linked tetrahedra (i.e. polyphosphates and metaphosphates), most ultraphosphates hydrolyse extranely rapidly in contact with moisture. This instability is usually associated with the triply linked tetrahedra and may arise because energy from resonance stabilisation is lacking. 

Hydrolytic instability arising from triply linked tetrahedra may be offset in some high polymers by insolubility resulting from complexity of structure. The best-known examples of this are the highly polymerised forms of P2O5 whose hydrolysis rates are considerably slower than that of the conunon form 1 (Chapter 4.2). 

Comparatively few ultraphosphates have as yet been synthesised and isolated as crystalline individuals, although an almost endless number of structures are theoretically possible with triply-linked PO4 tetrahedra. In this respect they rival the great variety of the much more stable silicate mineral structures which are based on condensed Si04 tetrahedra sharing up to all four of their comers with neighbouring tetrahedra. 

The simplest ultraphosphate anions are the branched isomers of the lower polyphosphates and metaphosphates. Individual isotetraphosphate and isotetrametaphosphate salts (5.186) have not so far been isolated as well-defined salts, but these anions are undoubtedly formed as intermediates during the break up of larger polymer units. The number of possible isomers increases rapidly with molecular weight, for example, isopentametaphosphates PsOfj. 

Among the earliest ultraphosphates to be characterised were CaP40n and Ca2P60i7 in 1944 [71]. These compounds were later shown to contain infinite two-dimensional anionic sheets. 

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The ultraphosphates are situated between P O q and the metaphosphates. These comparatively Htde-known, highly cross-linked polymers contain at least some of the phosphoms atoms as triply coimected branching points. This stmctural feature is quite unstable toward hydrolysis. Ultraphosphates undergo rapid decomposition upon dissolution. In amorphous ultraphosphates, the cross-linking is presumably scattered randomly throughout the stmctural matrix in contrast, crystalline ultraphosphates have a regular pattern. 

Condensed phosphates are derived by dehydration of acid orthophosphates. The resulting polymeric stmctures are based on a backbone of P—O—P linkages where PO tetrahedra are joined by shared oxygen atoms. The range of materials within this classification is extremely broad, extending from the simple diphosphate, also known as pyrophosphate, to indefinitely long-chain polyphosphates and ultraphosphates (see Table 1). Both weU-defined crystalline and amorphous materials occur among the condensed phosphates. 

It is known that ammonium polyphosphate tends to dissociate liberating ammonia above 300 C and the resulting free hydroxy groups condense giving crosslinked structures (ultraphosphate) with elimination of water (28.29) ... 

Above 410 C the ultraphosphate undergoes a relatively slow degradation process which is completed at about 650 C with formation of a residue (35% of original phosphate) mostly stable to 950 C... 

Figure 12). In this step the DTG curve shows a very broad peak with a narrower maximum superimposed (550 C) indicating the occurrence of different overlapping processes. This thermal behaviour cannot be explained on the basis of that of melamine condensation products or of ultraphosphates (e.g. ammonium salt). Indeed melamine condensate undergoes complete fragmentation to volatile products below 750 C (18) while ammonium ultraphosphate does so mostly below 700 C (29). in TG at 10 C/min. The presence of P in the material obtained at 650 C is shown by the solid state 31P NMR which however gives broad complex... 

Binary systems, evolving separation strategies for, 22 322-325 Binary ultraphosphate glasses, diffraction studies of, 72 574-575 Binders... 

Other Nd—O distances are Nd—0(3) =2.35 A and Nd—0(4) =2.33 A (129). The separation between two neod miiums is 4.234 A. However, the ultraphosphates (P5O14) of Nd(HI) and Sm(HI) containing MOg units have a different structure (129,130). The coordination polyhedra is best described as being bicapped trigonal prism (see next section). 

Gouy method, 32 4, 10 Graham s salt, 4 4, 5, 6, 7, 10-11, 39 glasslike, 4 41-50 tetrametaphosphates from, 4 20 trimetaphosphate from, 4 44-45 ultraphosphates from, 4 56 Grain coarsening, 31 14 Graphite... 

Ultraphosphates are defined as phosphates in which the ratio Me1 is less than unity. The existence of such compounds, e.g. Ag2P60,o, PbP6Oio and Ca2PeOi7, was detected in the corresponding melts some time ago (165) and the compounds CaP4Ou and CasPeOn were described as crystalline solid phases (136). In such compounds P04 tetrahedra must be present which are linked by three of their O atoms with neighboring P atoms (349). 

When crystallization is completed the fibrous phosphate can be extracted from the ultraphosphate matrix by leaching the system with hot water. 

According to the current classification, condensed phosphates are divided into cyclophosphates, polyphosphates and branched inorganic phosphates (or ultraphosphates ). 

Ultraphosphates, which contain branching units. The ultimate ultraphosphate is P4Oio, consisting entirely of branch units. 

Phosphoms pentoxide may be regarded as a neutral ultraphosphate containing only three-connected phosphate tetrahedra. Two extended forms of P2O5 are known, one containing infinite sheets, and the other a three-dimensional... 

The short, strong P=0 bond in ultraphosphates gives the highest IR absorption of all phosphates, at 1300-1400 cm-. Otherwise the spectra of ultraphosphates (which consist of fused rings) resemble those of cyclophosphates. 

Vitreous P2O5 and phosphate-rich glasses are difficult to prepare, owing to the volatility of P2O5, and are very moisture sensitive. Vibrational and MASNMR spectra show that they contain three-dimensional phosphate networks comparable to those in the extended forms of P2O5 and ultraphosphates. 

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