Ammonium nitrate polymorphism

Polymorphism is a condition in which a specific chemical substance may crystallize iato different forms. For example, ammonium nitrate exhibits four changes in form (5) between —18° and 125°C ... 

A specific polymorph may be absolutely essential for a crystalline product, for example, one polymorph may have a more desirable color or greater hardness or disperse in water more easily than another polymorph. Often, one polymorphic form is more stable than another (for example, at 80°C the orthorhombic I form of ammonium nitrate is more stable than the trigonal form) at conditions to which a product is exposed. An interesting approach to... 

B. The curve exhibits sharp breaks.—Two possible causes, namely, a change of polymorphic form or a change of hydration,1 will give rise to a sudden break in the curve. The former case is illustrated by ammonium nitrate, which is capable of existing m no fewer than four crystalline forms. Of these the /3-rhombic passes into the a-rhombic variety at about 32° C.2 At this temperature a break occurs m the solubility curve.3... 

Alternatively, ammonium nitrate (AN, QCq2 = 20.0 %, begins decomposition at m.p. = 169.9 °C, complete decomposition at 210 °C) has been discussed, however this compound has severe burn rate issues. Furthermore, AN is hygroscopic and shows phase transitions from one polymorph to another at 125.2 °C, 84.2 °C, 32.3 °C and -16.9 °C. Phase stabilized ammonium nitrate (PSAN) and spray crystallized AN (SCAN) are special qualities provided by ICT. 

Fig. 4.1 An illustration from the original paper by Lehman (1877i>) on Physical Isomerism showing his hot stage microscope and various shapes of crystals observed. Another noteworthy feature of this figure is the time vs temperature cooling curve for ammonium nitrate in the upper left hand comer, showing four inflection points that indicate polymorphic transitions. (Reprinted by permission from Zeitschrift fur Kristallographie.)...

Ammonium nitrate is hygroscopic and readily soluble in water (the saturated solution contains about 65% NFI4NO3). Transitions from one polymorph to another take place at 125.2 °C, 84.2 °C, 32.3 °C and -16.9 °C. The product shows a great tendency to cake. The difficulties therefore involved are avoided by transformation into -+ Prills. Ammonium nitrate is marketed as dense prills and as porous prills. Both can be introduced in industrial explosives after milling except -> ANFO blasting agents, which need unmilled porous prills. 

The commonest polymorphic changes of this type are those associated with the onset of free rotation in a crystal structure. Many simple molecular compounds (HC1, HBr and CH4 are examples) show a transition with rising temperature from a complex structure to a simple close-packed arrangement in which the molecules effectively acquire spherical symmetry by free rotation. Similar effects are displayed by a number of ionic crystals containing complex ions of un-symmetrical shape. Thus NaCN, KCN and RbCN have complex structures at low temperatures but transform at higher temperatures to the sodium chloride structure in which the CN ions behave as spherical entities. In some cases rotation may take place only about one axis, so that the molecule or group acquires cylindrical rather than spherical symmetry, and in other cases rotations about different axes may be excited successively at different temperatures. An extreme example of this is ammonium nitrate, in which both cation and anion are capable of... 

Ammonium nitrate (melting point 169.6 °C) exhibits five polymorphs and four enantiotropic changes between —18 and 125 °C, as shown below ... 

Polymorphism is commonly encountered in crystalline substances. Calcium carbonate, for example, has three polymorphs, ammonium nitrate has five (section 1.8), and some organic compounds have many more. Aspirin, for example, was once thought to have 6 or 8 and phenobarbitone as many as 13, but it is always worth keeping in mind the somewhat provocative eomment, generally attributed to McCrone (Dunitz and Bernstein, 1995), that the number of polymorphs discovered often seems to be proportional to the time and money spent looking for them. 

Phase transformations can also occur in the solid state, and this mode is particularly common in organic solids held close to their melting point. The roles of both solid-state and solvent-mediated polymorphic transformations have been studied with ammonium nitrate by Davey, Guy and Ruddick (1985) and with oleic acid by Suzuki, Ogaki and Sato (1985). 

Polymorphism and mechanical stability of ammonium nitrate granules the influence of additives, in D BIWIC 1993, p. 51, Delft + Bremen 1993... 

FIG. 2 Polymorphism in ammonium nitrate (top) and effect of additives on crystal habit modifications (bottom). 

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