Polymorph Example

Variation of with atomic structure. Some materials are polymorphic examples are a- and y-iron, - and / -quartz, and the graphite and diamond forms of carbon. The chemical potential has a different value in each polymorph for example, at 298 K and 1 bar pressure, the values for graphite and diamond are ... 

Mineral polymorphs of this type are the usual sort of geochemical examples used to define metastable equilibria, and the usual definition centers on the fact that the metastable phase has a higher energy content than the stable phase. By focusing on the constraint that prevents the metastable phase from changing to the stable phase, we have not changed the concept of metastability but have simply broadened its applicability, as shown in the other examples. The perfect analogy between the partitioned gas example and the polymorph example should demonstrate the utility of the extra constraint definition we have used. 

Vibrational spectrosopy can be used to support structural elucidation by NMR and MS, but more typically it is used for identity testing, because IR and Raman spectra act as a fingerprint for molecular stmcture. However, both IR and Raman find their principal application in the investigation of polymorphism. Examples are described in this chapter, together with the benefits of coupling these techniques to microscopy. 

MAS Si speetnim of a sample of sodium disilieate (Na Si O,) erystallized from a glass is shown as an example. Whilst the statie speetnim elearly indieates an axial ehemieal shift powder pattern, it gives no evidenee of more than one silieon site. The MAS speetnim elearly shows four resolved lines from the different polymorphs present in die material whose widths are 100 times less than the ehemieal shift anisotropy. 

Crystalline Structures. Crystal shape of amino acids varies widely, for example, monoclinic prisms in glycine and orthorhombic needles in L-alanine. X-ray crystallographic analyses of 23 amino acids have been described (31). L-Glutamic acid crystallizes in two polymorphic forms (a and P) (32), and the a-form is mote facdely handled in industrial processes. The crystal stmeture has been determined (33) and is shown in Figure 1. 

Alkenoic acids also have polymorphic crystalline forms. For example, both oleic and elaidic acids are dimorphic with melting points of 13.6 and 16.3°C for oleic, and 43.7 and 44.8°C for elaidic acid (14). 

The primary phases all contain impurities. In fact these impurities stabilize the stmctures formed at high temperatures so that decomposition or transformations do not occur during cooling, as occurs with the pure compounds. For example, pure C S exists in at least six polymorphic forms each having a sharply defined temperature range of stability, whereas alite exists in three stabilized forms at room temperature depending on the impurities. Some properties of the more common phases in Portland clinkers are given in Table 2. 

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... 

An example of the use of polyacrylamide gels in an ief system in combination with immun obi otting is given in Reference 40 where this method is used to detect low quantities of group specific component (GC) subtypes. Another example of polyacrylamide in combination with ief is given in Reference 41 where the technique is used as a screening tool for inheritance of a certain polymorphic protein. 

Fig. 2.1. Some metals hove more than one crystal structure. The most important examples of this polymorphism ore in iron and titanium.

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

Allele variant forms oftheDNA sequence at a specified locus. For example, alleles at a single-nucleotide polymorphism (SNP) are characterized by the nucleotide that is changing. The combination of two alleles at a locus constitutes a genotype. 

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