Polymorphism and Phase Transformations

In Section 3.1.1 we encountered the crystallographer and chemist Eilhardt Mitscherlich who around 1818 discovered the phenomenon of polymorphism in some substances, such as sulphur. This was the first recognition that a solid phase 

The polymorphism of certain metals, iron the most important, was after centuries of study perceived to be the key to the hardening of steel. In the process of studying iron polymorphism, several decades were devoted to a red herring, as it proved this was the P-iron controversy. P-iron was for a long time regarded as a phase distinct from at-iron (Smith 1965) but eventually found to be merely the ferromagnetic form of ot-iron thus the supposed transition from P to a-iron was simply the Curie temperature, p-iron has disappeared from the iron-carbon phase diagram and all transformations are between a and y. 

Polymorphism in nonmetals has also received a great deal of study and is particularly clearly discussed in a book by two Indian physicists (Verma and Krishna 1966) which also links to the phenomenon of polytypism, discussed in Section 3.2.3.4. 

Of course, freezing of a liquid - or its inverse - are themselves phase transformations, but the scientific study of freezing and melting was not developed until well into the 20th century (Section 9.1.1). Polymorphism also links with metastability thus aragonite, one polymorphic form of calcium carbonate, is under most circumstances metastable to the more familiar form, calcite. 

The kinetics of diffusion-controlled phase transformations has long been a focus of research and it is vital information for industrial practice as well as being a fascinating theme in fundamental physical metallurgy. An early overview of the subject is by Aaronson et ai (1978). 

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Polymorphism and Phase Transformation of Poly (1-Butene)/Hydrogenated Oligo (Cyclopentadiene)... 

Metastable crystalline phases frequently crystallise to a more stable phase in accordance with Ostwald s rule of stages, and the more common types of phase transformation that occur in crystallising and precipitating systems include those between polymorphs and solvates. Transformations can occur in the solid state, particularly at temperatures near the melting point of the crystalline solid, and because of the intervention of a solvent. A stable phase has a lower solubility than a metastable phase, as indicated by the solubility curves in Figures 15.7a and 15.7/ for enantiotropic and monotropic systems respectively and,... 

Geophysicists have spent considerable efforts to develop experimental techniques to determine the phase diagram and physical properties of iron at high pressure and temperature since the 1950s. Much of our initial knowledge on the density and phase transformation of iron at high pressures is from dynamic shock wave experiments (e.g., Bancroft et al, 1956 McQueen and Marsh, 1966 Barker and Hollenbach, 1974 Brown and McQueen, 1986). However, structural information for high-pressure polymorphs of iron was obtained from static compression experiments combined with in situ X-ray diffraction measurements (e.g., Jamieson and Lawson, 1962 ... 

Kelly, R. Rodriguez-Hornedo, N. Solvent dependent nucleation and phase transformation of carbamazepine anhydrous polymorphs. 2006 (In preparation for submission). 

In this chapter, the effects of these thermodynamic, kinetic, and molecular recognition phenomena on crystallization and the role of solvent in these processes will be described. The role of solvent on crystallization, polymorphic outcome, and phase transformations will also be discussed. Experimental approaches for polymorph screening will be presented with an emphasis on the important considerations and strategies for solvent selection. 

Katrusiak, A. (1992). Stereochemistry and transformation of -OH-0= hydrogen bonds. Part I. Polymorphism and phase transition of 1,3-cyclohexanedione crystals. J. Mol. Struct. 269, 329-354. 

Theoretical and experimental studies of the role of solvent on polymorphic crystallization and phase transformations abound in the literature of the last few years and some pertinent examples are described here. For solvent-mediated transformations, the driving force is the difference in solubility between different polymorphs. An important earlier paper on the kinetics of such phase transformations [51 ] described a model featuring two kinetic processes in sohd to solid phase changes via a solution phase, namely dissolution of the metastable phase and growth of the stable one. 

The complexity of the typical solid deformation response can be further compounded by the presence of one or more polymorphic phase transformations, and a host of other phenomena typical of solids. Table 1.1 lists a number of such phenomena. 

The ideas developed in this chapter are descriptive of shock waves in fluids. Solids have many significant features that distinguish them from liquids and gases, such as shear strength, polymorphic phase transformations, heterogeneous structure, anisotropy, and viscoplastic behavior. The influences of these special properties of solids on shock compression are the topics of several of the other chapters, and for the most part are ignored in this introduction to the basic principles of shock compression. 

V. V. Murashov, I. M. Svishchev. Quartz family of silica polymorphs comparative simulation study of quartz, moganite, and orthorhombic silica, and their phase transformations. Phys Rev B 57 5639, 1998. 

Pressure-induced phase transitions in the titanium dioxide system provide an understanding of crystal structure and mineral stability in planets interior and thus are of major geophysical interest. Moderate pressures transform either of the three stable polymorphs into the a-Pb02 (columbite)-type structure, while further pressure increase creates the monoclinic baddeleyite-type structure. Recent high-pressure studies indicate that columbite can be formed only within a limited range of pressures/temperatures, although it is a metastable phase that can be preserved unchanged for years after pressure release Combined Raman spectroscopy and X-ray diffraction studies 6-8,10 ave established that rutile transforms to columbite structure at 10 GPa, while anatase and brookite transform to columbite at approximately 4-5 GPa. 

Secondary processing does not always lead to phase transformations, as was shown during studies of the polymorphs of ranitidine hydrochloride [92]. No solid-solid transformation could be detected during either the grinding or compression of metastable Form I, stable Form II, or of a 1 1 mixture of these forms. The dissolution rates of both forms were found to be equivalent, and the solution-mediated transformation of Form I to Form II was observed to be slow. 

Based on the reversibility of their phase transformation behavior, polymorphs can easily be classified as being either enantiotropic (interchange reversibly with temperature) or monotropic (irreversible phase transformation). Enantiotropic polymorphs are each characterized by phase stability over well-defined temperature ranges. In the monotropic system, one polymorph will be stable at all temperatures, and the other is only metastable. Ostwald formulated the rule of successive reactions, which states that the phase that will crystallize out of a melt will be the state that can be reached with the minimum loss of free... 

As it turns out, there are pharmaceutical implications associated with the polymorphism of glycerol esters, since phase transformation reactions caused by the melting and solidification of these compounds during formulation can have profound effects on the quality of products. For instance, during the development of an oil-in-water cream formulation, syneresis of the aqueous phase was observed upon using certain sources of glyceryl monostearate [13]. Primarily through the use of variable temperature X-ray diffraction, it was learned that... 

Another example of pressure-induced polymorphism is seen in the case of amiloride hydrochloride, where ball-milling Form-B causes a solid-state phase transformation into Form-A [43]. These workers deduced the phase relationship between two different pressure-induced polymorphs of the dihydrate, as well as the alternative route to one of those dihydrate forms that used the anhydrous form as the source material and effected the phase transformation through storage at high degrees of relative humidity storage. 

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