Displacive polymorphs

Fig. 3. The main crystallographic forms of siUca stable at atmospheric pressure. The vertical directions represent the facile, displacive polymorphic transitions, whereas the horizontal directions represent the sluggish reconstmctive transitions (44).

In addition to the frequency considerations attendant to the examination of polymorphisms or haplotypes, one must also consider the impact of possible differences in the magnitude of effects of any putative loci. The magnitude of any effects is denoted as "scale" effects based on the notion from quantitative genetics that there will be a displacement from the overall population mean for a trait that is dependent on genotype. To illustrate the effects of scale and frequency, consider two well-known examples of genetic effects. These are the effect of the apolipoprotein E (apo E) polymorphism on cholesterol levels and the impact of the familial hypercholesterolemia polymorphism on cholesterol levels. 

Polymorphic transitions render the thermodynamic description of feldspar end-members rather complex. According to Helgeson et al. (1978), the transition between the monoclinic and triclinic forms of NaAlSi308 may be regarded as the overlap of two high-order transitions (cf section 2.8), one associated with displacive structural modifications and the other with ordering on tetrahedral sites. The analysis of calorimetric data by Holm and Kleppa (1968) concerning the enthalpy... 

Crystal phase transitions are a possible target with present day computational means, when the transition is a smooth one and does not involve melting of the mother phase and subsequent recrystallization into the daughter phase. For crystalline OL-norleucine, an MD simulation has provided a detailed picture of the mechanism of a solid-solid second-order transition between two polymorphic crystal forms, showing concerted molecular displacements involving entire bilayers [61]. 

Atomic Coordinate Displacements in Amino Acid Residues of H-Sites of GST-pi Polymorphic Proteins3 Resulting from Docking of CDNB... 

It is also probable that under proper circumstances a helix could displace the water in the open channel of the B-structure, thus converting it to the A-structure. This conversion has also been observed. A mixture of the A and B unit cells in the same structure is also possible and this, in fact, accounts for the C-polymorph. 

Zircon, complete solid-solution behavior is observed, and a plot of the unit cell volume against x shows that Vdgard s Law is followed. When the end members are not is structural, a systematic change in the solubility range in both structures is found as A is varied, and the data have been systematized in terms of a simple, potentially predictive, structure-field map. The pervasive polymorphism of these ABO4 compounds, involving both reconstructive and displacive transformations and metastable structures produced by different sample preparation methods, indicates that the crystal structural stability of substituted compounds needs to be carefully evaluated as a function of temperature to assess the structural integrity of waste-form materials. 

According to Moissan,4 ferrous oxide can exist in two polymorphic modifications, according as it is produced at high or at lower temperatures. The variety obtained at the lower temperatures, namely below 600° C., is more chemically reactive, uniting with oxygen, upon exposure to air, with such rapidity that the whole mass becomes incandescent. It decomposes water, slowly in the cold but with considerable rapidity at the boiling-point. It is readily soluble even in dilute acids, such as acetic acid, and easily displaces ammonia from its salts. 

Yang, Q.-C., Richardson, M. F. and Dunitz, J. D. (1989). Conformational polymorphism of dimethyl 3,6-dichloro-2,5- dihydroxyterephthalate. I. Structures and atomic displacement parameters between 100 and 350 K for three crystal forms. Acta Crystallogr. B, 15, 312-23. [215]... 

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