Polymorphic transformations reconstructive

At the temperature limits of their stability ranges, the main forms of silica interconvert. The transformations involve a change in the secondary (nonnearest-neighbor) coordination and require the breaking and reformation of Si—O bonds. The transformation processes, known as reconstructive polymorphic transformations (44), are slow, as shown by the fact that the high temperature polymorphs can persist outside their normal stability range. 

As single-phase substances are heated or cooled, they can undergo a number of polymorphic transformations. Polymorphs are different crystalline modifications of the same chemical substance. These transformations are quite common and include crystallization of glasses, melting, and many solid-solid phase transformations, some of which are described below. In general, there are two types of polymorphic transformations, displacive and reconstructive. 

From the stmctural point of view, polymorphic transformations can be of two types [1, 2], viz. reconstructive transitions with change in of atoms (Fig. 9.1) and displacive transitions , in which the positions of atoms change insignificantly and Nc remains the same (Fig. 9.2). For stmctural chemistry, the former transformations are most important. As energies of phase transitions amount (at most) to several percent of the atomization energy of solids, exact calculations of the thermodynamic stability of phases are very difficult. At present, the most effective are crystal-chemical approaches to estimating the reasons and results of polymorphism. 

Figure 1. a) HRTEM image of polymorph B of zeolite beta along the [1-10] direction. Insets are (from left to right) Fourier transform and averaged images with p and p2 symmetries applied, b) The 3D potential map reconstructed from this HRTEM image. All nine unique Si atoms were obtained from this 3D map, and are shown superimposed on the potential map. 

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. 

Transformations between the polymorphs of crystalline amino acids often require the reconstruction of the extended hydrogen-bond network in the crystal, are kinet-ically controlled and related to recrystallization. Both the crystal growth and the polymorphic transitions in the crystals are therefore extremely sensitive to the interaction with molecules of gases or liquids that can be involved in the formation of hydrogen bonds with molecules at the surface. 

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