Polymorphic transformations displacive

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. 

The effect of temperature on polymorphic transformations that take place during compression has been studied. Tableting temperature was found to exert a definite effect on the polymorphic transformation of chlorpropamide during compression, and on the physical properties of the produced tablets [49]. Compression stress, distance, and energy were measured using a noncontact displacement transducer mounted... 

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. 

The low-pressure silica polymorphs include quartz, tridymite, and cristo-balite. The stable phase at room temperature is a-quartz or low quartz. This transforms to 3-quartz or high quartz at approximately 573°C at 1 bar. The transition from (3-quartz to tridymite occurs at 867°C and tridymite inverts to 3-cristobalite at 1470°C. P-Cristobalite melts to silica liquid at 1727 C. All three of these stable silica polymorphs experience displacive transformations that involve structural contraction with decreased temperature and all can be cooled stabily or metastabily to room temperature in glass-ceramics compositions. ... 

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. 

Another example of polymorphic change involving a change in co-ordination is that between the cubic body-centred and cubic close-packed structures of a- and y-iron. Although a change in co-ordination takes place, it should nevertheless be noted that the transformation can be achieved by a purely displacive mechanism, and accordingly takes place readily. In fig. 9.02 four unit cells of the cubic body-centred a-iron... 

The polymorphism of silicas is based on different linkages of the tetrahedral [SiOj4- units (2). Quartz has the densest structure, and tridymite and cristobalite have a much more open structure. All three forms exist in a- and / -forms, which correspond to low- and high-temperature modifications, respectively. The a- and / -modifications differ only slightly in the relative positions of the tetrahedral arrangements. This similarity is evident from the fact that the conversion a (3 is a rapid displacing transformation that occurs at relatively low temperatures. Quartz is the most stable modification at room temperature all other forms are considered to be metastable at this temperature (2). 

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