Polymorphism structurally related lattices

For the study of polymorphs that are structurally related, the structural relationships between the polymorphs should be established first second, it should be explained why a particular substance is able to arrange its structural units in two closely related lattices, and finally, there should be a description of the manner and conditions tmder which rearrangement of the units from one lattice type to another can happen. It is the physical form of the drugs that is responsible for its degradation in the solid state. Selection of a polymorph that is chemically more stable is a solution in many cases. Different polymorphs also... 

The classification of polymorphic substances into monotropic and enantiotropic classes from the standpoint of observed phenomena is not appropriate as a basis for the lattice theory of polymorphism. In the lattice theory, one begins by considering whether the lattices of the polymorphic forms are related in structure or not. Only in the case of lattices that are structurally related in structure or not. Only in the case of lattices that are structurally related will it be possible for mutual transformation to take place at a transition point fixed by the temperature and pressure of the system. If the lattices differ in such a way that atoms or molecules must be completely regrouped during the transformation (changing their state of bonding), no point of contact for mutual reversible transformation will exist. 

Generally speaking, the concepts of monotropy and enantiotropy in phase theory appear to coincide with the structural concepts of unrelated and related lattices. Nevertheless, one must avoid equating the two, for it is certainly possible that one of two related lattices of the same substance is less stable than the other under all conditions of temperature and pressure. This would indicate the existenced of monotropy in spite of the existence of related lattices. This situation becomes especially important for polymorphic organic compounds, which form molecular lattices. 

Finally, new polymorphic structures can arise from different ways of assembling closely related coordination polyhedra. Three possibilities are known that lead to polymorphism of this type, which are most commonly found in oxidic lattices. First, the coordination polyhedron of one lattice particle is the same, while that of the other is only slightly changed (e g., silicon dioxide). Second, the coordination polyhedron of one lattice particle is the same, and that of the other is considerably changed (e.g., titanium dioxide). Finally, the coordination polyhedra are both deformed to varying degrees, with the basic lattice type being the same but the type of distortion different (e.g., zirconium dioxide). 

The stability of polymorphs is thermod5mamicaUy related to their free energy. The more stable polymorph has the lower free energy at a given temperature. The aforementioned classification of polymorphic substances into monotropic and enantiotropic classes, from the lattice theory perspective is not always appropriate. There is a need to explore the way the crystal lattice structures of... 

Polymorphs are classified according to the following terminologies. Concomitant polymorphs crystallize simultaneously from the same solvent and crystallization flask under identical crystal growth conditions. They may be viewed as supramolec-ular isomers in a chemical reaction. Conformational polymorphs occur for flexible molecules, i.e. these molecules can adopt more than one conformation under ambient conditions. When different conformers of the same molecule are present in the same crystal structure the situation represents conformational isomorphs. Conformational isomorphism, the existence of multiple conformations in the same crystal structure, is closely related to the presence of more than one molecule in the asymmetric unit, i.e. Z >1. The exact reasons why some crystals have Z > 1 are still not properly understood even as several research groups are working to seek answers to this enigma [9]. Pseudopolymorphism, [10] the occurrence of the same molecule with different solvent molecules in the crystal lattice or the same solvent in a different stoichiometry, is closely related to polymorphism. 

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