Polymorphism

Polymorphism - 1 Polymorphism and Polytypism Definition of...

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1 Polymorphism and Polytypism Definition of polymorphism : A phenomenon which occurs whenever a given chemical compound exists in more than one structural form or arrangement. Reasons for polymorphism: 1- Borderline r + /r - values in ionic compounds (sometimes but not always) 2- Variations in P & T render one structure more stable than another; e.g., at higher T, the internal energies of the structure increase as a result of the increase in the frequency of vibration of the atoms. Another way to express this is to say that the degree of “disorder” of the structure increases with increasing T. Rules: 1- As P increases, structures with higher densities and larger coordination numbers are favored (i.e. become more stable). 2- As T increases, structures with low densities and smaller coordination numbers are favored. 3- As T increases, structural modification (from one polymorph to the other) frequently result in an increase in overall symmetry. Example: Quartz (trigonal) Cristobalite (cubic). Types of polymorphic transitions:
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This note was uploaded on 02/27/2012 for the course GLY 314 taught by Professor Staff during the Spring '09 term at Marshall.

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Polymorphism - 1 Polymorphism and Polytypism Definition of...

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