Phase transformations, general features

Determination of the influence of crystal structure and reactant environment on deammination and dehydration processes is complicated by the several solid phase transformations that are a characteristic feature of many ammonium salts. Sublimation and/or melting may also occur. Deammination and dehydration steps are generally reversible. At high temperatures, however, particularly in the presence of a residual oxide... 

Because of the crystal anisotropy, on the other hand, the sequence and the frequency of phase transformations depend on the character of compression. For example, the compression in z forces the molecules to develop more rapidly electrostatic interactions, which in turn affect the electronic charge of the nitro group and therefore result in a steeper decrease of the band gap with applied pressure. Since charge transfer is important in the description of atomic forces, we expect that its realistic description within the SCC-DFTB method captures the general features of the molecular response to external stress. 

Martensitic transformations in alloys are essentially order-disorder displacive transitions that take place very rapidly, because atomic diffusion does not occur. The discussion of the formation of martensite in the Fe-C system, in Section 8.2.5, is an example. This transition is the transformation of a cubic phase containing excess carbon in interstitial sites into a tetragonal phase. As any one of three cubic axes can be elongated, three orientations of the martensite c axis can occur. This is a general feature of martensitic transformations and the different orientations that can arise are called variants or domains of the martensitic phase. These variants are simply twins (see Section 3.4.10). 

The thermal decomposition of a solid, which necessarily (on the above definition) incorporates a chemical step, is sometimes associated with the physical transformations to which passing reference was made above melting, sublimation, and recrystallization. Aspects of the relationships between physical transitions and decomposition reactions of solids are discussed in a book by Budnikov and Ginstling [1]. Since, in general, phase changes exert significant influence upon concurrent or subsequent chemical processes, it is appropriate to preface the main survey of the latter phenomena with a brief account of those features of melting, sublimation, and recrystallization which are relevant to the consideration of thermal decomposition reactions. 

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