**5. Systems with complex phase behavior**

524 Advances in Crystallization Processes

Fig. 5. Phase diagram of succinonitrile (1) + pyrene (2) mixture system (Rai and Pandey, 2002).

A solid solution is a solid mixture in which one or more atoms and/or molecules of one of the components occupies sites in the crystal lattice of the other component without significantly changing its crystal structure, even though the lattice parameter may vary. So this kind of system has a homogenous crystalline structure and is also called isomorphic system, because the components are completely miscible in both the liquid and solid phases. Figure 1(B) shows the phase behavior of a binary mixture system that forms a solid solution. In the diagram, the curve ABC and ADC are the liquidus and solidus curves, respectively. The area above ABC curve represents the region of homogeneous liquid solutions and the area below ADC curve represents the region of homogeneous solid solution. The area enclosed by ABCD is the region of liquid + solid solution. For instance, a mixture M1 at temperature T1is cooled to temperature T2, the

**4. Solid solution** 

Due to the large molecular mass and complexity of the crystal structure of PAHs, i.e. polymorphism and racemate, the phase behavior of some of the PAH binary mixtures may be different from the above described three phase behaviors. Three complicated PAH binary mixture systems, i.e. anthracene + benzo[a]pyrene system (Rice and Suuberg, 2010), pyrene + 9,10-dibromoanthracene system (Fu et al., 2010), and anthracene + 2-bromoanthracene are introduced here.
