**7. Crystal growth**

When a solid is heated, some physical and chemical changes sintering, melting, and thermal decomposition can be observed. Sintering process results from crystal growth at the contact area between adjacent crystallites. As to conclude, the crystallites connect to each other, and the size increases [41–43].

In high temperatures, as a result of ion movements, melting occurs. The ordered lattice array is replaced by the short-range order of the liquid state. Crystallization may proceed in the light of several different ways:


It is well-known that there is a remarkable correlation between stability and energy. Stable states have low energy values. For the processes, final energy values of solids must be lower from the free energy of initial state of the systems. It should be noted that crystal formation process forms from two steps. First step is the formation of a new nucleus. Second step is the growth of the nucleus formed to form a particle of appreciable size. Crystals may contain some defects. In terms of the determination of crystal properties, these defects are quite important. In addition to these defects, distortion of lattice also may be possible. This condition is called as dislocation. Many important properties of crystals are due to the regions and numbers of these dislocations. Crystal growths can be via the following types:


In high-temperature solution growth, the constituents of the crystals are dissolved in a suitable solvent and then when the solution becomes over saturated, crystallization occurs. In flux growth, crystals like ceramics and ferrites have been grown by the slow cooling of a solution in a molten flux. Chemical transport reactions are widely considered in the preparation of single crystals like magnetite.
