Polymorphism is the ability of solids to form more than one crystalline lattice, that is stable for different pressures and temperatures. These lattices are called polymorphic forms of the solid structure. Stable modifications with normal and low temperature is denoted α, modifications stable with higher temperatures is denoted β, γ, etc.

Polymorphism is mostly a characteristic of technical materials. The clear example is calcium carbonate polymorphism (Figure 5). The image is courtesy of Thermo Fisher Solutions, Electron Microscopy Solutions.

Figure 5
Figure 5. Calcium Carbonate polymorphism. Transmission electron microscope (TEM) image

Carbon polymorphism is a practically interesting phenomena – transformation of diamond into graphite. Graphite is a more stable structure then diamond in normal conditions. However, graphite stability decreases, and diamond increases when the pressure goes up.

Amorphous structures
Crystalline structure is natural for most solid compounds, because energy of a system with periodic arrangement of atoms is the lowest. However, non-compounds tend to be arranged in a periodic structure. The reason, for example, can be a low diffusion rate of atoms during the cooling process. Solids with a chaotic arrangement of atoms and molecules are called amorphous. Amorphous solids are isotropic, and do not have an exact melting temperature. The example of amorphous structures are glasses and plastics. However, glasses have some order among the nearby atoms.

Solid band theory

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