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Last edited on 15. April 2016

Overview and Classification

Definition of a Solid

Solid State Physics deals - as the name already implies - with the physical properties of solid materials. A material is referred to as solid if it is composed of a larger number ($\propto 10^{23}$) of smallest constituents (atoms, molecules, …) which are in fixed positions and very tightly packed with a strong mutual attraction.[1] Ideal solids are incompressible just as liquids but in contrast to liquids solids show a strong resistance to shear stress which can be regarded as the main distinctive feature.[2] Due to these properties usually strong forces/torques are required to change the shape of a solid.

Classification of Solids

The most common classification of solids regards the extend of inner order on a large scale. Solids which exhibit a very regular, periodic structure are referred to as crystals while solids with no such positional large scale order are called amorphous.[3] [4] [5]

Many solids have their origin in a molten substance that is cooled down. When this cooling process is slow enough, the constituents have enough time to arrange themselves in the positions of lowest energy which usually produces a very regular pattern. Materials that consist only of single atoms or very simple molecules therefore often form crystals. An exception is glass which has a highly unordered (amorphous) structure (due to rather fast cooling processes). Large molecules as for instance in organic materials or plastics usually also form amorphous solids.[6]

Difference between crystalline, polycrystalline and amorphous materials.
The ideal crystal has a high periodic order which is maintained through the whole crystal whereas an amorphous material is completely unordered. A polycrystalline material is composed of small crystalline groups (crystallites) which have however different orientations. The boundaries between these groups are therefore called grain boundaries.[7]


[1] W. Demtröder Experimentalphysik 1 Springer 2013 (ch. 6.1)
[2] E. N. Economou The Physics of Solids Springer 2010 (ch. 3.1)
[3] S. Hunklinger Festkörperphysik De Gruyter 2014 (ch. 1)
[4] E. N. Economou The Physics of Solids Springer 2010 (ch. 3.1.1)
[5] J. S. Blakemore Solid-State Physics Cambridge University Press 2004 (pp. 4-5)
[6] E. N. Economou The Physics of Solids Springer 2010 (3.1.4)
[7] R. Gross, A. Marx Festkörperphysik De Gruyter 2014 (ch. 5.2.3)