Concept of Unit Cell

The crystalline phase is composed of smaller units, called unit cells. A unit cell is basically a small set of atoms, ions, or molecules that are arranged in a particular way, which is periodically repeated in three dimensions on a lattice. Unit cells are the building block of the crystallites. The entire crystalHte stracture can be constmcted by pure translational repetition of unit cells. The overall properties of a crystallite stracture also can be calculated, at least theoretically, by considering an individual unit cell. 

There are totally 14 different types of unit cells, which meet the following criteria  

In a real crystallite, each unit cell contains a few atoms, ions or molecules, depending on the type of the material. For example, cesium chloride (CsCl) has a 

Orthorhombic simple Base-Centered BodyCentered Face-Centered 

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This is the matrix notation, in which the matrix M uniquely characterizes the relationship between the unit cells (note that the concept of unit cell is not unique — different unit cells can describe the same lattice — and so different matrices M can characterize the relationship between two given lattices). 

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