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The structure of the crystal may be broken up into a number of parallelepipeds that are exactly the same. The name “unit cell” refers to each of these parallelepipeds. This parallelepiped is a full unit that may be used to produce the entire crystal via translation (stacking), similar to how individual bricks come together to build a wall. It is possible to think of a crystal as being made up of an unlimited number of unit cells like this. Therefore, a unit cell may be described as an imaginary parallelepiped, which is the starting point for the generation of the whole periodic crystal by the use of just translations.
The following are the two major categories into which unit cells can be placed:
Primitive Unit Cells (P): The constituent particles only occupy the four corners of the unit cell in this configuration.
Centered Unit Cells: One or more component particles are present at places within the unit cell that are not at the four corners of the structure. There are three distinct varieties of centred unit cells, which are as follows:
Body-Centered Unit Cells (I): This type of unit cell has a component particle in the middle, while others occupy the four corners of the cell.
Face-Centered Unit Cells (F): This type of unit cell has a component particle located in the middle of each face, while others occupy the corners of the faces.
Base-Centered (or End-Centered) Unit Cells (C): This type of unit cell is characterised by the presence of a component particle at the centre of any two parallel sides, while others occupy the cell’s corners.
Body-Centered Unit Cells (I):
This packing has the spheres in the first layer (type A) slightly separated, and the spheres in the second layer (type B) are inserted in the depressions between the spheres in the first layer. The packing is referred to as body-centered cubic, or BCC. The third layer is arranged such that it is parallel to the first. Therefore, it is an arrangement of the ABAB type, and the coordination number for each sphere is 8, with four neighbours in the layer above and four neighbours in the layer below. The space used up by this construction is merely 68 percent of the total. Metals such as chromium, molybdenum, potassium, and cesium are all examples of the bcc crystal structure.
Face-Centered Unit Cells (F):
Another sort of packing that may be created using the hexagonal close packing method. An octahedral hole is generated in the location where the triangular voids of the second layer are located above the triangular voids of the first layer. This vacuum is made up of three spheres originating from the bottom layer and three spheres originating from the higher layer. A regular octahedron is produced when the centres of these six spheres are joined together. If a third layer is placed on top of the second layer in such a way that it covers the octahedral holes, then the third layer will not align properly with either the first or the second layer. As a result, this layer is of type “C.” After the placement of the fourth layer, it will line with the first. Therefore, we may classify this structure as an ABCABC type . The term “facial centred cubic” refers to this structure (fcc). In this form, metals like aluminium, copper, lead, and gold, as well as ionic crystals like sodium chloride, lithium hydroxide, and magnesium oxide, can crystallise.
Base Centered (C)
Base Centered (C) is a type of lattice in which the points of the lattice are located on the corners of the cell, in addition to one additional point located at the centre of each face of one pair of parallel faces of the cell. One other name for it is end-centered. As a result, it possesses particles at each of the four corners and a single particle in the middle of each of the opposing faces.
Conclusion:
The tiniest part of a crystal lattice that displays the three-dimensional pattern of the complete crystal is called a unit cell. One can conceptualise a crystal as being composed of identical unit cells that are arranged in a three-dimensional grid. A crystal lattice is broken up into increasingly smaller sections called unit cells. There is a large variety of distinct types of unit cells. To give just one illustration, the basic cubic, face-centered cubic, and body-centered cubic unit cells that make up the cubic crystal system are the three distinct types of unit cells that make up the system. There are three different unit cells that make up the cubic crystal system. Each sphere is meant to stand in for either an atom or an ion. The only locations that atoms or ions can occupy in a basic cubic system are the four corners of the unit cell.
