Types of Unit Cell

A regular three-dimensional arrangement of atoms or ions makes up crystals. Unit cells are the smallest structures that are repeated continuously in three dimensions to create crystalline solids (or crystals). A unit cell can be compared to a wall brick; just as the shape of a brick relies on its shape, so does the shape of a crystal. Consequently, a unit cell is a crystalline solid’s basic elementary pattern. Identification of the crystal’s unit cell is necessary for crystal classification.

Types of cubic unit cell

Three different types of unit cells are formed by cubic crystals in these 14 crystal systems.

Primitive or simple unit cell

Body-centered unit cell

Face-centered unit cell

Primitive cubic unit cell

in a simple or primitive cubic crystal structure where atoms are only found in the corners. Because each cube has eight corners, each atom is shared evenly by eight atoms.

As a result, each atom contributes 1/8 to the unit cell. As a result, there are only one atom and eight atoms in each unit cell,

= 8 × (1/8) = 1 only

Body centered cubic unit cell

The unit cell is the only unit to which the body-centered atom belongs. As a result, there are a total of 2 atoms per unit cell

= 1 (for the body) + 1 (for corner) = 2

Examples of body-centered cubic crystal lattice –

Body-centered cubic lattice is used to create alkali metals like lithium, sodium, potassium, rubidium, and cesium in the periodic table. This particular sort of cubic crystal structure has eight atoms at each cube corner and one atom in the center.

The corner and body components of cesium chloride are distinct from one another. In the cubic crystal, the cesium cation is in the center and all the chlorine ions are in the corners, or the other way around.

Face centered cubic unit cell

Atoms are found at the six faces and the eight corners of a face-centered cubic lattice. However, two unit cells share the face atoms equally.

As a result, there are 4 atoms overall per unit cell

= 3 (for face) + 1 (for corner)

= 4

As a result, face-centered unit cells have a higher element density than primitive or body-centered unit cells.

Example of face-centered cubic crystal lattice-

Numerous chemical elements found in our environment’s periodic tables, such as copper, silver, gold, nickel, and platinum, as well as solidified inert gases (helium, neon, argon, krypton, and xenon), have face-centered cubic crystal structures.

Crystal lattice

The systematic placement of atoms, ions, or molecules in space or a three-dimensional system result in the formation of a crystal lattice, also known as a cubic crystal lattice. Every crystalline solid has three or four lattice planes, and the distance between them shows where the components are located or where they are located at.

Examples of crystal lattice

In sodium chloride, the sodium atom is ionised and loses one electron to create a sodium ion; the chlorine atom then gains this electron back to produce a chloride ion. With the release of lattice energy, they are held together by the electrostatic force of attraction to create the ionic body-centered cubic sodium chloride crystal.

But water molecules establish hydrogen bonds with one another to form ice crystals. Different forms of crystals develop as a result of various forces, including electrostatic, hydrogen bonding, and van der Waals..

Law of rational indices

We need to talk about how the plane that passes through the lattice point of the cubic crystal is oriented in order to describe the crystallographic structure of a crystalline solid. However, the lattice plane’s orientation is described by its intercepts on the three lattice basis vectors.

Any plane’s intercepts along any of the three crystallographic structures are either equal to the unit cell ratio or a multiple of that ratio that is an integral. This is referred to as the rotating indices law.

Conclusion

Crystal structure is the phrase that is used to describe the organised arrangement of atoms, ions, and molecules that may be found in a crystalline substance. When the properties of particles arrange themselves in a pattern that is symmetrical and which repeats with the fundamental direction that matter takes in three-dimensional space, the result is an ordered structure. The recurrent translation of the unit cell along its primary axis creates the structure and symmetry of the whole crystal, which is reflected in the unit cell’s own structure and symmetry. The lengths of the major axes and the angles formed by the unit cell are referred to as the lattice constants. Cell parameters or lattice parameters are the names given to these elements. The idea of the space group provides the framework for describing the symmetrical characteristics of the crystal. Crystals’ cleavage, optical transparency, and electronic band structures are only few of the many physical qualities that are heavily influenced by the structure and symmetry of the crystals themselves.