# Crystal Structure

## Crystal structure and its Types

The arrangement of the atoms, molecules, or ions to form a significant shape of an element or compound is called its crystal structure. It is generally found in solid chemical substances. It is a pattern that depicts the intrinsic properties of the particles constituting a solid. A repeating structure in a crystal acting as a building block is called a unit cell. In this article, we will find out the different crystal structures and their properties.

### What is a Lattice Structure?

A lattice or a crystal lattice is the formation of a symmetrical 3D structural arrangement of ions, atoms, and molecules to form a unit cell. The significant and distinct geometrical shape of a unit cell defines the type of crystal lattice.

The features of a crystal structure are:

• In a unit cell of a crystal structure, every ion, atom, or molecule represents a point in a 3D space. It means all the constituent particles in that unit can be considered points.

• Every point in a crystal structure is defined as a lattice point or lattice site.

• Two or more lattice points can be joined to form a straight line depending on the shape of a unit cell.

• When two or more straight lines are joined, it will form a 3D unit cell design representing a crystal structure. This arrangement of lattice points in a 3D space is called Bravais Lattices.

### What is a Unit Cell?

The smallest unit of a solid crystal structure is called a unit cell. It can be defined as the structural unit of a solid crystal, and a lattice is generated by its repetition in a particular format.

To identify a unit cell of a crystalline structure, the following parameters are considered.

• Three edges a, b, and c

• The angles between these edges ∝, β, and  $\gamma$

The angles between these edges can vary. When high-energy electromagnetic waves are passed through a crystal, the unit cells absorb and send signals. These signals can be interpreted to understand the shape of a unit cell. Hence, the mutual arrangements of multiple unit cells can be identified, leading to the expression of a crystal structure.

### Types of Lattice Structures

1.  Primitive Unit Cells

In this arrangement, the constituent lattice points hold the corner positions only.

2.  Centered Unit Cells

In this arrangement, the particles occupy the centre along with the corner positions. They are of the following types.

• Body-Centred Cubic (BCC) Lattice structure

This is a typical lattice structure where the atomic planes or lattice planes lie within the gaps of the lower planes of the respective atoms. In this type, the unit cell is shaped like a cube. It means that one atom is directly in contact with eight other atoms or lattice points. This type of arrangement can be found in vanadium, tungsten, and chromium.

• Hexagonal Closest Packed (HCP) Lattice Structure

The superposed atoms or lattice points create gaps where the underlying plane can fit into the upper one. The atomic levels are closely packed, creating a hexagonal closest packed (HCP) lattice structure. Metals like cobalt, zinc, titanium, and magnesium show such types of lattice arrangement. A point is connected with 12 neighbouring points making its coordination number 12.

• Face-Centered Cubic (FCC) Lattice Structure

This lattice structure also packs the maximum number of atomic planes in a stacking sequence. The second layer of atoms is primarily stacked following the HCP structure in the underlying layer gaps. The third layer, on the contrary, lies in the free gaps. The metals like copper, lead, and nickel shows such types of solid crystal lattice arrangements. The coordination number of this crystal structure is 12.

### Types of Crystal Systems

1. Triclinic System

In this system, all three axes are inclined to each other and share the same length. Based on their inclination angles, the shape of the crystals varies. Example – kyanite, amazonite, feldspar, etc.

2. Monoclinic System

In this crystalline structure, there are three axes. Two of them are perpendicular to each other, whereas the 3rd one is inclined to form a different angle. The best examples of monoclinic systems are gypsum, petalite, diopside, etc.

3. Orthorhombic System

The crystal lattice structure of this type comprises three axes that are perpendicular to each other but the lengths of these axes vary. Examples of this lattice structure are topaz, iolite, zoisite, etc.

4. Trigonal Structure

The unit cell of this system is basically a three-sided pyramid. It can also attend to shapes like rhombohedral and scalenohedral. Examples are calcite, ruby, agate, etc.

5. Hexagonal System

Now that we know what crystal structure is, this system seems a little different from the rest. It has four axes where three of them share the same length. The 4th one intersects these three at right angles. The examples are beryl, apatite, etc.

6. Tetragonal System

This system also consists of three axes. Only the main axis varies in dimension. The other two are of the same length. An example of this system is perovskite.

7. Cubic System

This is a lattice system where all three angles are at right angles to each other with the same length. This crystal structure of metals can be found in gold and silver. Among nonmetals, diamond shows this characteristic crystal shape.

### Conclusion

This is all about the crystal lattice structures and their examples. Follow the shapes of these types and remember the examples to correlate.