Crystal Structures | Types with Explanation

A crystal structure refers to the repeating arrangement of particles in a solid. It displays the arrangement of atoms in a three-dimensional structure. There are 4 basic types of crystal structures, which are further divided into a total of 7 crystal systems with 14 Bravais lattices.

The 4 types of crystal structures are as follows:

  1. Simple Cubic structure (SC)
  2. Body-Centered Crystal structure (BCC)
  3. Face-Centered Crystal structure (FCC)
  4. Hexagonal Close Packed structure (HCP)

Types of Crystal Structures

The types of crystal structures are explained in terms of the cubic crystal system:

1. Simple Cubic Structure (SC)

The simplest type of crystal structure has atoms present at each corner of a cube. This pattern is repeated for a lattice. Since a cube has 8 corners, there are 8 atoms present to be displayed in one unit cell. 

However, since each corner of the cube is shared between 8 other cubes in a 3-dimensional lattice, the total contribution of the combined 8 atoms to the reference unit cell is 8 (atoms) x 1/8 (contribution of each atom). Therefore, the average number of atoms in a unit cell of a simple cubic structure is 1.

Simple Cubic Crystal Structure

Contrary to the BCC and FCC (explained below), the simple cubic structure does not have any atom(s) present at the center of the cube or at the center of each of the faces of the cube. This is the simplest type of crystal structure and is not associated with any of the materials used for engineering purposes.

2. Body-Centered Cubic Structure (BCC)

In a body-centered crystal structure, there are 8 atoms present, one at each corner of a cube. Additionally, there is an atom present at the center of the cube, hence the name body-centered. This feature makes the BCC a little more complex than the SC structure.

As the atom fixed at the center of the reference unit cell/cube is not shared with any other cube, it belongs to this cell alone. So the contribution of this atom to the unit cell is one. Adding this figure with the contribution of one more provided by the 8 atoms at the corners of the reference cell (as in SC), we get an average of 2 atoms per BCC unit cell.

Body Centered Crystal Structure

Some of the metals that assume this crystal structure are lithium, potassium, sodium, and vanadium. 

3. Face-Centered Cubic Structure (FCC)

The face-centered crystal structure contains an atom occupying the center of each of the 6 faces of a cube. This corresponds to 6 atoms in this manner. However, each face of a cube is shared with one other cube. So the contribution of these 6 atoms to the unit cell is given by 6 (atoms) x 1/2 (contribution) = 3 atoms on average.

Moreover, there are also a total of 8 atoms present, one at each corner much like the SC. The contribution of those 8 atoms to this unit cell is 1, as previously given.

This makes a total of 4 atoms on average for each unit cell of the FCC structure.

Face Centered Crystal Structure

Further, there is no atom present at the center of the cube, like that in the BCC. A few of the metals that possess this type of arrangement are calcium, copper, silver, aluminum, and platinum. 

4. Hexagonal Close Packed Structure (HCP)

As the name indicates, the shape in question is a hexagon. The atoms are arranged in such a way that there are 12 atoms present in the 12 corners of the hexagon, one at each. The division occurs as each corner in a hexagon is shared with 6 other similar figures. This makes the contribution of these 12 atoms to the reference unit cell: 12 (atoms) x 1/6  (contribution) = 2 atoms.

Moreover, there is an atom present at the center of each of the two faces, one above and one below the hexagonal figure. Each of these two faces is shared with another hexagon. So the contribution of these 2 atoms to the reference cell is 2 (atoms) x 1/2 (contribution) = 1 atom.

Furthermore, there are three unshared atoms present in the central cavity of the hexagon in a trigonal planar shape. Since the three atoms are unshared, their contribution is 3 atoms to this unit cell.

Hexagonal Close Packed Crystal Structure

The sum of all of these contributions provides us with a total of 6 atoms on average per HCP unit cell.

Beryllium, magnesium, zinc, copper, and cadmium are some of the metals that have this crystal structure in the solid state.

Crystal Systems – Reference to Crystal Structures

In crystallography, there are a total of seven crystal systems, namely cubic, rhombohedral (trigonal), tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic.

Crystal systems of crystalline structures

A representation of the 14 Bravais lattices is given above. These lattices stem from the 7 crystal systems.

Starting with the cubic system, it is the most symmetric of all. Each of the sides/axes are of equal length i.e. a = b = c. Moreover, the sides are perpendicular to each other, which means all of the angles equal 90°. The types of crystal structures included in the cubic system are simple, face-centered, and body-centered.

Moving on to the second, the rhombohedral, or the trigonal system, has all sides of equal lengths, much like the cubic system. However, the sides are not exactly perpendicular. Only the simple trigonal system exists.

In the tetragonal system, two of the three axes are of equal length i.e. a = c. It implies that the third side has a different length. Furthermore, the three axes are perpendicular to each other. Simple and body-centered crystal structures are included in the tetragonal crystal system.

The hexagonal crystal system possesses three sides of equal lengths, with an angle of 120° between them. However, there exists a fourth side having a different length, perpendicular to the plane of the other three.

Furthermore, the orthorhombic crystal system has all axes of different lengths. On the other hand, each of the sides has an angle of 90° between them. The types of crystal structures included are simple, base-centered, face-centered, and body-centered.

In the fifth crystal system, monoclinic, all of the sides are unequal in length. Additionally, only two of the three sides are perpendicular to each other. The types of crystal structures involved are simple and base-centered.

The final crystal system, triclinic, has all sides with unequal lengths, as in the monoclinic system. In addition, neither of the sides is perpendicular; and the angles between the three axes are all different. The crystal structure associated with the triclinic crystal system is the simple one only.

Read about Crystalline vs Amorphous solids here.

Key Takeaway(s)

the seven crystal system or unit cells

Concepts Berg

Do rocks have a crystal structure?

Crystalline rocks do contain minerals and microscopic crystals. However, the rocks themselves are neither of those.

What is the difference between crystals minerals and rocks?

A mineral is defined as a solid with a definite chemical composition that occurs naturally. It possesses a highly ordered arrangement of atoms. On the other hand, a crystallized rock may be made of multiple crystallized minerals. Contrary to this, a crystal refers to any solid material that contains its constituent atoms arranged in a definite pattern.

Why are there only 7 crystal systems?

The seven primitive crystal systems are the basic unit cells, either of which any substance has its particles arranged in.

What are the 4 types of crystal structures?

The four types of crystal structures are simple cubic (SC), body-centered crystal structure (BCC), face-centered crystal structure (FCC), and hexagonal close-packed structure (HCP).

What is BCC and FCC structure?

In the Body-Centered Cubic (BCC) arrangement, 8 atoms occupy the corners of a cubic structure. In addition, there is another atom present in the exact center of the cube. There is an average of 2 atoms for one BCC unit cell.

On the other hand, the Face-Centered Cubic (FCC) structure has 6 atoms at the center of the 6 faces of a cube, one at each. Additionally, there are 8 atoms present at the vertices of the cube. This makes an average of 4 atoms per FCC unit cell.

What is the crystal structure of steel?

Steel is an alloy of iron and carbon. Also, pure iron exists as ferrite, which is arranged in the body-centered cubic form.

What is the difference between crystal structure and microstructure?

Microstructure explains the arrangement of atoms in materials on a larger scale (than nm) as compared to the crystal structure which normally refers to the arrangement in nanometers (nm).

Do all minerals have crystal structures?

Yes, all minerals have a definite chemical composition and an orderly arrangement of atoms, which is the requirement for a crystal.

References

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