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What is the crystal structure?

The crystal structure refers to the way in which are arranged or packed atoms, ions or molecules in the crystalline solid such as diamonds, salt (sodium chloride) or sugar (sucrose).

Crystalline solids are made up of three-dimensional networks of identical units called unit cells. These resemble small identical building blocks (like legos) that are joined together to form crystals that we can see with the naked eye.

The crystal structure consists of a description of the shape of this unit cell and the exact position of each atom within said cell.

Characteristics of the crystal structure

They are formed by unit cells that are repeated

The crystalline structure consists of the repetition of a unit cell in all directions. These cells consist of three-dimensional parallelepipeds whose shape and size are defined by three vectors called a, b and c , and by the angles between these vectors, called α, β and γ.

They are highly ordered structures

The crystalline structure is characterized by being one of the most ordered states of matter that exists. Crystals are so ordered that with a good understanding of the structure of the unit cell that contains only a handful of atoms, the entire structure of a crystal made up of millions of atoms, ions or molecules can be reconstructed.

They present symmetry

Most of the unit cells with which you can build crystalline solids exhibit some type of symmetry. This means that the content of one part of the cell is repeated in another part of the cell after carrying out a rotation, reflection or inversion.

For example, if a unit cell has a plane of symmetry, this means that one half of the cell is a reflection of the other.

Relationship between properties of matter and crystalline structure

There are many physical and chemical properties that depend on the crystal structure:

Density

Depending on the way the atoms are packed into the crystal structure, more compact structures or structures in which the atoms are further away from each other can be obtained. In the first case, a denser and heavier solid will be obtained, since more atoms are packed into a smaller volume .

  • Example

If we compare the three cubic crystal structures called simple cubic (P), face-centered cubic (FCC), and body-centered cubic (BCC) for equal atoms, the BCC structure is 2.6 times denser than FCC, and FCC is 1.4 times denser than P.

Crystalline form

The shape of the crystals that we can see with the naked eye is a reflection of the crystalline structure, and in particular, of the unit cell. Depending on the structure of the unit cell, the crystals will grow more in one direction than in others, giving rise to crystals with different shapes such as needles, quotas, hexagonal crystals, etc.

Anisotropy

Physical properties such as thermal or electrical conductivity and magnetic properties of matter are often greater along one direction of the material than along another.

This is called anisotropy, and these effects are particularly strong in crystal structures because they are very orderly and regular structures.

Types of crystal structure

Crystal structures can be classified according to the type of unit cell by which they are formed. These are characterized by the three sides that connect to one of their edges (called a, b and c) and by the angles between these sides (called α, β and γ).

This gives rise to 7 crystal systems. Within each of these crystal systems, different classes of unit cells can be distinguished in turn. In total, there are 14 different unit cells that are called the 14 Bravais networks and are represented below:

Cubic system

As the name implies, the unit cell consists of a perfect cube. All three sides, a, b, and c are equal to each other, and their three angles are all 90 °. This system is made up of cells:

  • Simple or primitive cubic.
  • Body Centered Cubic (BCC).
  • Face Centered Cubic (FCC).

Tetragonal system

In this type of crystal structure, a and b are the same, but c is different, but they are all perpendicular to each other. Tetragonal cells can be:

  • Simple or primitive tetragonal.
  • Tetragonal centered in the body.

Orthorhombic system

In this crystal system, a, b, and c are all different, but they are still perpendicular to each other. There are 4 different orthorhombic cells:

  • Primitive or simple orthorhombic.
  • Body-centered orthorhombic.
  • Orthorhombic centered on the bases.
  • Face-centered orthorhombic.

Monoclinic system

Primitive monoclinic cell
Base-centered monoclinic cell

The monoclinic system is similar to the hexagonal, except that all of its sides are different. There are two cells for the monoclinic system:

  • Primitive monoclinic.
  • Base-centered monoclinic.

Triclinic system

Triclinic cell

In the triclinic system there is no symmetry. All the angles are different from each other and some do not necessarily have to be 90 °. All its sides are also different.

Hexagonal system

Hexagonal cell

This cell contains equal sides a and b and different side c. The angles α and β are both 90 ° while γ = 120 °.

Trigonal or rhombohedral system

Trigonal or rhombohedral cell

This is a particular type of structure similar to taking a cube and stretching it along two opposite vertices. All sides are equal and so are their angles, but these are not 90 °.

Examples of crystal structures

Sodium chloride

Its crystalline structure consists of a face-centered cubic lattice with 4 NaCl units per unit cell,

Alpha iron

Iron crystallizes into alpha iron at 768 ° C. Its structure is cubic centered in the body with an edge of 2.86 Å (or 286 pm)

Gamma iron

It is a form of iron that occurs between 910 ° C and 1400 ° C and has a face-centered cubic crystal structure of 3.64 Å (364 pm) edge.

Diamond

Diamond is one of the most valuable forms of carbon and has a face-centered cubic (FCC) crystal structure containing two carbon atoms and a 3.75 Å (375 pm) edge.

Graphite

Graphite is another very common form of carbon. In this case, it is a question of sheets of carbon atoms joined together forming 6-membered rings, which gives rise to a hexagonal crystalline structure.

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