Learn Different Crystal Structures with Examples

A crystal is a solid structure composed of atoms, molecules and ions arranged in a 3D lattice. Since most crystals are solid, they are often referred to as crystalline solids. But there are also liquid crystals. Crystals are made up of different types of bonds like covalent, ionic, and metallic bonds. Crystallization is the process by which crystals develop.

Continue reading to learn more about crystal structures, their types and examples in this Chemistry article.

Crystal Structure

Crystal is a solid material whose particles are arranged in different manners and form a crystal lattice. Atoms, clusters of atoms, or molecules are attached to form the crystal structure. The unit cell is the building block of the crystal structure. It provides a detailed explanation of the crystal’s overall symmetry and atom locations along with its primary axes. Lattice constants or lattice parameters are the lengths, edges of major axes, and angles between unit cells. Crystals can have certain special electrical, optical, and mechanical features that glass and polycrystals normally cannot.

In this structure, it is visible that all the atoms are packed tightly and form a solid structure.

Read about defects in crystal structure here.

Crystal Structure Example

Table salt, sugar, and snowflakes are a few examples of commonplace materials that are crystals. Crystals make up several gemstones, including diamonds and quartz.

A lot of substances resemble crystals but are actually polycrystals. When tiny crystals combine to form a solid, polycrystals are created. These materials don’t have a set structure like a lattice. Ice, several metal samples, and ceramics are all examples of polycrystals. Amorphous solids, which have disorganized interior structures, exhibit even less structure.

Types of Crystal Structure

There are different types of crystal structure:

1. Triclinic System

Crystalline solids can be classified structurally into the triclinic system. The unit cell is defined by three axes of varying lengths, each with a different angle. None of these angles is equal to 90 degrees, and alpha = beta = gamma is not equal to 90 degrees. The triclinic system’s crystals have a one-fold symmetry, which is the same as having no symmetry at all.

The resulting lattice will consist of an ordered stacking of blocks, or unit cells if the atoms or atom groups in the solid are represented by points that are connected. Of all unit cells, the triclinic unit cell is the least symmetrical. The triclinic system is where turquoise and other minerals, such as microcline, crystallize.

In a triclinic system, all three axes are not equal and all these axes are inclined towards each other.

2. Monoclinic System

One of the structural types that crystalline solids might belong to is the monoclinic system. Three axes of different lengths—say, let’s a, b, and c. Out of these, a is perpendicular to b and c, but b and c are not perpendicular to each other. Then, they are referred to as crystals in this system.While the third pair of vectors forms an angle other than 90°, the first two vector pairs are perpendicular.

The monoclinic unit cell can be rotated by 180 degrees along a single axis, known as an axis of twofold symmetry, without the cell’s appearance changing. The monoclinic system is home to more solids than any other. The monoclinic system is used for the crystallization of beta-sulfur, gypsum, borax, orthoclase, kaolin, muscovite, clinoamphibole, clinopyroxene, jadeite, azurite, and spodumene.

3. Orthorhombic System

One of the structural categories to which crystalline solids can be categorized is the orthorhombic system. In this system, three axes with varying lengths that are mutually perpendicular are referred to as crystals. The resulting lattice will consist of an ordered stacking of blocks, or unit cells, if the atoms or atom groups in the solid are represented by points and the points are connected. The three lines that make up the axes of twofold symmetry that define the orthorhombic unit cell allow it to be rotated by 180 degrees without affecting its appearance. The edges of the unit cell may be any length, but this property demands that any two edges be at right angles.

4. Trigonal Structure

Crystalline solids can be classified structurally into the trigonal system, also known as the rhombohedral system. Some people believe that the trigonal system is a division of the hexagonal system.

Similar to the hexagonal system, the trigonal system locates its crystallographic constituents using four axes: three of equal length with 120° intersections, and one perpendicular to the plane of the other three.

The primitive vectors must have the form a=b, alpha = beta = 90 degree, gamma =120 in order to satisfy the symmetry. We may use alpha = 60, but in that case, it would be difficult to determine the three-fold rotation axis from the basic vectors. A limiting example of the simple monoclinic Bravais lattice, with gamma =120, is the trigonal system.

5. Hexagonal System

One of the main structural types to which a particular crystalline solid can be categorised is the hexagonal system. Four axes—three of equal length arranged at 120° to one another, and a fourth perpendicular to the plane of the other three—are used in this technique to locate the components of crystals.The combination of the hexagonal crystal system and the trigonal crystal system results in the hexagonal crystal family, which comprises of the 12 point groups where at least one of their space groups has the hexagonal lattice as the underlying lattice.

. Arsenic, calcite, dolomite, quartz, apatite, tourmaline, emerald, ruby, cinnabar, and graphite are examples of hexagonal systems.

6. Tetragonal System

Crystalline solids can be classified structurally into the tetragonal system. In this system, crystals are defined by three axes, two of which are equal in length and are mutually perpendicular. The tetragonal unit cell is distinguished by an axis of fourfold symmetry, about which the atoms coincide with their initial positions when the cell is rotated 90 degrees. Tetragonal crystals can occur in the boron and tin elements as well as in some minerals like zircon.

7. Cubic System

The most symmetrical of the systems is the cubic lattice. Every angle is exactly 90 degrees, and every side is the same length (a = b = c). only one of the sides’ lengths. The cubic lattice also comprises body-centred cubic and face-centred cubic in addition to basic cubic. In a body-centred cubic, an atom (or ion) is also present in the centre of the cube in addition positioned at the cube’s vertices. Similar to this, a face-centred cubic demands the existence of atoms (ions) in the centre of each cube face in addition to those that are positioned at the cube’s vertices.

Application of Crystal Structure

These are the applications of crystal structure:

• They are used in studies about organic molecules such as polymorphism of pharmaceutical molecules.
• They are used in optics, and lasers.
• They are used in light-emitting diodes.
• They are used in biomedical technology.

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