Cubic crystal system

In crystallography, the cubic (or isometric) crystal system is a special way that crystals can form, where the basic building block, called the unit cell, is shaped like a cube. This is one of the most common and simplest shapes seen in real crystals and minerals.

There are three main types of cubic crystals. The first is called primitive cubic, sometimes also known as simple cubic. The second type is body-centered cubic, which has atoms in the center of the cube as well as at the corners. The third type is face-centered cubic, where atoms sit at the corners and the centers of each face of the cube.

Even though these crystals are described using cubes, the smaller parts that repeat to make the crystal, called primitive unit cells, might not always look like perfect cubes. This system helps scientists understand how atoms are arranged in many everyday materials.

Bravais lattices

Further information: Bravais lattice

The cubic crystal system has three main types of patterns called Bravais lattices.

The primitive cubic lattice (cP) has points only at the corners of the cube. Each point is shared among eight cubes, so each cube contains one full point.

The body-centered cubic lattice (cI) adds a point in the middle of the cube, giving a total of two points per cube.

The face-centered cubic lattice (cF) adds points to the centers of each face of the cube, giving a total of four points per cube.

These lattices have different ways of arranging atoms and different densities of packing.

Crystal classes

Further information: Crystallographic point group

The isometric crystal system has special names and groups that describe how crystals can be arranged. These groups are written in different ways, such as Schönflies notation, Hermann–Mauguin notation, orbifold, and Coxeter notation. There are 36 different ways these cubic shapes can be organized, and they are shown in a table.

Other names for one type of this system include normal class, holohedral, ditesseral central class, and galena type.

Single element structures

Atoms in solids usually stick together tightly, so arrangements where atoms are packed closely together are more common. The simplest cubic shape, called primitive cubic, has very few atoms packed closely and is rare in nature, but it is found in a material called polonium. Two other cubic shapes, called body-centered cubic (bcc) and face-centered cubic (fcc), have more atoms packed closely and are found often in nature. Examples of bcc include iron, chromium, tungsten, and niobium. Examples of fcc include aluminium, copper, gold and silver.

Another important cubic shape is the diamond cubic structure, seen in materials like carbon, silicon, germanium, and tin. This structure is different because it has many atoms in its basic building block. There are also other cubic shapes, like the A15 structure in tungsten, and a very complex shape in manganese.

See also: Periodic table (crystal structure)

Multi-element structures

Compounds that have more than one kind of building block, like binary compounds, often arrange themselves in shapes based on the cubic crystal system. These structures can be thought of as two or more sets of points that fit together, with each set sitting in the spaces of the others.

Caesium chloride structure

See also: Category:Caesium chloride crystal structure

One common structure is called the "caesium chloride" or B2 structure. It looks similar to a body-centered cubic shape, but it is made of two different kinds of atoms. In this structure, moving in a certain direction changes the type of atom you see. You can imagine it as two simple cubic shapes stacked on top of each other, one for each kind of atom. In caesium chloride, each atom is surrounded by eight atoms of the other kind, forming a cube around it.

This structure appears in some compounds made from alkali metals and their partners when made under special conditions. It is common when the two kinds of atoms are about the same size.

Rock-salt structure

See also: Category:Rock salt crystal structure

Another common structure is the rock-salt structure, named after table salt. Here, each kind of atom forms its own shape, and the two shapes fit together like a 3D checkerboard. Each atom is surrounded by six atoms of the other kind, arranged like the points of a star.

Fluorite structure

Main article: Fluorite structure

See also: Category:Fluorite crystal structure

The fluorite structure is similar to the rock-salt structure but has twice as many of one kind of atom compared to the other.

Zincblende structure

See also: Category:Zincblende crystal structure

The zincblende structure is named after a mineral form of zinc sulfide. In this structure, each atom is surrounded by four atoms of the other kind, arranged like the points of a pyramid. This structure is also found in many important materials used in electronics.

This group is also known as the II-VI family of compounds.

This group is also known as the III-V family of compounds.

Heusler structure

Main article: Heusler compound

The Heusler structure is a common way for three different elements to arrange themselves, especially when one of them is a transition metal.

Iron monosilicide structure

See also: Category:Iron monosilicide structure type

The iron monosilicide structure has a special twisty shape and is found in some compounds made from transition metals and silicon or germanium.

Weaire–Phelan structure

A Weaire–Phelan structure has a special kind of balance and shape. It is made from three different ways of stacking tetradecahedrons with pyritohedral shapes in between. You can find this pattern in chemistry, where it is called a "type I clathrate structure". Some gases like methane, propane, and carbon dioxide form structures at cold temperatures where water molecules are connected by hydrogen bonded links, and the gas molecules fit into the special shapes of this structure.