# Crystal structure

The crystal structure describes the arrangement of atoms, ions or molecules in a crystalline material. Given for solid elements under standard conditions in the thermodynamically most stable form. Otherwise, the information refers to the melting point of the element.

### Hexagonal

The hexagonal unit cell (HEX) describes a hexagonal prism with one atom at each corner. It includes all point groups with a six-fold rotation or rotation inversion axis.

### Hexagonal close-packed

Hexagonal close-packed (HCP) unit cells define hexagonal prisms with one atom at each vertex and 3 atoms in the center. It is one of the most common structures for metals.

### Body-centered cubic

In the body-centered cubic (BCC) arrangement of atoms, the unit cell consists of eight atoms at the corners of a cube and one atom at the center of the cube.

### Rhombohedral

The rhombohedral unit cell (RHO) is a cube distorted in the direction of the space diagonal. Although it is primitive, it is not conventional, as the threefold axis does not lie in the direction of a lattice vector, but in the direction of the space diagonal.

### Simple cubic

The simple cubic unit cell (SC) is the simplest form of the unit cell. Here, there is one atom at each corner of an equilateral cube.

### Face-centered cubic

The face-centered cubic unit cell (FCC) defines a cube with one atom at each corner of a cube and on each face.

### Diamond structure

The diamond structure (DC) describes the combination of two interpenetrating face-centered cubic lattices that are offset by 1/4 of the space diagonal.

### Orthorhombic

In the orthorhombic lattice system (ORTH), lattice axes lie in the direction of the 3 perpendicular symmetry axes.

### Tetragonal

The tetragonal crystal system (TETR) includes all point groups that have a fourfold rotation or rotation inversion axis in exactly one direction.

### Double hexagonal close-packed

The double hexagonal close-packed (DHCP) crystal structure resembles two regular HCP unit cells stacked on top of each other. This structure is common in several lanthanides and actinides.

### Monoclinic

The three axes of the unit cell in the monoclinic (MON) crystal system (a, b and c) are unequal. The monoclinic unit has a rectangular shape with a parallelogram as a base.