the (3-d) crystal structure of h2
o ice ih (c) consists of bases of h2
o ice molecules (b) located on lattice
points within the (2-d) hexagonal space lattice (a). the values for the h–o–h angle and o–h distance have come from physics of ice
with uncertainties of ±1.5° and ±0.005 Å
, respectively. the white box in (c) is the unit cell defined by bernal and fowler
in crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns that repeat along the principal directions of three-dimensional space in matter.
the smallest group of particles in the material that constitutes this repeating pattern is the unit cell of the structure. the unit cell completely reflects the symmetry and structure of the entire crystal, which is built up by repetitive translation of the unit cell along its principal axes. the translation vectors define the nodes of the bravais lattice.
the lengths of the principal axes, or edges, of the unit cell and the angles between them are the lattice constants, also called lattice parameters or cell parameters. the symmetry properties of the crystal are described by the concept of space groups. all possible symmetric arrangements of particles in three-dimensional space may be described by the 230 space groups.
the crystal structure and symmetry play a critical role in determining many physical properties, such as cleavage, electronic band structure, and optical transparency.