# Centred lattice

(Difference between revisions)
 Revision as of 07:30, 25 February 2015 (view source) (→See also: Now directly in the table)← Older edit Revision as of 13:37, 24 November 2016 (view source)m (link)Newer edit → Line 3: Line 3: == Definition == == Definition == - When the unit cell does not reflect the symmetry of the lattice, it is usual in crystallography to refer to a '[[conventional cell|conventional]]', non-primitive, crystallographic basis, '''ac''', '''bc''', '''cc''' instead of a [[primitive_cell| primitive basis]], '''a''', '''b''', '''c'''. This is done by adding lattice nodes at the center of the unit cell or at one or three faces. The vectors joining the origin of the unit cell to these additional nodes are called 'centring vectors'. In such a lattice '''ac''', '''bc''' and '''cc''' with all their integral linear combinations are lattice vectors again, but there exist other lattice vectors '''t''' ∈ '''L''', '''t''' = ''t1'' '''ac''' + ''t2'' '''bc''' + ''t3'' '''cc'''; with at least two of the coefficients ''t1'', ''t2'', ''t3'' being fractional. The table below gives the various types of centring vectors and the corresponding types of centring. Each one is described by a letter, called the Bravais letter, which is to be found in the Hermann-Mauguin symbol of a space group. + When the unit cell does not reflect the symmetry of the lattice, it is usual in crystallography to refer to a '[[conventional cell|conventional]]', non-primitive, crystallographic basis, '''ac''', '''bc''', '''cc''' instead of a [[primitive_cell| primitive basis]], '''a''', '''b''', '''c'''. This is done by adding lattice nodes at the center of the unit cell or at one or three faces. The vectors joining the origin of the unit cell to these additional nodes are called 'centring vectors'. In such a lattice '''ac''', '''bc''' and '''cc''' with all their integral linear combinations are lattice vectors again, but there exist other lattice vectors '''t''' ∈ '''L''', '''t''' = ''t1'' '''ac''' + ''t2'' '''bc''' + ''t3'' '''cc'''; with at least two of the coefficients ''t1'', ''t2'', ''t3'' being fractional. The table below gives the various types of centring vectors and the corresponding types of centring. Each one is described by a letter, called the Bravais letter, which is to be found in the [[Hermann-Mauguin symbols|Hermann-Mauguin symbol]] of a space group. The 'multiplicity', ''m'', of the centred cell is the number of lattice nodes per unit cell (see table). The 'multiplicity', ''m'', of the centred cell is the number of lattice nodes per unit cell (see table).

## Revision as of 13:37, 24 November 2016

Réseaux centrés (Fr). Zentrierte Gitter (Ge). Redes centradas (Sp). Reticoli centrati (It). 複合格子 (Ja)

## Definition

When the unit cell does not reflect the symmetry of the lattice, it is usual in crystallography to refer to a 'conventional', non-primitive, crystallographic basis, ac, bc, cc instead of a primitive basis, a, b, c. This is done by adding lattice nodes at the center of the unit cell or at one or three faces. The vectors joining the origin of the unit cell to these additional nodes are called 'centring vectors'. In such a lattice ac, bc and cc with all their integral linear combinations are lattice vectors again, but there exist other lattice vectors tL, t = t1 ac + t2 bc + t3 cc; with at least two of the coefficients t1, t2, t3 being fractional. The table below gives the various types of centring vectors and the corresponding types of centring. Each one is described by a letter, called the Bravais letter, which is to be found in the Hermann-Mauguin symbol of a space group.

The 'multiplicity', m, of the centred cell is the number of lattice nodes per unit cell (see table).

The volume of the unit cell, Vc = (ac, bc, cc) is given in terms of the volume of the primitive cell, V = (a, b, c), by:

Vc = m V

## Centred cells vs. "centred lattices"

A lattice being an infinite, symmetric and periodic collection of zero-dimensional nodes, rigorously speaking it is neither primitive nor centred. The expression "centred lattice" has to be considered as a shortcut for "lattice whose conventional cell is centred".

## Types of centred lattices

Bravais letter Centring type Centring vectors Multiplicity
(number of nodes per unit cell)
Unit-cell volume Vc
P Primitive 0
1
V
A A-face centred ½bccc
2
2V
B B-face centred ½ccac
2
2V
C C-face centred ½acbc
2
2V
I body centred
(Innenzentriert)
½acbccc
2
2V
F All-face centred ½acbc
4
4V
½bccc
½ccac
R Primitive
(rhombohedral axes)
0
1
V
R Rhombohedrally centred
(hexagonal axes)
ac+⅓bc+⅓cc
3
3V

ac+⅔bc+⅔cc

H Hexagonally centred ac+⅓bc
3
3V
ac+⅔bc
D Rhombohedrallycentred ac+⅓bc+⅓cc
3
3V
ac+⅔bc+⅔cc

The letter S is also used to indicate a single pair of centred faces. This happens in the monoclinic and orthorhombic crystal families.

• In the monoclinic crystal family, b-unique axis, the centred cells mA, mC, mI and mF are equivalent in the sense that a different choice of axes in the (010) plane interchanges these centrings. The letter mS is sometimes used to collectively indicate any of these cells. The cell mB is instead equivalent to the cell mP.
• In the orthorhobic crystal family, the centred cells oA, oB and oC are transformed one into the other when the axes are permuted. The symbol oS is sometimes used to collectively indicate these three equivalent cells.