Bravais class

From Online Dictionary of Crystallography

Classe Bravais (Fr). Bravais-klasse (Ge). Classe di Bravais (It). ブラベー類 (Ja). Clase de Bravais (Sp).

Definition

Space groups that are assigned to the same Bravais arithmetic class belong to the same Bravais class of space groups.

The introduction of the concept of Bravais classis necessary in order to classify space groups on the basis of their Bravais type of lattice, independently from any accidental metric of the lattice.

An orthorhombic crystal may have accidentally a = b and have a tetragonal lattice not because of symmetry restrictions but just by accident. If the Bravais types of lattices were used directly to classify space groups, such a crystal would belong to another category with respect to an orthorhombic crystal without specialized metric. The concept of Bravais class solves this ambiguity.

A space group G is assigned to a Bravais arithmetic class on the basis of the corresponding point group P and the arithmetic crystal class associated with it:

• If the arithmetic crystal class of G is a Bravais arithmetic class, then G is assigned to that Bravais arithmetic class;
• if the arithmetic crystal class of G is not a Bravais arithmetic class, then the Bravais arithmetic class to which G is assigned is obtained as follows:
• those Bravais arithmetic classes are retained whose point group B is such that P is a subgroup of B;
• G is assigned to that Bravais arithmetic class, among those selected above, for which the ratio of the order of the point groups of B and of P is minimal.

For example, a space group of type I41 belongs to the arithmetic crystal class 4I, to which two Bravais arithmetic classes can be associated, 4/mmmI and $m{\bar 3}mI$. The second condition uniquely assigns G to the Bravais classof 4/mmmI, despite the fact that the Bravais arithmetic class of the lattice may be $m{\bar 3}mI$ as a result of accidental symmetry.

In earlier editions of Volume A of the International Tables for Crystallography, Volume A Bravais classes were called Bravais flocks.