International
Tables for Crystallography Volume B Reciprocal space Edited by U. Shmueli © International Union of Crystallography 2010 |
International Tables for Crystallography (2010). Vol. B, ch. 1.5, pp. 178-179
Section 1.5.3.4. Irreducible representations of space groups and the reciprocal-space group^{a}Departamento de Fisíca de la Materia Condensada, Facultad de Cienca y Technología, Universidad del País Vasco, Apartado 644, 48080 Bilbao, Spain , and ^{b}Institut für Kristallographie, Universität, D-76128 Karlsruhe, Germany |
Let be a conventional basis of the lattice L of the space group . From (1.5.3.6), and , equation (1.5.3.4) can be written Equation (1.5.3.12) has the same form if a primitive basis of L has been chosen. In this case, the vector k is given by with rational coefficients .
Let a primitive basis be chosen for the lattice L. The set of all vectors k (known as wavevectors) forms a discontinuous array. Consider two wavevectors k and , where K is a vector of the reciprocal lattice . Obviously, k and describe the same irrep of . Therefore, to determine all irreps of it is necessary to consider only the wavevectors of a small region of the reciprocal space, where the translation of this region by all vectors of fills the reciprocal space without gap or overlap. Such a region is called a fundamental region of . (The nomenclature in literature is not quite uniform. We follow here widely adopted definitions.)
The fundamental region of is not uniquely determined. Two types of fundamental regions are of interest in this chapter:
Let k be some vector according to (1.5.3.12) and W be the matrices of . The following definitions are useful:
Definition. The set of all matrices for whichforms a group which is called the little co-group of k. The vector k is called general if ; otherwise and k is called special.
In words: The k vector is called general if its little co-group is the identity of . Otherwise, is a non-trivial subgroup of the point group of , , and k is called a special vector of the reciprocal space.
Equation (1.5.3.14) for k resembles the equation by which the fixed points of the symmetry operation of a symmorphic space group are determined. Indeed, the orbits of k defined by (1.5.3.13) correspond to the point orbits of , the little co-group of k corresponds to the site-symmetry group of that point X whose coordinates have the same values as the vector coefficients of k.
Consider the coset decomposition of relative to .
Definition. If is a set of coset representatives of relative to , then the set is called the star of k and the vectors are called the arms of the star.
The number of arms of the star of k is equal to the order of the point group divided by the order of the symmetry group of k. If k is general, then there are vectors from the orbit of k in each fundamental region and arms of the star. If k is special with little co-group , then the number of arms of the star of k and the number of k vectors in the fundamental region from the orbit of k is .
The analogue of the little group is rarely considered in crystallography.
All symmetry operations of may be obtained as combinations of an operation that leaves the origin fixed with a translation of L, i.e. are of the kind . We now define the analogous group for the k vectors. Whereas is a realization of the corresponding abstract group in direct (point) space, the group to be defined will be a realization of it in reciprocal (vector) space.
Definition. The group which is the semidirect product of the point group and the translation group of the reciprocal lattice of is called the reciprocal-space group of .
By this definition, the reciprocal-space group is isomorphic to a symmorphic space group . The elements of are the operations with and with the coefficients of K. In order to emphasize that is a group acting on reciprocal space and not the inverse of a space group (whatever that may mean) we insert a hyphen `-' between `reciprocal' and `space'.
From the definition of it follows that space groups of the same type define the same type of reciprocal-space group . Moreover, as does not depend on the column parts of the space-group operations, all space groups of the same arithmetic crystal class determine the same type of ; for arithmetic crystal class see Section 1.5.3.2. Following Wintgen (1941), the types of reciprocal-space groups are listed for the arithmetic crystal classes of space groups, i.e. for all space groups , in Appendix A1.5.1.
References
Wintgen, G. (1941). Zur Darstellungstheorie der Raumgruppen. Math. Ann. 118, 195–215.