Some algebraic properties of crystallographic sublattices

Abstract

In this article, a number of the results relevant to the concept of sublattices of a basic crystallographic lattice are reviewed, emphasizing particularly previously unpublished work on the algebraic aspects. A three-dimensional geometric lattice L can be considered as an infinite Abelian group under addition. A sublattice S of L, which is also three-dimensional, is a subgroup of L such that the finite quotient group, G approximately equals L/S, is an Abelian group of order the index of S in L. The sublattice itself in its standard form is represented by an upper triangular matrix. The index of the sublattice is given by the determinant of this matrix. It is first noted that a sublattice described by an arbitrary basis set in L may be converted to this standard form. Next the sublattice is expressed as the intersection of a set of sublattices of individual index a power of a distinct prime, i.e. S(n = p(a)(1)p(b)(2)...) = S(1)(p(a)(1)[cap]S(2)(p(b)(2)...[cap]... = [bigcap](i)S(i)(p(alpha(i)), where p(1), p(2) etc. are prime numbers and n = Pi(i)p(alpha)(i) is the Euclidean factorization of n. This decomposition is important because it corresponds to the Sylow decomposition of the corresponding quotient group G approximately equals (i)[sign: see text] A(p)(i). It is also useful to be able to carry out two commutative binary operations on sublattices of L; these are to find their common sublattice of lowest index in L, which is their intersection S(cap) = S(a)(m)[cap]S(b)(n) and their common superlattice of highest index in L, given by S(< >) = <S(a)(m), S(b)(n)>, where < > indicates the span of the sublattices.

Surprises and pitfalls arising from (pseudo)symmetry

“…Twinning by reticular merohedry. Reticular merohedral twinning can be understood as merohedral twinning on a collection of unit cells, a so-called sublattice (Rutherford, 2006). In this type of twinning, only a fraction of the reflections will overlap with their twin-related counterpart.…”

Autoindexing the diffraction patterns from crystals with a pseudotranslation

“…where M is a transformation matrix whose integer determinant n is the ratio of unit-cell volumes, n = |A 0 |/|A|. Using the terminology of Rutherford (2006), we call L 0 a sublattice of L and n the index of the sublattice. Although there are an infinite number of index-n sublattices of L, a key result from group theory (Billiet & Rolley Le Coz, 1980) is that the number of distinct sublattices is finite and small.…”