Atomic modulation function

Fonction de modulation atomique (Fr.)

Definition

A modulated structure is a structure that may be obtained from a crystalline system with space group symmetry, and therefore with lattice periodicity, by a regular displacement of atoms (displacive modulation) and/or change in the occupation probability of a site in the basic structure. The deviation from the positions in the basic structure are given by

$r(n,j) = n~+~r_j+u_j((n+r_j).$

The occupation probability to find an atom of species A at the position $n+r_j$ is $p_A(n,j)$, where the sum over the species of the functions $p_A$ is one. Instead of a different species, one may have a vacancy. The functions $u(n,j)$ and $p_A(n,j)$ are the atomic modulation functions. For a crystal they should have Fourier modules of finite rank, i.e. the functions have Fourier transforms with delta peaks on wave vectors k of the form

$k~=~\sum_{i=1}^n h_i a_i^*,~~(h_i~~{\rm integers},~n~{\rm finite}.)$

Modulation functions may be continuous or discontinuous.