Atomic modulation function

From Online Dictionary of Crystallography

Fonction de modulation atomique (Fr). Atomare Modulationsfunktion (Ge). Funzione di modulazione atomica (It). 原子変調関数 (Ja). Función de modulación atómica (Sp).


A modulated crystal 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

[math]r(n,j) = n~+~r_j+u_j (n+r_j).[/math]

The occupation probability to find an atom of species A at the position [math]n+r_j[/math] is [math]p_A(n, j)[/math], where the sum over the species of the functions [math]p_A[/math] is one. Instead of a different species, one may have a vacancy. The functions [math]u(n, j)[/math] and [math]p_A(n, j)[/math] 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

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

Modulation functions may be continuous or discontinuous.

See also