  The static diffraction intensity distribution from large material system
conceived as perfectly homogeneous system made inhomogeneous, though
substitution of groups of atoms, small particles, by other groups of atoms, is
explicitly expressed in terms of all possible partial contributions subsequent
to the particle exchange operation. This gives the diffracted intensity as that
from the homogeneous system modified by additional part and, thus, yields
result that may prove to be quite useful in comparative studies of materials
with the same sort of atomic network. The additional part embodies the effects
of the presence of the structural disturbances and consists in two main
components. One of these represents the inhomogeneity coupling with the system
in its homogeneous state and, the other, represents the inhomogeneity
self-coupling, the coupling of the inhomogeneities with one another. Two
particular cases of deviation from homogeneity are considered in more details:
the case of non-interacting inhomogeneities and that of interacting
inhomogeneities. The effects on the small angle scattering (SAS) due to
possible structural differences (chemical nature, atomic distribution and
dimensions) between the deficient and substitute particles are given. The SAS
distribution does not obey the very attractive Guinier law in some cases of
non-interacting inhomogeneities.
