Superlattices are characterized by diffraction patterns consisting of
maxima with alternating intensities (the so-called miniband spectrum).
The observation of such a miniband spectrum can be regarded as a phys
ical criterion of the existence of a superlattice. A characteristic st
ructural feature of superlattices is pseudosymmetry, due to which a co
nsiderable part of the electron density is invariant with respect to t
he operations of a certain translation supergroup of the space group o
f the crystal. We introduce a quantitative measure of the translationa
l pseudosymmetry eta(t)[rho(x)] and define the quantity eta above whi
ch all the structures having translation symmetry would be characteriz
ed by a miniband spectra and, hence, would be superlattices (the struc
tural criterion of the existence of a superlattice). The criterion of
the existence of a superlattice is established for natural crystals of
the same structure type and for the one-dimensional Kronig-Penney mod
el.