Phase diagrams of systems exhibiting incommensurate structures

The global phase diagram of a simple model exhibiting disorder-incommensurate (D-I) transition is analyzed using mean-field approximation. This phase diagram is expected to be qualitatively correct in d=3 dimensions. It is shown that depending on the parameters of this model, the D-I transition either (a) is associated with a small order parameter and may thus be described by a Landau-Ginzburg-Wilson model, or (b) has no small order parameter, and the transition takes place via condensation of solitons. The two segments of the transition line are connected by a line of first-order transitions. Applications to intercalate systems and to magnetic spirals are considered.