Phase diagrams of systems exhibiting disorder-incommensurate transitions

It is shown that a continuous disorder-incommensurate (DI) 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 parameters, and the transition takes place via condensation of solitons. The global phase diagram of a simple model exhibiting D-I transition is analysed. It is shown that the phase diagram exhibits both types of continuous transitions. These 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.