  A model of phase transitions with coupling between the order parameter and
its gradient is proposed. It is shown, that this nonlinear model is suitable
for the description of phase transitions accompanied by the formation of
spatially inhomogeneous distributions of the order parameter. Exact solutions
of the proposed model are obtained for the special cases which can be related
to the spinodal decomposition or cosmological scenario. The proposed model is
analogical to the mechanical nonlinear oscillator with the coordinate-dependent
mass or velocity dependent elastic module. Based on this analogy, the existence
of the limit cycles is established.
