  We show, that the standard model of phase transition can be unified with the
gradient model of phase transitions using the description in terms of the
gradient of order parameter. The generalization of the gradient theory of phase
transitions with regard to the fourth power of the order parameter and its
gradient is proposed. Such generalization makes it possible to described wide
class of phase transitions within a unified approach. In particular it is
consistent with the nonlinear models, that can be used to describe a phase
transition with the formation of spatially inhomogeneous distribution of the
order parameter. Typical examples of such structures (with or without defects)
are considered. We show that formation of spatially inhomogeneous distributions
of the order parameter in the course of a phase transitions is a characteristic
feature of many nonlinear models of phase transitions.
