  We show that an arbitrary spatial distribution of complex refractive index
inside an object can be exactly represented as a sum of two "monomorphous"
complex distributions, i.e. the distributions with the ratios of the real part
to the imaginary part being constant throughout the object. A priori knowledge
of constituent materials can be used to estimate the global lower and upper
boundaries for this ratio. This approach can be viewed as an extension of the
successful phase-retrieval method, based on the Transport of Intensity
equation, that was previously developed for monomorphous (homogeneous) objects,
such as e.g. objects consisting of a single material. We demonstrate that the
monomorphous decomposition can lead to more stable methods for phase retrieval
using the Transport of Intensity Equation. Such methods may find application in
quantitative in-line phase-contrast imaging and phase-contrast tomography.
