  We propose a simple phenomenological picture to explain the unusual
dielectric properties of the proto-typical ferroelectric relaxor lead magnesium
niobate-titanate (PMN-PT). Our model assumes a specific, slowly changing,
displacement pattern of the lead ion, which is indirectly controlled by the
low-energy acoustic phonons of the system. The model qualitatively explains in
great detail the temperature, pressure, and electric field dependence of
diffuse neutron- and x-ray scattering, as well as the existence of hierachy in
the relaxation times of these materials. We furthermore show that the widely
used concept of polar nanoregions as indiviudal static entities is incompatible
with the available body of experimental diffuse scattering results.
