  The Rashba parameter ${\alpha}$$_R$ is usually assumed to scale linearly with
the amplitude of polar displacements by construction of the spin-orbit
interaction. On the basis of first-principles simulations, ferroelectric phases
of SrTiO$_3$ reached under epitaxial compressive strain are characterized by
(i) large Rashba effects at the bottom of the conduction band near the
paraelectric-ferroelectric boundary and (ii) an unexpected suppression of the
phenomena when the amplitude of polar displacements increases. This peculiar
behavior is ascribed to the inverse dependance of the Rashba parameter with the
crystal field $\Delta$$_{CF}$ induced by the polar displacements that
alleviates the degeneracy of Ti t$_{2g}$ states and annihilates the Rashba
effects. Although ${\alpha}$$_R$ has intrinsically a linear dependance on polar
displacements, these latter becomes antagonist to Rashba phenomena at large
polar mode amplitude. Thus, the Rashba coefficient may be bound to a upper
value.
