  Ferroelectric Rashba semiconductors (FERSC), in which Rashba spin-splitting
can be controlled and reversed by an electric field, have recently emerged as a
new class of functional materials useful for spintronic applications. The
development of concrete devices based on such materials is, however, still
hampered by the lack of robust FERSC compounds. Here, we show that the
coexistence of large spontaneous polarisation and sizeable spin-orbit coupling
is not sufficient to have strong Rashba effects and clarify why simple
ferroelectric oxide perovskites with transition metal at the B-site are
typically not suitable FERSC candidates. By rationalizing how this limitation
can be by-passed through band engineering of the electronic structure in
layered perovskites, we identify the Bi$_2$WO$_6$ Aurivillius crystal as the
first robust ferroelectric with large and reversible Rashba spin-splitting,
that can even be substantially doped without losing its ferroelectric
properties. Importantly, we highlight that a unidirectional spin-orbit field
arises in layered Bi$_2$WO$_6$, resulting in a protection against
spin-decoherence.We highlight moreover that a unidirectional spin-orbit field
arises in Bi$_2$WO$_6$, in which the spin-texture is so protected against
spin-decoherence.
