  We explore from first-principles calculations the ferroelectric material
Pb5Ge3O11 as a model for controlling the spin-orbit interaction (SOC) in
crystalline solids. The SOC has a surprisingly strong effect on the structural
energy landscape by deepening the ferroelectric double well. We observe that
this effect comes from a specific Pb Wyckoff site that lies on the verge of a
natural cavity channel of the crystal. We also find that a unique cavity state
is formed by the empty 6p states of another Pb site at the edge of the cavity
channel. This cavity state exhibits a sizeable spin splitting with a mixed
Rashba-Weyl character and a topologically protected crossing of the related
bands. We also show that the ferroelectric properties and the significant SOC
effects are exceptionally robust against n-doping up to several electrons per
unit cell. We trace the provenance of these original effects to the unique
combination of the structural cavity channel and the chemistry of the Pb atoms
with 6p orbitals localizing inside the channel.
