  Ferroelectric domain walls are a rich source of emergent electronic
properties and unusual polar order. Recent studies showed that the
configuration of ferroelectric walls can go well beyond the conventional
Ising-type structure. N\'eel-, Bloch-, and vortex-like polar patterns have been
observed, displaying strong similarities with the spin textures at magnetic
domain walls. Here, we report the discovery of antiferroelectric domain walls
in the uniaxial ferroelectric Pb$_{5}$Ge$_{3}$O$_{11}$. We resolve highly
mobile domain walls with an alternating displacement of Pb atoms, resulting in
a cyclic 180$^{\circ}$ flip of dipole direction within the wall. Density
functional theory calculations reveal that Pb$_{5}$Ge$_{3}$O$_{11}$ is
hyperferroelectric, allowing the system to overcome the depolarization fields
that usually suppress antiparallel ordering of dipoles along the longitudinal
direction. Interestingly, the antiferroelectric walls observed under the
electron beam are energetically more costly than basic head-to-head or
tail-to-tail walls. The results suggest a new type of excited domain-wall
state, expanding previous studies on ferroelectric domain walls into the realm
of antiferroic phenomena.
