  Despite their ionic nature, many layered diatomic crystals avoid internal
electric polarization by forming a centrosymmetric lattice at their optimal
anti-parallel van-der-Waals stacking. Here, we report a stable ferroelectric
order emerging at the interface between two naturally-grown flakes of
hexagonal-boron-nitride, which are stacked together in a metastable
non-centrosymmetric parallel orientation. We observe alternating domains of
inverted normal polarization, caused by a lateral shift of one lattice site
between the domains. Reversible polarization switching coupled to lateral
sliding is achieved by scanning a biased tip above the surface. Our
calculations trace the origin of the phenomenon to a subtle interplay between
charge redistribution and ionic displacement, and our minimal cohesion model
predicts further venues to explore the unique "slidetronics" switching.
