  A crystalline solid is a periodic sequence of identical cells, each
containing one or more atoms. If the constituting unit cell is not
centrosymmetric, charge may distribute unevenly between the atoms, resulting in
internal electric polarization. This effect serves as the basis for numerous
ferroelectric, piezoelectric, and pyroelectric phenomena. In nearly all polar
materials, including multilayered van der Waals stacks that were recently found
to exhibit interfacial polarization, inversion symmetry is broken by having two
or more atomic species within the unit cell. Here, we show that even elemental
crystals, consisting of one type of atom, and composed of non-polar
centrosymmetric layers, exhibit electric polarization if arranged in an
appropriate three-dimensional architecture. This concept is demonstrated here
for inversion and mirror asymmetric mixed-stacking tetra-layer polytypes of
non-polar graphene sheets. Furthermore, we find that the room temperature
out-of-plane electric polarization increases with external electrostatic
doping, rather than decreases owing to screening. Using first-principles
calculations, as well as tight-binding modeling, we unveil the origin of
polytype-induced polarization and its dependence on doping. Extension of this
idea to graphene multilayers suggests that solely by lateral shifts of
constituent monolayers one can obtain multiple meta-stable interlayer stacking
sequences that may allow for even larger electrical polarization.
