  First-principles calculations on monolayer 8-{\it Pmmn} borophene are
reported to reveal unprecedented electronic properties in a two-dimensional
material. Based on a Born effective charge analysis, 8-{\it Pmmn} borophene is
the first single-element based monolayered material exhibiting two sublattices
with substantial ionic features. The observed Dirac cones are actually formed
by the p$_z$ orbitals of one of the inequivalent sublattices composed of
uniquely four atoms, yielding an underlying hexagonal network topologically
equivalent to distorted graphene. A significant physical outcome of this effect
includes the possibility of converting metallic 8-{\it Pmmn} borophene into an
indirect band gap semiconductor by means of external shear stress. The
stability of the strained structures are supported by a phonon frequency
analysis. The Dirac cones are sensitive to the formation of vacancies only in
the inequivalent sublattice electronically active at the Fermi level.

Materials Science