  Based on variable components global optimization algorithm, we predict a
stable two-dimensional (2D) phase of boron phosphide with 1:5 stoichiometry,
i.e. boron pentaphosphide (BP_5) monolayer, which has a lower formation energy
than that of the commonly believed graphitic phase (g-BP). BP_5 monolayer is a
multiferroic material with coupled ferroelasticity and ferroelectricity. The
predicted reversible strain is up to 41.41%, which is the largest among all
reported ferroelastic materials. Due to the non-centrosymmetric structure and
electronegativity differences between boron and phosphorus atoms, an in-plane
spontaneous polarization of 326.0 pC/m occurs in BP_5. Moreover, the recently
hunted negative Poisson's ratio property, is also observed in BP_5. As an
indirect semiconductor with a band gap of 1.34 eV, BP_5 displays outstanding
optical and electronic properties, for instance strongly anisotropic
visible-light absorption and high carrier mobility. The rich and extraordinary
properties of BP_5 make it a potential nanomaterial for designing
electromechanical or optoelectronic devices, such as nonvolatile memory with
conveniently readable/writeable capability. Finally, we demonstrate that AlN
(010) surface could be a suitable substrate for epitaxy growth of BP_5
monolayer.

Materials Science