  Silver and copper monohalides can be viewed as a class of compounds in the
neutral zone between predominantly covalent and ionic compounds, thereby
exhibiting neither strong ionicity nor strong covalency. We show ab initio
calculation evidence that silver and copper monohalides entail relatively low
transition barriers between the non-polar rock-salt phase and the polar
zinc-blende phase, due largely to their unique chemical nature of modest
iconicity or covalency. Notably, the low transition barriers endow both
monohalides with novel mechanical and electronic properties, i.e., coupled
ferroelasticity and ferroelectricity with large polarizations and relatively
low switching barriers at ambient conditions. Several halides even possess very
similar lattice constants and structures as the prevailing semiconductors such
as silicon, thereby enabling epitaxial growth on silicon. Moreover, based on
extensive structural search, we find that the most stable two-dimensional (2D)
polymorphs of the monolayer halides are close or even greater in energy than
their bulk counterparts, a feature not usually seen in the family of rock-salt
or zinc-blende semiconductors. The low transition barrier between zinc-blende
phase and 2D phase is predicted. Moreover, several 2D monolayer halides also
exhibit multiferroicity with coupled ferroelasticity or ferroelectricity,
thereby rendering their potential applications as high-density integrated
memories for efficient data reading and writing. Their surfaces, covered by
halides, also provide oxidation resistance and give low cleave energy from
layered structure, suggesting high likelihood of experimental synthesis of
these 2D polymorphs.
