Phase transitions and ferroelasticity-multiferroicity in bulk and two-dimensional silver and copper monohalides

We show ab initio calculation evidence that silver and copper monohalides have relatively low transition barriers between the non-polar rock-salt phase and the polar zinc blende phase. 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 under ambient conditions. Several halides even possess very similar lattice constants and structures to 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 monolayer halides have cohesive energies close to or even greater than their bulk counterparts, a feature not usually seen in the family of rock-salt or zinc blende semiconductors. The low transition barrier between the zinc blende phase and the layered bulk phase is predicted. Moreover, several 2D monolayer halides also exhibit multiferroicity with coupled ferroelasticity/ferroelectricity, thereby endowing them with potential for applications as high-density integrated memory devices for efficient data reading and writing. Their surfaces, covered with halides, also provide oxidation resistance. The low cleavage energy of their layered bulk structure suggests a high likelihood of producing these 2D polymorphs through experimental exfoliation.