Bulk and Two-dimensional Silver and Copper Monohalides: A Unique Class of Materials with Modest Ionicity/Covalency and Ferroelasticity/Multiferroicity

TL;DR

This paper investigates silver and copper monohalides, revealing their unique position between covalent and ionic compounds, and uncovers their potential for ferroelasticity, ferroelectricity, and 2D polymorphs with promising applications.

Contribution

The study provides ab initio evidence of low transition barriers and multiferroic properties in silver and copper monohalides, highlighting their potential for novel electronic and mechanical applications.

Findings

Low transition barriers between phases enable ferroelasticity and ferroelectricity.

Several halides have lattice constants similar to silicon, allowing epitaxial growth.

Stable 2D polymorphs exhibit multiferroicity and are experimentally feasible.

Abstract

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.