Two-dimensional ionic crystals: The cases of IA-VII alkali halides and IA-IB CsAu

TL;DR

This study investigates the structural, vibrational, and electronic properties of 2D ionic crystals, specifically alkali halides and CsAu, using first-principles calculations to identify stable configurations and electronic characteristics.

Contribution

It introduces a comprehensive first-principles analysis of 2D alkali halides and CsAu, proposing a hard sphere model to explain their stability and properties, which is novel in the context of 2D ionic materials.

Findings

Most alkali halides are dynamically stable in hexagonal and tetragonal structures.

Electron energy gaps vary from 3.9 eV to 6.8 eV in different compounds.

CsAu in 2D is stable with a band gap of 2.6 eV, larger than in 3D form.

Abstract

The alkali halides, known as ionic crystals, have the NaCl-type or CsCl-type structure as the ground state. We study the structural, vibrational, and electronic properties of two-dimensional (2D) ionic crystals from first-principles. Two potential structures that are hexagonal and tetragonal are investigated as structural templates. Through phonon dispersion calculations, 8 and 16 out of 20 alkali halides in the hexagonal and tetragonal structures are dynamically stable, respectively. The electron energy gaps range from 6.8 eV for LiF to 3.9 eV for RbI and CsI in the tetragonal structure within the generalized gradient approximation. By considering the Madelung energy and the core-core repulsion, we propose a hard sphere model that accounts for the nearest-neighbor bond length and the cohesive energy of 2D alkali halides. The 2D CsAu in the tetragonal structure is also predicted to be stable as an ionic crystal including only metallic elements, showing a band gap of 2.6 eV that is higher than that of the 3D counterparts.