Two-Dimensional Ionic Crystals: The Cases of IA-VII Alkali Halides and IA-IB CsAu

The alkali halides, known as ionic crystals, have the NaCl-type or CsCI-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, 4 and 12 out of 20 alkali halides in the hexagonal and tetragonal structures are dynamically stable, respectively. The electronic band 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. We also predict that the band gap of 2D CsAu is larger than that of the 3D counterpart by a factor of more than two, whereas the 2D structures are unstable for the long wavelength phonons in the acoustic branch.