High thermal insulation and optical conductivity of the 2D phase of MgX2 (X=Cl, Br, and I): A DFT and AIMD study

Abstract

This work investigates the electronic structure, dynamics, thermal, and optical properties of 2D MgX${}_2$ (where X=Cl, Br, and I) using density functional theory, DFT, and ab initio molecular dynamics, AIMD, simulations. The 2D structures exhibit natural planar buckling, with larger buckling observed in MgI${}_2$ compared to MgBr${}_2$ and MgCl${}_2$, correlating with bond length and lattice constant trends. The band gap in MgX${}_2$ is primarily influenced by planar buckling in addition to differences in electronegativity. Among them, MgCl${}_2$ shows the largest direct band gap, while MgI${}_2$ has the smallest indirect band gap. MgBr${}_2$ lies in between, demonstrating a direct band gap. The stability analysis indicates that all three considered MgX${}_2$ are energetically, dynamically and thermally stable. Thermal properties indicate that MgI${}_2$ has the highest heat capacity, attributed to degenerate acoustic phonon modes. In contrast, MgCl${}_2$ exhibits superior thermal conductivity due to high phonon group velocity. Optical studies show that MgI${}_2$ absorbs in the Mid-UV region, while MgCl${}_2$ and MgBr${}_2$ absorb in the Deep-UV. A higher plasmon frequency is found for lower buckled structures compared to the more buckled ones, with potential applications in optoelectronics and plasmonics. This comprehensive study highlights the significant role of structural distortions, such as planar buckling, in determining the electronic, dynamic, thermal, and optical behavior of MgX${}_2$.