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

This work investigates the electronic structure, dynamics, thermal, and optical properties of 2D MgX₂ (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 MgI₂ compared to MgBr₂ and MgCl₂, correlating with bond length and lattice constant trends. The band gap in MgX₂ is primarily influenced by planar buckling in addition to differences in electronegativity. Among them, MgCl₂ shows the largest direct band gap, while MgI₂ has the smallest indirect band gap. MgBr₂ lies in between, demonstrating a direct band gap. The stability analysis indicates that all three considered MgX₂ are energetically, dynamically and thermally stable. Thermal properties indicate that MgI₂ has the highest heat capacity, attributed to degenerate acoustic phonon modes. In contrast, MgCl₂ exhibits superior thermal conductivity due to high phonon group velocity. Optical studies show that MgI₂ absorbs in the Mid-UV region, while MgCl₂ and MgBr₂ 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 MgX₂.