Thermally stable 2D AlTlF4: A combined DFT and AIMD study of structural, electronic, thermal, and optical properties

This study offers the first comprehensive investigation of the structural, electronic, thermal, and optical characteristics of the mixed metal fluoride AlTlF${}_4$ by density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. The negative formation energy of -$3.11\mathrm{eV}$ for AlTlF${}_4$ signifies energetic stability, whereas phonon dispersion analysis demonstrates the lack of negative frequencies, thereby affirming its dynamic stability. Furthermore, the AIMD simulation confirms its thermal stability. Electronic structure simulations reveal a substantial indirect band gap of 5.42 eV (GGA) and 6.75 eV (HSE06), characterized by flat valence bands predominantly comprised of F-$2p$ states and narrow conduction bands derived from Tl-$6p$ orbitals, indicative of a robust ionic insulator. Thermal study demonstrates distinct regimes in heat capacity and entropy, influenced by low-frequency Tl–F vibrations and high-frequency Al–F phonons. The smooth, continuous nature of these curves confirms the absence of phase transitions within the studied temperature range, underscoring the material’s thermodynamic stability. Analysis of optical properties reveals a broad transparency window below 5.92 eV using GGA functional, confirming AlTlF${}_4$’s potential as a UV-transparent material for protective coatings and optical components. The pronounced anisotropic absorption in the ultraviolet (6-8 eV) and deep-ultraviolet (12-14 eV) regions, particularly the enhanced response for $y$/$z$-polarized light, suggests specific applicability in polarization-sensitive deep-UV optoelectronics. This directional absorption behavior, combined with the material’s thermal stability, enables the design of specialized photodetectors for extreme environment applications.