First-principles analysis of structural stability, lattice dynamics, and physical properties of alkali fluorides: LiF, NaF, and KF

A comprehensive first-principles investigation was carried out on the physical properties of alkali halides: LiF, NaF, and KF, using density functional theory (DFT) within the GGA-PBEsol framework. Comparative analysis revealed that LiF possesses the smallest lattice constant, the highest cohesive energies, and the widest band gap compared to NaF and KF. Phonon dispersion and formation enthalpy trends confirmed dynamic and thermal stability across all compounds, with LiF exhibiting the highest phonon frequencies. Mechanical parameter analysis indicated a systematic decline in bulk, shear, and Young’s moduli from LiF to KF, with LiF being the hardest and most brittle, and KF the softest yet most ductile. Anisotropy and acoustic velocity evaluations revealed an increase in elastic anisotropy and anharmonicity along the series, consistent with the rise in Grüneisen parameters. Temperature-dependent thermodynamic functions, including enthalpy, T⁎ entropy, free energy, and heat capacity, demonstrate that LiF maintains superior vibrational stability at elevated temperatures. Among the studied compounds, LiF possessed the highest Debye temperature 718 K. Likewise, the temperature-dependent lattice thermal conductivity decreased with increasing atomic mass, following the trend LiF < NaF < KF, confirming LiF’s highest heat transfer efficiency. The optoelectronic analysis shows that with the highest optical band gap and lowest refractive index, LiF is more suitable for UV-based optoelectronic and photonic applications, which also ensures its UV absorptivity and transparency in visible range is better than the other two computed compounds. Collectively, this comparative analysis shows that LiF exhibits the highest thermal stability, mechanical robustness, and suitable optical behavior among the studied compounds, highlighting its potential relevance for applications requiring thermally stable and optically transparent materials. • LiF, NaF, and KF are confirmed to be both dynamically and thermodynamically stable in their crystalline phases. • A clear elastic anisotropy trend is observed across the series, increasing systematically from LiF to NaF to KF. • Analysis of temperature-dependent thermodynamic behavior, Debye temperature, and lattice thermal conductivity reveals that LiF exhibits superior thermophysical performance among the studied compounds. • The materials show strong UV absorption combined with high transparency in the visible range, indicating their potential for photonic and optical applications. • Overall, LiF emerges as the most mechanically robust and thermally stable compound, making it a strong candidate for high-temperature, optoelectronic, and nuclear applications.