Band‐gap and phonon distribution in alkali halides

A systematic first-principles study is performed to calculate the structural, electronic, dynamical, and thermodynamic properties of alkali halides NaF, NaCl, NaBr, and NaI by means of both full-potential linear augmented plane-wave, and plane wave pseudopotential methods. The obtained structural parameters agree favourably with experimental findings. The calculated band gaps are underestimated by GGA functional compared to experiment. The gap values are improved from the modified Becke–Johnson exchange potential, which gives band gaps in perfect agreement with the measured values. In addition, linear-response approach to the density functional theory is used to derive several quantities such as the Born effective charges, high-frequency dielectric constant, phonon band structure, and phonon density of states. The dynamical properties of Na-based alkali halides are also analysed and discussed. Finally the temperature dependence of various quantities such as the mean-squared displacement, and the heat capacity are computed using the quasi-harmonic approximation.