First-principles study of electronic, absorption, and thermodynamic properties of crystalline styphnic acid and its metal salts.

The electronic structure, absorption spectra, and thermodynamic properties of crystalline styphnic acid and its metal salts (potassium, barium, and lead styphnates) have been studied using density functional theory within the local density approximation. The results show that the metal states affect the electronic structure of styphnic acid by modifying the density of states of the O atoms of hydroxyls. The C-O bond fission may be favorable in the decomposition of styphnic acid and its metal salts. The absorption spectra of the four crystals display a few strong bands in the fundamental absorption region. Compared with styphnic acid, potassium, barium, and lead styphnates decrease its enthalpy, entropy, free energy, and heat capacity as the temperature increases. However, the differences of the thermodynamic functions between each metal salt are very small. As the temperature increases, the decomposition reactions of the four crystals are more and more favorable thermodynamically. It is also found that there is a relationship between the band gap and impact sensitivity for the four crystals.