Lattice dynamics and thermodynamic properties of alkaline earth metal nitrates M(NO3)2 (M = Sr, Ba): A first principles study

Abstract

Metal nitrates are widely used as oxidizers and in light-producing compositions in both civilian and military explosive applications. In this work we have explored the isomorphous divalent metal nitrates M(NO3)2 (M = Sr, Ba) by using dispersion-corrected density functional theory methods. The equilibrium results calculated with Grimme (G06) (for Sr(NO3)2) and Ortmann-Bechstedt-Schmidt (for Ba(NO3)2) functionals reproduced the experimental lattice parameters. We present the effect of cations on the lattice dynamical properties conjointly with their elastic and thermodynamic properties. The linear response approach within density functional perturbation theory was used to calculate the zone-center vibrational frequencies, phonon dispersion relation, and phonon density of states. The infrared spectrum of these compounds in their fundamental state is studied in the whole 0–1500 cm−1 range, and is critically analyzed in the light of previous experimental investigations. We observed that most of the high-frequency vibrational modes emerge because of the NO3 group. The calculated phonon dispersion curves do not show any vibrational anomaly, confirming the dynamical stability of the compounds in the cubic (Pa3¯) phase. The calculated shear anisotropic factors of 1.8 and 2.1 for Sr(NO3)2 and Ba(NO3)2, respectively, indicate that both crystals studied possess considerable mechanical anisotropy. As the thermal behavior of these materials could play a key role in the growth of sustainable smoke compositions, thermodynamic properties such as the entropy, Debye temperature, heat capacity, and enthalpy were calculated. The results reveal that the compounds are thermodynamically stable up to 750 K. This work demonstrates that Sr(NO)2 and Ba(NO3)2 can be reliably used in pyrotechnics as they possess considerable thermal conductivity, leading to a high burn rate. The results presented in this work could open a way to understand the lattice dynamics of materials of this type and could provide necessary input from the pyrotechnic applicability point of view, which would help experimental researchers in the future.