Comparative study of electronic structure, optical properties, lattice dynamics and thermal expansion behaviour of energetic ammonium and potassium dinitramide salts

In this work, we present a detailed first principles study addressing the electronic structure, optical properties, lattice dynamics and thermal expansion studies of energetic salts ammonium dinitramide (ADN) and potassium dinitramide (KDN). From the electronic structure, it is vivid that both the compounds are wide band gap insulators with an indirect band gap of value of 4.3 eV for KDN. Optical constants reveal that ADN and KDN possess considerable optical anisotropy. Analysis of the vibrational modes reveal that the N(NO2) group contributions are considerably large in the high-frequency region in KDN. Phonon dispersion relations computed within the harmonic approximation unambiguously shows that both the compounds are dynamically stable in the reported monoclinic structure. In addition, the optical and acoustical modes are highly interactive as evident from the dispersion spectra, which might lead to low thermal conductivity, quite desirable for an energetic material. Thermal expansion studies within the quasi-harmonic approximation reveal a large positive thermal expansion in both the compounds with KDN having relatively larger thermal expansion coefficient when compared to ADN, in agreement with experiments.