Abstract
High pressure experiments utilizing Raman spectroscopy indicate that the a phase of sodium azide undergoes a polymeric phase transition at high pressure. In this work, the structural and vibrational properties, including the first order Raman and infrared spectra, of the a phase of sodium azide are calculated using first-principles density functional theory up to 92 GPa. The equation of state of α NaN3 is obtained within the quasi-harmonic approximation at various temperatures. Each Raman-active mode blue shifts under compression whereas the doubly degenerate IR-active azide bending mode red-shifts under compression. However, at 70 GPa, the intensity of the Bu IR-active bending mode decreases substantially, and a new distorted azide bending lattice mode appears in the IR spectrum. In contrast to the bending mode, this new mode blue-shifts under compression. No new modes appear in the Raman spectra at high pressure, indicating that the changes in the Raman spectrum seen in experiment at high pressure are signs of new high nitrogen content structures, but not due to sodium azide.
Highlights
All-nitrogen non-molecular solids attracted substantial interest during the last several decades [1, 2, 3, 4, 5, 6] due to their potential use as a powerful energetic material (EM)
Hexagonal β − NaN3 is the ground state crystal structure at room temperature and ambient pressure and is composed of parallel planes of N−3 ions that are directed perpendicular to planes of sodium ions in the R3m space group. β − NaN3 undergoes a polymorphic phase transition to the α − NaN3 phase at 0.25 GPa [12]
The isothermal equation of state (EOS) is shown in figure 1 which compares the pressure versus volume dependence calculated with the pure PerdewBurke Ernzerhof (PBE) functional at 0 K and that calculated using the PBE functional with Grimme’s van der Waals (vdW) correction at 293 K
Summary
All-nitrogen non-molecular solids attracted substantial interest during the last several decades [1, 2, 3, 4, 5, 6] due to their potential use as a powerful energetic material (EM). Measurements of the Raman spectrum at high pressures and temperatures indicate the possible formation of a non-molecular phase of nitrogen when using sodium azide (NaN3) as a precursor material [7, 8, 9]. When hydrostatically compressed to pressures greater than 15 GPa, several new peaks appear in the experimental Raman spectrum [7], in addition to those observed at low pressure [11, 13].
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