Calculations and Experimental Studies of TAGzT under High Pressure
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Calculations and Experimental Studies of TAGzT under High Pressure
I.G. Batyrev and R.C. Sausa US Army Research Laboratory, Aberdeen Proving Ground, MD 21005
ABSTRACT We studied TAGzT theoretically using density functional perturbation theory within the plane-wave-pseudo-potential formalism and experimentally by Raman and IR spectroscopy at ambient and high pressure. The modeled spectra predict reasonably well the experimental spectra at ambient pressure and the Raman vibrational modes at pressures up to 25 GPa. We report the effects of pressure on volume, Raman and IR vibrational modes, and charge distribution of TAGzT.
INTRODUCTION Bis-triaminoguanidium azotetrazolate (TAGzT) is a high-nitrogen compound. It has a high heat of formation and generates a high volume of gas when it decomposes. TAGzT comprises of two monovalent triaminoguanidinium cations and one azo tetrazole anion. Its molecular formula is C4H18N22. The vibrational spectra of TAGzT under isothermal compression has been examined recently by in situ Raman spectroscopy to near 17 GPa to determine its stability[1]. Here, we present new spectroscopic and X-ray diffraction data of TAGzT at ambient pressure, and model this data and the Raman data using density functional theory (DFT). We report the frequency shift of the vibrational modes with an increase in pressure and the pressureinduced evolution of Hirshfeld atomic charges. THEORY
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The crystalline structure of TAGzT was obtained by optimizing its 88- atom unit cell at various pressures by means of Broyden–Fletcher–Goldfarb–Shanno (BFGS) algorithm for relaxation of crystals, based on quasi-Newton method [2]. Ultra-soft projector augmented waves (PAW) with Grime dispersion corrections [3] were used to calculate the XRD patterns. This type of dispersion correction was successfully used previously to calculate the elastic constants of energetic materials [4]. The plane wave code CASTEP with norm-conserving pseudo potentials was used to calculate the IR and Raman spectra at zero temperature [5]. The calculations employed a cut-off energy of 900 eV for the plane waves and a 10 irreducible point in BZ (3x3x3 grid). After relaxation with accuracy 10-6 eV/atoms, the forces were less 0.005 eV/Å. The Raman spectra were calculated at 10K using a laser wavelength of 514.5 nm and a Lorentzian width of 20 cm-1. The phonon frequencies were determined from the dynamical matrix, dielectric constant, and Born effective charges. The Born effective charge tensor of an ion was found from the partial derivatives of the macroscopic polarization with respect to a periodic displacement of all the periodic images of that ion at zero macroscopic electric field according to formalism derived previously [6]. The following equation relates Raman cross section the for i-th eigenmode wi
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