School of Physics

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Browsing School of Physics by Author "Abraham, B. Moses"
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    A comparative study of the structure, stability and energetic performance of 5,5′-bitetrazole-1,1′-diolate based energetic ionic salts: Future high energy density materials
    ( 2018-01-01) Abraham, B. Moses ; Ghule, Vikas D. ; Vaitheeswaran, G.
    Developing novel energetic materials of high detonation performance and low sensitivity is one of the primary objectives related to explosive research. By employing ab initio calculations, a series of energetic ionic salts based on 5,5′-bitetrazole-1,1′-diolate (BTO) were thoroughly investigated to understand the structure-property-performance interrelationship. The physicochemical and detonation characteristics of these energetic ionic salts including structural, electronic, vibrational and performance parameters (heats of formation, detonation pressures, and detonation velocities) were discussed in detail. The strong intermolecular hydrogen bonding environment between the BTO2- anion and various cations is mainly responsible for prominent detonation performance and enhanced molecular stability. Such strong intermolecular hydrogen bonds are observed in hydrazine and hydroxylammonium cations compared to other cations. To predict an accurate band gap, electronic band structures of the studied energetic ionic salts (EIS) were calculated using the HSE06 hybrid functional and they are found to be wide band gap insulators with a bandwidth ranging from 4.33-5.05 eV. Careful inspection of various EIS revealed that the hydroxylammonium and hydrazine cations produce the highest density relative to other cations when combined with the BTO anion. The detonation characteristics of BTO2- are computed using EXPLO5 code. In particular, HA-BTO and TKX-50 exhibit high detonation pressure (38.85 and 40.23 GPa) and detonation velocities (9.94 and 9.91 km s-1), superior to those of traditional nitrogen-rich energetic materials with moderate sensitivities. These results highlight the importance of hydrogen bonding interactions in designing energetic salts for next-generation explosives, propellants, and pyrotechnics.
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    First principles study of pressure induced polymorphic phase transition in trimethylamine
    ( 2018-04-10) Abraham, B. Moses ; Vaitheeswaran, G.
    The pressure induced variations on the crystal structure of various polymorphs of Trimethyamine (TMA-I, TMA-II, TMAIII) has been studied theoretically using first principles calculations up to 5 GPa. The obtained equilibrium lattice parameters using standard PBE-GGA functional for the ambient and high pressure phases are found to be in good agreement with the experimental values. We calculated the enthalpies of each phase to assess their relative stability. Our results also supports the existence of additional phase transitions of TMA into two new polymorphs under external pressure. The TMA-I to TMA-II transition is found to occur at 1.41 GPa and the TMA-II to TMA-III transition at 3.33 GPa. The electronic band structure calculations using Tran Blaha-modified Becke Johnson (TB-mBJ) potential show that these polymorphs of TMA are indirect band gap insulators.
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    From van der Waals interactions to structures and properties of 3,3 < sup > ′ < /sup > -dinitro-5,5 < sup > ′ < /sup > -bis-1,2,4-triazole-1,1 < sup > ′ < /sup > -diolate based energetic materials
    ( 2020-01-15) Abraham, B. Moses ; Vaitheeswaran, G.
    As a continuation for the search of precise correlation between performance and sensitivity of high energy density materials (Phys. Chem. Chem. Phys. 2018, 20, 29693), we present structure analysis and quantum mechanical calculations for similar energetic ionic salts (EIS) based on 3,3′-dinitro-5,5′-bis-1,2,4-triazole-1,1-diolate anion with ammonium (DA-DNBTO) and hydrazinium (DH-DNBTO) cations. The geometry optimization demonstrate the importance of van der Waals correction when studying the structure and properties of EIS, and highlights the significance of DFT-D2 method proposed by Grimme in reproducing the experimental crystal structures of energetic salts. The IR spectrum of DA-DNBTO contains more number of intense peaks in the high frequency range (above 2850 cm−1) compared to DH-DNBTO. Especially, the electronic band gap of DNBTO salts obtained using the HSE06 hybrid functional are reduced to 50% when compared with that of BTO based energetic salts due to NO2 group attached to the DNBTO anion. Besides, we find that the O.O contacts that generally exist in most of the energetic materials do not show much impact in both the studied energetic salts. The strong intermolecular interactions of DA-DNBTO represents its inferiority in molecular stability, which is in good agreement with the experimentally measured impact sensitivity ( > 40 J) and friction sensitivity (360 N) values and also explains why it has highest stability compared to DH-DNBTO. Our calculations re-verifies the importance of intermolecular hydrogen bonding in the construction of high performance and low sensitive energetic materials.
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    High-Pressure Structural and Electronic Properties of Potassium-Based Green Primary Explosives
    ( 2021-04-01) Abraham, B. Moses ; Yedukondalu, N. ; Vaitheeswaran, G.
    Recently synthesized green primary explosives potassium 4,4′-bis(dinitromethyl)-3,3′-azofurazanate (K2BDAF) and potassium 1,1′-dinitramino-5,5′-bistetrazolate (K2DNABT) offer fast, powerful initiation capacity and high-performance detonation characteristics to replace the long-standing toxic primary explosives. In the present study, we report the structural and electronic properties of the emerging green primary explosives K2BDAF and K2DNABT under hydrostatic pressure up to 10 GPa. We observed that dispersion correction methods are important for capturing weak intermolecular interactions in order to accurately describe the geometries of the primary explosives. The computed ground state structural properties using optB86b-vdW method are in good agreement with the experimental results. The pressure-dependent lattice constants and bond parameters show anisotropic nature and also a sharp discontinuity around 4–5 GPa. The obtained equilibrium bulk modulus shows that K2BDAF is softer than K2DNABT. Bonding analysis revealed that the C-NO2 energetic functional groups are more sensitive than the stable ring structure under hydrostatic pressure. The calculated electronic structure of K2BDAF using the Tran–Blaha-modified Becke–Johnson (TB-mBJ) potential shows a direct-to-indirect band gap transition around 5 GPa, which is consistent with the aforementioned discontinuity of the structure, while K2DNABT is found to be a direct-band-gap insulator in the studied pressure range of 1–10 GPa. The abnormal trends in the structural and electronic properties suggest that K2BDAF may undergo a structural transition/distortion around 4–5 GPa of pressure.
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    Structural, electronic and lattice dynamical properties of perovskite CaZrO < inf > 3 < /inf > under high pressure
    ( 2017-05-19) Abraham, B. Moses ; Yedukondalu, N. ; Vaitheeswaran, G.
    We report the structural, electronic and lattice dynamical properties of perovskite CaZrO3 (CZO) under pressure up to 30 GPa using density functional theory calculations. The obtained lattice parameters and bulk modulus using standard PBE-GGA functional are in good agreement with the experimental data. The computed phonon dispersion curves at 0 and 30 GPa pressures show the dynamical stability of ambient phase of CZO under high pressure. The calculated electronic structure using Tran-Blaha modified Becke-Johnson (TB-mBJ) potential shows that CZO is a direct band gap insulator with a band gap of 4.93 eV which is closely comparable with the experimental value of 5.7 eV and it is found to increase with pressure.
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    Structural, vibrational and bonding properties of hydro-nitrogen solids under high pressure: An ab-initio study
    ( 2016-10-01) Vaitheeswaran, G. ; Yedukondalu, N. ; Abraham, B. Moses
    Hydro-nitrogen solids are potential high energy density materials (HEDMs) due to high mass ratio of nitrogen which find wide range of applications as propellants and explosives. In the present work, we report the structural and vibrational properties of Tetramethyl Ammonium Azide (TMAA) and HydroZonium Azide (HZA) using density functional theory calculations by treating weak intermolecular interactions. The obtained ground state parameters using vdW-TS method are in good agreement with the experimental data. The pressure dependent lattice constants, compressibility and equation of state are discussed. The obtained equilibrium bulk moduli show the soft nature of these materials. The compressibility curves reveal that these compounds are highly compressible along crystallographic a-axis. We have also calculated the zone-center phonon frequencies and made a complete analysis of vibrational spectra at ambient as well as at high pressure. Contraction and elongation of C-H and N-H (NH3 stretching) bonds under pressure lead to blue- and red-shift of the frequencies in the mid-IR region for TMAA and HZA compounds, respectively. [Figure not available: see fulltext.]