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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.116

Prediction of amorphous structure and stability of P-N and N-CO extended solids under pressure I.G. Batyrev US Army Research Laboratory, Aberdeen Proving Ground, MD 21005

ABSTRACT

The amorphous structures of poly-CO, P-N and N-CO extended solids at high pressures were predicted using density functional theory (DFT) and evolutionary algorithms employing variable and fixed concentrations of components methods. Compression of random network of poly-CO up to 45 GPa results in elimination of small rings of the amorphous network. The amorphous structure with stoichiometry N9P was found to be dynamically stable (no imaginary frequencies in phonon-dispersion curve), stable relative transformation to solid nitrogen and phosphorus, but metastable according to convex hull calculations. The amorphous structure of the N-CO extended solid was obtained with various concentrations of N atoms under isotropic compression up to 50 GPa and release of pressure down to 5 GPa calculated using DFT. The higher concentration of CO is found to be favourable for stabilization of an amorphous covalent N-C-O network consisting of chains and a cage of the network. Upon lowering the pressure and decomposition of the compressed extended solid, atoms are disconnected first from the ends of polymeric chains, while rings of random network are sustained almost intact. Results of a calculated Raman spectra are compared with available experimental results.

INTRODUCTION It was demonstrated more than a decade ago that an extended solid derived from poly-CO is indeed a high energy density material [1]. It contains amorphous and crystalline phases. It has been proposed to be an alternative (disruptive) high-energy-density material that could release the energy exceeding that of HMX (cyclo-tetramethylene tetranitramine, a commonly used conventional high explosive). The nitrogen molecule because of the N≡N triple bond is one of the most stable chemical bonds known (second after the C=O bond). A search of new structures and synthesis methods of mixtures of N2 and CO extended solids revealed new promising structures [2 - 5]. For example, the N-CO structure with P43 space group found in [5] has mass density of 3.983 g/cm3 comparable with that of cubic gauche

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nitrogen (cg-N). The cg-N, a polymeric form of nitrogen made of 3-fold coordinated single bonds, is predicted to contain large chemical energies (‫׽‬33 kJ/cm3). It is 3 times that of HMX (11 kJ/cm3) and might be obtained at substantially lower pressure−temperature condition than those required for converting pure nitrogen [5]. Phosphorous is also a prospective element for mixing with nitrogen in high energy extended solids. Smokeproducing, self-igniting white phosphorus was commonly used in