Modeling of Early Stages of Formation of Poly-CO
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Modeling of Early Stages of Formation of Poly-CO I. G. Batyrev, W. D. Mattson and B. M. Rice US Army Research Laboratory, Aberdeen Proving Ground, Maryland ABSTRACT We studied the early stages of polymerization of CO under pressure. We performed DFT simulations of 128 and 432 atom models. Structures of random networks found at zero temperature were used for equilibration at 100 K by employing first principles MD. We found that the polymerization begins at 7 - 8 GPa and slightly depends on the size of the model. It turned out that there are several metastable phases of the extended CO solid, corresponding to different compression pressures from 7 - 8 GPa to 15-18 GPa with different numbers of CO fragments, not connected to the random network. We also found that the transition to the phases is irreversible which results in hysteresis loops. Random network structures obtained, say, under 18 GPa could exist at 3 GPa, whereas compression to 3 GPa results in the delta phase of CO crystal, with intact CO fragments and minor distortion of the cubic phase. To analyze the random structure fragments we calculated normal modes and IR intensities using the dipole approximation. Contributions from the main motifs of the random network are identified and compared with experimental IR measurements. INTRODUCTION The random network of polymeric carbon monoxide (poly-CO) is a high energy density material [1]. A set of images following laser initiation may be considered as support of the explosive behavior of poly-CO [1]. Quantum mechanical modeling of possible structures indicates the presence of lactone–like fragments in the structure of poly-CO [2] and a reasonable agreement of the vibration analysis of the five-fold lactones with IR spectroscopy measurements. In this paper we systematically perform first principles calculations of early stages of polymerization of CO molecules starting from the δ phase of solid CO [3]. Our calculations show that the phase plays an important role in the formation of poly-CO. It turns out that there is a set of irreversible transitions to poly-CO structures corresponding to different applied pressures, which is consistent with the qualitative model suggested by Brazhkin in [4]. Starting from 7-8 GPa, some of the CO molecules are intact as in the δ crystal phase, but with different densities, and certain parts of the molecules form chains, four- and fivefold rings with O in the cage and carbonyl groups decorating the rings. Compression to 15-18 GPa results in an almost complete transformation of the crystal phase to a random network of four, five, and six cycles decorated by carbonyl groups and connected by C-O-C, C-C, and C-O(C=O)-(C=O)-O-C bridges. There are ~ 1.5% of CO and CO2 type molecules within the random network. Calculations in which pressure is released starting from the random structure obtained at 15-18 GPa indicate a significant pressure stabilization and densification of the poly-CO structure. We found that the poly-CO structure may exist down to a fraction of 1 GPa. We calculated norma
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