Modeling of the Amorphous Phase of Poly-CO with He and N
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Modeling of the Amorphous Phase of Poly-CO with He and N Iskander G Batyrev Army Research Laboratory, Aberdeen Proving Grounds, MD 21005, USA
ABSTRACT Density functional theory (DFT) and variational density functional perturbation theory (DFPT) simulations of amorphous poly-CO structures were performed to understand the stability of the polymerized structure at low pressures and to study the mechanism of destruction of the extended network at low pressures. Charge population analyses accompanied the search of the “ weakest link” in the covalently bonded network. IR and Raman spectra of amorphous p-CO, calculated at 15 and 5.02 GPa, show significant contributions of CO molecules, carbonyl groups fragments decorating chains, and lactones of amorphous p-CO structures. DFT simulations of formation of amorphous polymeric structures were also done with the addition (as a result of replacement of CO molecules) of N or He atoms to the crystalline delta phase of CO. For the CO-N mixtures, the concentration of N was varied in the range from 6.25 % to 50% with different distribution patterns of N atoms in the unit cell. For all studied CO-N concentrations, isotropic compression led to CO polymerization beginning at a pressure of 11 GPa; the N was incorporated in the random network in low concentration. In CO-He mixtures He atoms appear to facilitate complete formation of the random structure which is almost completely polymerized at a pressure of 18 GPa. He atoms also help stabilize the structure at low pressures.
INTRODUCTION The high pressure phase of polymeric CO has been of interest since the first publication of experimental evidence that it is a high energy material [1]. Theoretical searches for possible crystalline [2] and amorphous structures [3,4] of polymeric CO are a subject of intense ongoing research. Most likely, polymeric CO is mainly a crystalline (or polycrystalline) material with a certain concentration of amorphous phase, according to X-ray diffraction experiments [5]. Previously we found that transition of the polymeric phase starts at 7-8 GPa, with almost full connectivity of CO molecules to a random network at 18 GPa [4]. The transition has a hysteresis in density and the polymeric structure is stable down to low pressures [4]. Here we present results of DFT modeling of amorphous poly-CO structure obtained by isotropic compression of the molecular crystal phase with certain fractions of He and N atoms replacing CO molecules. THEORY
Molecular crystal and amorphous poly- CO structures were obtained through 128 and 432 atom cell optimization (conjugate gradient method) and atomic positions relaxation (LBFGS method) at zero temperature using Quickstep/CP2k package (versions 2.3 and 2.4) [6] with GTH pseudopotentials within the PBE approximation. The polymeric structure was also optimized using norm-conserving pseudo potentials for IR and Raman calculations by the plane wave code CASTEP [7]. The Born effective charge tensor of an ion is found from partial derivatives of the macroscopic polarization with re
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