Structure, Elastic Constants and XRD Spectra of Extended Solids under High Pressure
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.277
Structure, Elastic Constants and XRD Spectra of Extended Solids under High Pressure I.G. Batyrev1, a), S.P. Coleman1, J.A. Ciezak-Jenkins1, E. Stavrou2 and J.M. Zaug2 1
US Army Research Laboratory, Aberdeen Proving Ground, MD 21005
2
Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA 94550
ABSTRACT
We present results of evolutionary simulations based on density functional calculations of a potentially new type of energetic materials called extended solids: P-N and N-H. Highdensity structures with covalent bonds generated using variable and fixed concentration methods were analysed in terms of thermo-dynamical stability and agreement with experimental X-ray diffraction (XRD) spectra. X-ray diffraction spectra were calculated using a virtual diffraction algorithm that computes kinematic diffraction intensity in threedimensional reciprocal space before being reduced to a two-theta line profile. Calculated XRD patterns were used to search for the structure of extended solids present at experimental pressures by optimizing data according to experimental XRD peak position, peak intensity and theoretically calculated enthalpy. Elastic constants has been calculated for thermodynamically stable structures of P-N system.
INTRODUCTION Extended solids [1] have been proposed to be alternative (disruptive) highenergy-density materials that could release up to 4–9 times the energy of TNT on a per mass basis [2]. X-ray diffraction has been widely used to characterize high-pressuretemperature synthesized extended-solid-state materials; however, unambiguous molecular structure refinements are often difficult to achieve. Three-dimensional network structural refinements absolutely require theory-based computations constrained by experimental results. Density functional theory (DFT) calculations have been helpful in interpretation of XRD results [3-5] and they offer a deeper understanding of the nature of atomic bonding. Here, we present the application of a virtual diffraction method [6] to simulate XRD patters of numerous high-pressure N-H and N-P structures that were calculated using the USPEX evolutionary algorithm approach [7], based on the planewave DFT code VASP [8]. Chain structures for N-H system were predicted in [3] for medium N concentrations using USPEX. Enthalpy per P3N5 formula unit was calculated as a function of pressure up to 100 GPa in [5] using the same evolutionary method. The calculations showed that phosphorous nitrides with the Imm2 γ-phase are the lowest enthalpy phases in the range of 5-35 GPa. Here we predict new chain structure of N-H system at high N concentration at 40 GPa and new metallic phase P 3N4 with C2/M
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(C2H+3) symmetry that is found to be thermodynamically sta
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