is transferred to another nitramine of different stoichiometry: hexanitrohexaazaisowurtzitane (CL-20 or HNIW). The modification of a single parameter alongside a very small number of add-ons related to carbon-carbon bonds, angles and dihedrals lead to two SB FF variants denoted SB-CL20 and SB-CL20 + CCNN. These flexible-molecule FFs should inherit the predictive capabilities of SB FF. For this purpose, we perform Molecular Dynamics simulations at ambient temperature and selected pressures. The modeled structures of the various CL-20 polymorphs are consistent with experimental data. Focusing on the 3-polymorph, we determine an equation of state which consolidates the general trend underpinned by most published results, and we confirm the increasing stiffness of the crystal under pressures up to 90 GPa. Moreover, we link some subtle pressure-induced changes of the elastic and structural properties to the flexibility and mobility of well-identified nitro groups. Finally, the simulations of the g 4 z phase transition suggest different multiple-step direct and reverse thermodynamic paths. † Electronic supplementary information (ESI) available: FF parameter and conguration les to get started with 3-CL20 simulations using LAMMPS and our SB-CL20 FFs. Model/experiment comparison of the pressure-induced variations of 3-CL20 lattice parameters. Wag angle distributions of a, b, g and z polymorphs modeled with our SB-CL20 FFs at ambient conditions (or 3.3 GPa for z), and comparison with experimental data. Replication-induced defects in supercell-framed 3-CL20 at 30 GPa, using SB-CL20 + CCNN FF. Two animations of the compression of 3-CL20 modeled with SB-CL20 + CCNN FF from 0 to 90 GPa. Density and pressure during the decompression of the g sample modeled with SB-CL20 + CCNN FF, up to its disintegration. One animation of the disintegration at negative pressure of the z sample modeled with SB-CL20 + CCNN FF. See Fig. 1 Phase transformation diagram (left) using g-polymorph as starting material (data from ref. 3), corresponding molecular conformations showing the characteristic orientations of the nitro groups, and (right) crystal symmetries. Fig. 2 Molecules of HMX (left) and CL-20 (middle), and CCNN dihedral interaction for SB-CL20 FF (right).39650 | RSC Adv., 2019,9,[39649][39650][39651][39652][39653][39654][39655][39656][39657][39658][39659][39660][39661] This journal is
Doping material with nanoparticles is increasingly used as an effective method for improving their mechanical, optical, and sturdiness properties in many fields. More specifically, effective material development will depend on our ability to control nanoparticles’ shape, composition, and size. While crystalline nanophase can be examined easily, characterization of amorphous nanoparticles remains a challenge. Here, we investigate the chemical composition of sub-20-nm oxide nanoparticles grown in rare-earth doped silicate glass through the phase separation mechanism occurring under heat treatment. Using a combination of analytical techniques, we demonstrate that nanoparticle composition and, therefore, the chemical environment of encapsulated rare-earth ions, is nanoparticle size dependent. This new experimental evidence of composition change contributes unique insights on the phase separation mechanism that will lead to better comprehension and will guide development of future materials.
A simple transferable adaptive model is developed and it allows for the first time to simulate by molecular dynamics the separation of large phases in the MgO-SiO2 binary system, as experimentally observed and as predicted by the phase diagram, meaning that separated phases have various compositions. This is a real improvement over fixed-charge models, which are often limited to an interpretation involving the formation of pure clusters, or involving the modified random network model. Our adaptive model, efficient to reproduce known crystalline and glassy structures, allows us to track the formation of large amorphous Mg-rich Si-poor nanoparticles in an Mg-poor Si-rich matrix from a 0.1MgO-0.9SiO2 melt.
Developing new rare-earth-doped optical glasses with "enhanced" spectroscopic properties requires the elaboration of new glass compositions. To overcome some typical limitations of silica glass, a strategy consists in encapsulating rare-earth (RE) ions within oxide nanoparticles (NPs) through a phase separation mechanism. In this paper, Molecular Dynamics simulations were performed using an interatomic potential reproducing the phase separation within a MgO-SiO2 binary melt to obtain RE-codoped glass models with RE = Eu or Er. In these structures, we observed that Mg-rich regions, included into a silica-rich matrix and identified as NPs, are amorphous and exhibit a large range of sizes. We showed that such nanoparticles are the host of a depolymerization phenomenon of the NPs' SiO4 tetrahedral network leading to a release of nonbridging oxygen atoms. In the NPs, the MgO concentration increases and the doping RE ions are mainly located into the NPs where they are over-concentrated compared with the nominal doping concentration.However, the induced clustering effect is limited because of the non-bridging-oxygen-rich environment encountered in the NPs. This numerical analysis allows to give an insight on the chemical composition of the NPs, and especially on the local environment of the encapsulated rare-earth ions.
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