An efficient computational procedure to obtain a more stable glass structure

A critical assessment of interatomic potentials for modelling lattice defects in forsterite Mg 2 SiO 4 from 0 to 12 GPa

Five different interatomic potentials designed for modelling forsterite Mg2 SiO4 are compared to ab initio and experimental data. The set of tested properties include lattice constants, material density, elastic wave velocity, elastic stiffness tensor, free surface energies, generalized stacking faults, neutral Frenkel and Schottky defects, in the pressure range 0 - 12  GPa relevant to the Earth's upper mantle. We conclude that all interatomic potentials are reliable and applicable to the study of point defects. Stacking faults are correctly described by the THB1 potential, and qualitatively by the Pedone2006 potential. Other rigid-ion potentials give a poor account of stacking fault energies, and should not be used to model planar defects or dislocations. These results constitute a database on the transferability of rigid-ion potentials, and provide strong physical ground for simulating diffusion, dislocations, or grain boundaries.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00269-021-01170-6.

Structural origin of thermal shrinkage in soda-lime silicate glass below the glass transition temperature: A theoretical investigation by microsecond timescale molecular dynamics simulations

Microscopic dynamical features in the relaxation of glass structures are one of the most important unsolved problems in condensed matter physics. Although the structural relaxation processes in the vicinity of glass transition temperature are phenomenologically expressed by the Kohlrausch-Williams-Watts function and the relaxation time can be successfully interpreted by Adam-Gibbs theory and/or Narayanaswamy's model, the atomic rearrangement, which is the origin of the volume change, and its driving force have not been elucidated. Using the microsecond time-scale molecular dynamics simulations, this study provides insights to quantitatively determine the origin of the thermal shrinkage below T_g in a soda-lime silicate glass. We found that during annealing below T_g, Na ions penetrate into the six-membered silicate rings, which remedies the acute O-O-O angles of the energetically unstable rings. The ring structure change makes the space to possess the cation inside the rings, but the ring volume is eventually reduced, which results in thermal shrinkage of the soda-lime silica glass. In conclusion, the dynamical structural relaxation due to the cation displacement evokes the overall volume relaxation at low temperature in the glassy material.

Self-assembly of the cationic surfactant n-hexadecyl-trimethylammonium chloride in methyltrimethoxysilane aqueous solution: classical and reactive molecular dynamics simulations

A flexible aerogel polymerized from methyltrimethoxysilane (MTMS) shows great promise as a high-performance insulator owing to its substantially low thermal conductivity and mechanical flexibility, attributed to its porous microstructure and organic-inorganic hybridization, respectively, which promote its industrial applications. Conventionally, the cationic surfactant n-hexadecyltrimethylammonium chloride (CTAC) is utilized to experimentally control the nanoscale microstructure and, consequently, the flexibility of the MTMS aerogel; however, the mechanism through which CTAC prevents MTMS aggregation in the solution is not yet fully understood. This study unravels the role of CTAC in preventing MTMS aggregation in aqueous solution using both classical and reactive molecular dynamics simulations. We found that CTAC molecules can form self-aggregates even when the polymerization of MTMS progresses and then the MTMS-derived oligomer turns to be hydrophobic in aqueous solution. In summary, the self-assemblies of CTAC disperse among the MTMS associations and effectively prevent MTMS clustering, and this is considered as the key mechanism underlying the formation of a flexible microstructure of the hybrid aerogel.

Disclosing crystal nucleation mechanism in lithium disilicate glass through molecular dynamics simulations and free-energy calculations

Unraveling detailed mechanism of crystal nucleation from amorphous materials is challenging for both experimental and theoretical approaches. In this study, we have examined two methods to understand the initial stage of crystal precipitation from lithium disilicate glasses using molecular dynamics simulations. One of the methods is a modified exploring method to find structurally similar crystalline clusters in the glass models, enabling us to find three different embryos, such as Li₂Si₂O₅ (LS₂), Li₂SiO₃ (LS) and Li₃PO₄ (LP), in the 33Li₂O·66SiO₂·1P₂O₅ glass (LS₂P1), in which P₂O₅ is added as a nucleating agent. Interestingly, LS₂ and LP crystals were found inside the LS₂P1 glass while LS crystal appeared on the glass surface, which agrees with experimental observations. The other method is free energy calculation using a subnano-scale spherical crystal embedded in the glass model. This method, which we called Free-Energy Seeding Method (FESM), allows us to evaluate free energy change as a function of crystal radius and to identify critical size of the crystal precipitation. The free energy profiles for LS and LS₂ crystal nuclei in the LS₂ glass models possess maximum energy at a critical radius as expected by classical nucleation theory. Furthermore, the critical radius and the energy barrier height agree well with recent experimental investigation, proving the applicability of this method to design glass–ceramics by atomistic modeling.

Ionic self-diffusion and the glass transition anomaly in aluminosilicates

The glass transition temperature (Tg) is the temperature after which a supercooled liquid undergoes a dynamical arrest. Usually, glass network modifiers (e.g., Na2O) affect the behavior of Tg. However, in aluminosilicate glasses, the effect of different modifiers on Tg is still unclear and shows an anomalous behavior. Here, based on molecular dynamics simulations, we show that Tg decreases with increasing charge balancing cation field strength (FS) in the aluminosilicate glasses, which is an anomalous behavior as compared to other oxide glasses. The results show that the origins of this anomaly come from the dynamics of the supercooled liquid above Tg, which in turn is correlated to pair excess entropy. Our results deepen our understanding of the effect of different modifiers on the properties of the aluminosilicate glasses.