Understanding the fracture behavior of glasses at the atomic scale is of fundamental impor-tance for improving the mechanical properties of these materials. Using an interaction potentialthat has been parametrized via ab initio calculations [1,2], we have carried out molecular dy-namics simulations in order to investigate the influences of system size, sample geometry, andstrain rate on the fracture behavior of sodium silicate glasses. In contrast to earlier simulationstudies on fracture, in which the bulk glass sample has often been put directly under stress, wehave applied here uniaxial tension to a glass sample that has stress-free surfaces, i.e. we adopta setup that is close to the one used in experiments on fracture studies. Our results show thatthe used interatomic potential captures reasonably well the brittle fracture on the nanoscale ofsilica glass and the enhanced ductility when sodium oxide is added to the glass network. Wehave found that below a critical strain rate of around 0.5/ns the stress-strain curve remainsbasically unchanged. By investigating the formation, growth and coalescence of cavities in thestrained glass samples we find that in sodium silicate glasses the formation of cavities is beingmore pronounced than in silica glass. The analysis of atomic energy, local stress and strain, andatomic displacements have confirmed the presence and the development of mechanically weakzones in the glass network, resulting in paths along which the cracks advance.A. Carré, S. Ispas, J. Horbach, and W. Kob, ”Developing empirical potentials from ab initiosimulations: The case of amorphous silica,” Comput. Mater. Sci., vol. 124, pp. 323–334, Nov.2016.S. Sundararaman, L. Huang, S. Ispas, and W. Kob, ”New optimization scheme to obtain inter-action potentials for oxide glasses”, to be sumitted (2017)

We propose a new scheme to parametrize effective pair potentials that can be used to simulate oxideglasses. As input data for the optimization we use the radial distribution functions of the liquid andthe vibrational density of state of the glass, both obtained from ab initio simulations, as well asexperimental data on the pressure and/or composition dependence of the density and the elasticmoduli of the glass [1].For the case of silica we find that this new scheme allows to find potentials that are significantlyaccurate than previous ones even if the functional form is the same, thus demonstrating that evensimple two-body potentials can be superior to more complex three-body potentials. We have testedthe new potential by calculating the pressure dependence of the elastic moduli and find a goodagreement with the corresponding experimental data.For binary alkali (lithium, sodium, potassium) silicate glasses, the new potentials allow to reproducethe composition dependence of both density and elastic moduli. Further, we examine thecapabilities of these potentials for studying ternary compositions containing two alkali oxides, andwe find that they are reliable even if they have been developed for binary compositions.Simplicity of the functional form also makes these potentials computationally more efficient thanpotentials with more complex functional forms, and hence more suitable for simulations involvinglarge length and/or time scales scales.[1] S. Sundararaman, L. Huang, S. Ispas, and W. Kob, ”New optimization scheme to obtaininteraction potentials for oxide glasses”, J. Chem. Phys. 148 194504 (2018)

The vibrational properties of three sodosilicate glasses have been investigated in the framework of Density Functional Theory. The Raman spectra are calculated and the results compare well with the experimental observations. The vibrational analysis confirms the presence of Si-O-Si bending as well as breathing modes of small rings at intermediate frequencies. The Qn Raman-feature at high frequency is decomposed into Q2, Q3, and Q4 vibrational species. Interestingly, the results suggests a large overlap between the three contributions as well as spectral shapes that are not necessarily unimodal nor Gaussian, as frequently assumed in the treatment of the experimental data. Raman scattering of ternary alumino-silicate glasses with alkali and alkaline-earth cations (Mg, Ca, Sr, Ba, Na,...) has also been performed. In these glasses, vibrational signatures of cations are clearly evidenced at low frequency in the depolarized (VH) spectra. Raman scattering by cations at low-frequency is confirmed by the computational data in the sodo-silicates. In the alumino-silicate glasses the responses associate to different types of motions. The analysis allows separating the contribution arising from network modifier cations to that originating from charge compensator ones.

Hybrid organic-inorganic perovskites are intensively studied as promising materials for various applications, taking advantage of their low cost and easy processing. Although much recent attention has been devoted towards unraveling their microscopic optoelectronic properties, the low frequency dynamics (e.g. their softness) is currently still lacking a comprehensive understanding as compared to that already reached for classic semiconductors. Here we investigate the low frequency dynamics in four different hybrid perovskite single crystals: MAPbBr3, FAPbBr3, MAPbI3 and α-FAPbI3. Using coherent inelastic neutron scattering spectroscopy and Brillouin light scattering, we investigate the low frequency acoustic phonons [1,2]. A very small shear C44 elastic constant is found for all the compounds as well as sizeable elastic anisotropy. The extremely low bulk modulus and negative C12 in α-FAPbI3 substantiates its very unstable nature and in FAPbBr3, a tendency towards an incipient ferroelastic transition, is interpreted as further evidence of the influence of plasticity in hybrid perovskites. We observe a systematic lower sound group velocity in the technologically important iodide-based compounds compared to the bromide-based ones [2]. The findings suggest that low thermal conductivity and hot phonon bottleneck phenomena are expected to be enhanced by low elastic stiffness, particularly in the case of the ultrasoft α-FAPbI3 (Fig. 1a).The low-frequency dynamics is further investigated by Raman spectroscopy. The scattering from the low-frequency optical phonons has been collected across the cubic to tetragonal transition. With the resolution and contrast available by our experiment (~4-5 cm-1 at the bottom of the elastic line) we do not observe the divergence of a central peak. However, the lowest frequency vibration, almost temperature independent in the cubic phase, hardens on cooling in the tetragonal phase. These conclusions apply in three of the four samples (analysis in α-FAPI still in progress) suggesting a common framework for the analysis of the low-frequency vibrational dynamics of this family compounds (Fig. 1b). In CH3NH3PbI3 the Raman spectroscopy across the full set of phase transitions down to 20 K will also be discussed.

Raman scattering of ternary alumino-silicate glasses with alkali and alkaline-earth cations(M=Mg, Ca, Sr, Ba, Na,...) has been performed. The concentrations range from the peral-caline (MO/Al2O3> 1) to the peraluminate (MO/Al2O3< 1) domain keeping constant thesilica content. The vibrational signature of the cations is clearly evidenced at low frequencyin the depolarized (VH) spectra. Between one and three bands are identifed. They associate to different types of motions depending on the charge carried by the cation (alkali oralkaline-earth atoms). In addition, glasses with magnesium exhibit a very peculiar behavior.Very interestingly, some bands disappear for the tectosilicate glasses (MO/Al2O3=1) andin the peraluminate domain revealing a clear vibrational contrast between network modifer cations and charge compensator cations. A preliminary experiment in a quaternaryalumino-silicate system combining two types of alkali atoms highlights the possibility by sim-ple Raman spectroscopy to define the preferential role of each cation, i.e. network modieror charge compensator, and to make a semi-quantitative treatment as well.

We use computer simulations to probe the thermodynamic and dynamic properties of a glass-former that undergoes an ideal glass-transition because ofthe presence of randomly pinned particles.We find that even deep in the equilibrium glass state the system relaxes to some extent because ofthe presence of localized excitations that allow the systemto access different inherent structures,giving thus rise to a non-trivial contribution to the entropy. By calculating with high accuracy thevibrational part of the entropy, we show that also in the equilibrium glass state thermodynamicsand dynamics give a coherent picture and that glasses shouldnot be seen as a disordered solid inwhich the particles undergo just vibrational motion but instead as a system with a highly nonlinearinternal dynamics.

We propose a new scheme to parametrize effective pair potentials that can be used tosimulate oxide glasses. As input data for the optimization we use the radial distributionfunctions of the liquid and the vibrational density of state of the glass, both obtained fromab initio simulations, as well as experimental data on the pressure and/or composition de-pendence of the density and the elastic moduli of the glass [1].For the case of silica we find that this new scheme allows to find potentials that are signifi-cantly accurate than previous ones even if the functional form is the same, thus demonstratingthat even simple two-body potentials can be superior to more complex three-body poten-tials. We have tested the new potential by calculating the pressure dependence of the elasticmoduli and find a good agreement with the corresponding experimental data.For binary alkali (lithium, sodium, potassium) silicate glasses, the new potentials allowto reproduce the composition dependence of both density and elastic moduli. Further, weexamine the capabilities of these potentials for studying ternary compositions containing twoalkali oxides, and we find that they are reliable even if they have been developed for binarycompositions.Simplicity of the functional form also makes these potentials computationally more efficientthan potentials with more complex functional forms, and hence more suitable for simulationsinvolving large length and/or time scales scales.S. Sundararaman, L. Huang, S. Ispas, and W. Kob, ”New optimization scheme to obtaininteraction potentials for oxide glasses”, submitted (2018)