Density effects on the structure of irradiated sodium borosilicate glass: A molecular dynamics study

International audienceWe have carried out Molecular Dynamics simulations on a sodium borosilicate glass in order to analyze how the structure of the glass during irradiation is affected by the choice of the density in the liquid state before cooling. In a pristine form generated through the usual melt-and-quench method, both short- and medium-range structures are affected by the compressive or tensile environment under which the glass model has been generated. Furthermore, Na-rich areas are much easier to compress, producing a more homogeneous glass, in terms of density, as we increase the confinement during the quench. When the glass is subjected to displacement cascades, the structural modifications saturate at a deposited energy of approximately 8 eV/atom. Swelling appears for the glasses that were initially prepared under compression, while contraction is evident for the ones prepared under tension. We have equally prepared glass models using a fast quench method, and we have found that they present an analogous disorder as the glasses submitted to displacement cascades. Compared to the irradiated glass, we found that the magnitude of the modifications for the fast quenched glass is lower, most notably in terms of boron and sodium coordination, the percentage of non-bridging oxygens and in the ring distributions. This later result agrees with statements extracted from recent experimental works on nuclear glasses

Vibrational properties of sodosilicate glasses from first-principles calculations

The vibrational properties of three sodosilicate glasses have been investigated in the framework of density functional theory. The pure vibrational density of states has been calculated for all systems and the different vibrational modes have been assigned to specific atoms or structural units. It is shown that the Na content affects several vibrational features as the position and intensity of the R band or the mixing of the rocking and bending atomic motions of the Si-O-Si bridges. The calculated Raman spectra have been found to agree with experimental observations and their decomposition indicated the dominant character of the nonbridging oxygen contribution on the spectra, in particular for the high-frequency band, above 800cm−1. The decomposition of the high-frequency Raman feature into vibrations of the depolymerized tetrahedra (i.e., Qn units) has revealed spectral shapes of the partial contributions that cannot be accounted for by simple Gaussians as frequently assumed in the treatment of experimentally obtained Raman spectra

A study on bimetallic nanoparticles

In this work we focused on small AgmCun, AgmPdn and AgmZrn nanoclusters with m+n=2-5. The calculations were carried out using Density Functional Theory (DFT) combined with a multitude of exchange-correlation functionals. Geometry optimization for the Ag-Cu clusters showed that, both pure and bimetallic, prefer planar structures. More specifically, for bimetallic structures we observed that Ag atoms prefer to occupy edge positions, while the two elements can be mixed at this scale. Electronic properties like ionization potentials, electron affinities and HOMO-LUMO gaps were found to have an even-odd oscillation, depending on the total number of electrons in the cluster. The calculation of the density of states showed that there are common qualitative characteristics for all Ag-Cu clusters with small localized s-d peaks close to the Fermi level and a d band at lower energies. On the other hand, pure Pd clusters are three dimensional, while for Ag-Pd bimetallic ones the substitution of two Pd atoms by Ag atoms is sufficient for the shift to planar geometries. The two elements do not occupy specific positions but they do mix. Mulliken population analysis showed that, in order to improve bonding, Pd atoms promote charge from the full 4d to the empty 5s orbital. The structure of the density of states for Pd atoms is different than for Ag or Cu atoms since the former has a wide d band where the small s charge hybridizes. Any addition of Ag atoms in the cluster results in a shift of the d band to lower energies and the appearance of the characteristic localized peaks of Ag. Pure Zr clusters have three dimensional structures, similar to those of Pd. In contrast to Ag-Pd clusters though, bimetallic Ag-Zr clusters change to planar structures for medium or low Ag content. Moreover, the long Ag-Zr bonds that are formed result in the positioning of Ag atoms in peripheral positions, in order to reduce any structural distortions. The structure of the density of states for pure Zr and bimetallic Ag-Zr clusters is similar to that of Pd and Ag-Pd. A secondary part of this work aimed at the appropriateness of various exchange-correlation potentials for this type of calculations. For this reason we used totally six potentials deriving from three different approximations: SVWN5 from LSDA, BPW91, PBE and BLYP from GGA and the hybrid potentials B3LYP and PBE0. Our results showed that the LSDA potential is inappropriate for this use and the best results derive from the GGA and hybrid potentials, particularly BPW91, PBE and PBE0Στην παρούσα διατριβή ασχοληθήκαμε με τη μελέτη μικρών νανοσωματιδίων (ΝΣ) AgmCun, AgmPdn και AgmZrn με m+n=2-5. Οι υπολογισμοί έγιναν με χρήση της Θεωρίας Συναρτησιακού Πυκνότητας Φορτίου (DFT) σε συνδυασμό με μια πλειάδα δυναμικών ανταλλαγής-συσχετισμού. Η γεωμετρική βελτιστοποίηση για τα ΝΣ Ag-Cu έδειξε ότι οι δομές και των μονομεταλλικών αλλά και των διμεταλλικών είναι επίπεδες. Για τα διμεταλλικά ΝΣ Ag-Cu συγκεκριμένα παρατηρήθηκε ότι τα άτομα του Ag προτιμούν περιφερειακές θέσεις, ενώ φάνηκε ότι τα δύο συστατικά είναι αναμείξιμα σε αυτήν την κλίμακα. Οι ηλεκτρονιακές ιδιότητες των ΝΣ αυτών όπως τα δυναμικά ιονισμού, οι ηλεκτροσυγγένειες και οι διαφορές HOMO-LUMO παρουσιάζουν συμπεριφορά μονού-ζυγού, εξαρτώμενες από τον συνολικό αριθμό ηλεκτρονίων του συστήματος. Η μελέτη της πυκνότητας ηλεκτρονιακών καταστάσεων έδειξε ότι εμφανίζονται παρόμοια ποιοτικά χαρακτηριστικά σε όλα τα ΝΣ Ag-Cu με μικρές εντοπισμένες κορυφές s-d χαρακτήρα κοντά στην ενέργεια Fermi και μία d ζώνη χαμηλότερα. Από την άλλη, τα μονομεταλλικά ΝΣ Pd είναι τρισδιάστατα, ενώ για τα διμεταλλικά Ag-Pd η αντικατάσταση δύο ατόμων Pd από άτομα Ag αρκεί ώστε να ανακτήσουμε επίπεδες γεωμετρίες. Τα δύο στοιχεία δεν καταλαμβάνουν συγκεκριμένες θέσεις, αλλά είναι ταυτόχρονα αναμείξιμα. Η ανάλυση πληθυσμών Mulliken έδειξε ότι τα άτομα του Pd προωθούν φορτίο από την συμπληρωμένη 4d στην κενή 5s στοιβάδα ώστε να δημιουργήσουν δεσμούς. Η δομή του διαγράμματος της πυκνότητας ηλεκτρονιακών καταστάσεων για τα άτομα του Pd είναι διαφορετική από αυτά των Ag και Cu αφού παρουσιάζουν μία φαρδιά d ζώνη όπου υβριδίζει με το μικρό s φορτίο. Προσθήκη ατόμων Ag στο ΝΣ έχει ως αποτέλεσμα τη μετατόπιση της ζώνης σε χαμηλότερες ενέργειες και την εμφάνιση των χαρακτηριστικών εντοπισμένων κορυφών του Ag. Τα μονομεταλλικά ΝΣ Zr εμφανίζουν και αυτά τρισδιάστατες δομές σαν του Pd. Σε αντίθεση όμως με τα Ag-Pd, τα διμεταλλικά ΝΣ Ag-Zr αποκτούν επίπεδες δομές για χαμηλή ή μέση περιεκτικότητα σε Ag και μεγάλου μήκους δεσμοί Ag-Zr που σχηματίζονται έχουν ως αποτέλεσμα την τοποθέτηση των ατόμων του Ag σε περιφερειακές θέσεις. Η ηλεκτρονιακή δομή των ΝΣ Zr και Ag-Zr είναι παρόμοια με αυτήν των ΝΣ Pd και Ag-Pd. Δευτερεύον σκέλος της μελέτης μας αφορούσε την καταλληλότητα διαφόρων δυναμικών ανταλλαγής-συσχετισμού για τέτοιου τύπου υπολογισμούς. Έτσι χρησιμοποιήσαμε συνολικά έξι δυναμικά από τρεις τύπους προσεγγίσεων: το SVWN5 από την LSDA, τα BPW91, PBE και BLYP από την GGA, καθώς και τα υβριδικά δυναμικά B3LYP και PBE0. Τα αποτελέσματα έδειξαν ότι το LSDA δυναμικό είναι τελείως ακατάλληλο και ότι τα καλύτερα αποτελέσματα δίνονται από τα GGA και υβριδικά δυναμικά, ειδικότερα τα BPW91, PBE και PBE0

Behavior of sodium borosilicate glasses under compression using molecular dynamics

International audienceWe have performed classical molecular dynamics simulations in order to study the changes under compression in the local and medium range structural properties of three sodium borosilicate glasses with varying sodium content. These glasses have been isostatically compressed up to 20 GPa and then decompressed in order to analyze the different mechanisms that affect densification, alongside with the permanent modifications of the structure after a full compression/decompression cycle. The results show that the atomic packing is the prominent characteristic that governs the amount of densification in the glass, as well as the setup of the permanent densification. During compression, the bulk modulus increases linearly up to approximately 15 GPa and more rapidly for higher pressures, a behavior which is reflected on the rate of increase of the average coordination for B and Na. Radial distribution functions at different pressures during the cycle help to quantify the amount of distortions in the elementary structural units, with a pronounced shortening of the Na–Na and Na–O bond lengths during compression. A subsequent decomposition of the glassy matrix into elementary Voronoi volumes verifies the high compressibility of Na-rich regions

Nanoindentation of the pristine and irradiated forms of a sodium borosilicate glass: Insights from molecular dynamics simulations

We have carried out classical molecular dynamics simulations in order to get insight into the atomistic mechanisms of the deformation during nanoindentation of the pristine and irradiated forms of a sodium borosilicate glass. In terms of the glass hardness, we have found that the primary factor affecting the decrease of hardness after irradiation is depolymerization rather than free volume, and we argue that this is a general trend applicable to other borosilicate glasses with similar compositions. We have analyzed the changes of the short- and medium-range structures under deformation and found that the creation of oxygen triclusters is an important mechanism in order to describe the deformation of highly polymerized borosilicate glasses and is essential in the understanding of the folding of large rings under stress. We have equally found that the less polymerized glasses present a higher amount of relative densification, while the analysis of bond-breaking during the nanoindentation has showed that shear flow is more likely to appear around sodium atoms. The results provided in this study can be proven to be useful in the interpretation of experimental results

Deformation of sodium borosilicate glasses under load using Molecular Dynamics simulations

International audienceIn this study we present the results of nanoindentation and isostatic compression simulations of sodium borosilicate glasses using Molecular Dynamics, alongside experimental results obtained by Raman spectroscopy. For nanoindentation, three glasses with varying sodium content were simulated both in their pristine form, as well as a “disordered” one, which is analogous to the real irradiated glass and exhibits a swollen and more depolymerized network. The results indicate a very low increase of coordination of the silicon atoms, in contrast to boron and sodium. Densification occurs by the decrease of free volume and takes place in regions rich in glass formers. On the contrary, we observed evidence of high shear flow around sodium atoms. In the compression simulations, the pristine forms of the glasses were brought to a pressure of 20 GPa and then decompressed again. We found that the amount of free volume has a major influence on the amount of densification, alongside the percentage of three-coordinated boron. At the same time, sodium creates soft regions in the glass that can be more easily compressed. The results are then compared to Raman spectra collected on the imprint marks of experimentally indented borosilicate glasses

Raman spectra of binary sodo-silicate glasses from first principles calculations

International audienceSilicate glasses possess a central role in glass technology due to their multiple applications ranging from optical devices to the immobilization of nuclear waste. Raman spectroscopy is a key method for the investigation of their structure and the study of their properties. However, the inherent structural disorder found in glasses results in very broad and overlapping peaks in their spectra. In this context, an accurate theoretical modeling of the spectra can be proven to be invaluable in order to optimize their performance and tailor their fabrication method to match requirements for future applications. In this work, we present results on binary sodo-silicate glasses [xNa2O- (1-x)SiO2], which have been prepared by combining classical and ab-initio Molecular Dynamics. The Raman spectra (polarized and unpolarized), as well as the IR spectra, have been obtained using the Quantum Espresso package [1] in the GGA framework by employing norm-conserving pseudopotentials. Calibration runs have been carried out on the crystalline α-quartz and Na2SiO3 systems in order to optimize the choice of pseudopotentials. Concerning the glasses under study, we focused on the effect of local structural units, such as Q-species and their interconnection, alongside the role of sodium atom content in order to assign the corresponding bands. The initial results show very good agreement with previous theoretical and experimental spectra [2-5], while the ongoing decomposition relative to the exact contributions from each of the structural units of the glasses is expected to provide insight to the interpretation of the experimental spectra.References[1] P. Gianozzi et al., J. Phys.: Condens. Matter 21, 395502 (2009).[2] S. Ispas, N. Zotov, S. de Wispelaere, W. Kob, J. Non-Cryst. Solids 351, 1144 (2005).[3] N. Zotov, H. Keppler, Phys. Chem. Minerals 25, 259 (1998).[4] B. Hehlen, D.R. Neuville, J. Phys. Chem. B 119, 4093 (2015).[5] B.O. Mysen, J.D. Frantz, Contrib. Mineral. Petrol. 117, 1 (1994)

Raman spectra of simple sodo-silicate glasses from first principles

International audienceSilicates possess a central role in glass technology due to their multiple applications ranging from optical devices to the immobilization of nuclear waste. In this context, an accurate theoretical modeling of their spectra can be proven to be invaluable in order to optimize their performance and tailor their fabrication method to match requirements for future applications. In this work, we present results on simulated Raman spectra of simple sodo-silicate glasses, which have been prepared by combining classical and Car-Parrinello Molecular Dynamics. We focus on the effect of local structural units, such as SiO4 tetrahedra and their interconnection, alongside the role of sodium atom content in order to assign the corresponding bands. The obtained information is then used in order to help interpret the experimental spectra obtained for more complex sodium-borosilicate glasses

Uniaxial compression of silicon nanoparticles: an atomistic study on the shape and size effects

International audienceMolecular dynamics simulations were carried out to investigate the mechanical properties of silicon nanoparticles during uniaxial compression by a flat-punch indenter. We considered a large set of systems, with dimensions in the range 10 nm to 50 nm, and various shapes like cubic (perfect and blunt), spherical, truncated spherical, and Wulff-shaped, as well as two compression orientations and two temperatures. Thorough analyses of the simulations first revealed that the relation between nanoparticle size and strength, usually termed as 'smaller is stronger', is critically dependent on the nanoparticle shape, at least for the investigated size range. For instance, a significant and size-dependent strength decrease is determined for facetted Wulff-like nanoparticles, but not for cubic or spherical systems for compression along . We also found that the nanoparticle shape greatly influences plasticity. Several original plasticity mechanisms are obtained, among which the nucleation of half-loop V-shaped dislocation contained in two different {111} planes, dislocations gliding in unusual {110} planes, or the nucleation of partial dislocations in shuffle {111} planes. Our investigations suggest that plasticity properties are mainly governed by the localization of shear stress build up during elastic loading, and the geometry of surfaces in contact with indenters, these two characteristics being intimately related to the nanoparticle shape