MOLECULAR DYNAMICS OF SILICATE GLASS STRUCTURE

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Outline. Introduction - silicate glass structure Q-species, {Si}5 defectsAim of this study - application of MD in glass scienceExperimental - classical Molecular Dynamics simulationResults - Q-species with respect to- alkali concentration- type of alkali ionDiscussion
MOLECULAR DYNAMICS OF SILICATE GLASS STRUCTURE

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1. MOLECULAR DYNAMICS OF SILICATE GLASS STRUCTURE J. MACH?CEK1*, O. GEDEON1, M. LI?KA2 1Institute of Chemical Technology Prague; Prague, Czech Republic, 2Vitrum Laugaritio - Joint Glass Center of Institute of Inorganic Chemistry SAS, Alexander Dubcek University of Trenc?n and RONA Lednick? Rovne, Trenc?n, Slovak Republic, *contact author, e-mail: Jan.Machacek@vscht.cz Mr/Ms Chairman, ladies and gentlemen, I will speak about ?Molecular dynamics simulation of the silicate glass structure?. Mr/Ms Chairman, ladies and gentlemen, I will speak about ?Molecular dynamics simulation of the silicate glass structure?.

2. Outline Introduction - silicate glass structure Q-species, {Si}5 defects Aim of this study - application of MD in glass science Experimental - classical Molecular Dynamics simulation Results - Q-species with respect to - alkali concentration - type of alkali ion Discussion & Conclusion - is it useful? Outlook - quantum simulations - In the first part of my talk, I will try to introduce the silicate structure described by so called Q-species and Si5 defects - Thereafter, I will specify the goals of my work and the method used. - The rest of my lecture covers results and their discussion.- In the first part of my talk, I will try to introduce the silicate structure described by so called Q-species and Si5 defects - Thereafter, I will specify the goals of my work and the method used. - The rest of my lecture covers results and their discussion.

3. Introduction - silicate glass structure non-periodic ? scattering gives only 1D projection of particle density - RDF Atom ordering: SRO - crystal-like, basic building units LRO - visible by microscopy, phases MRO - inherent to the glassy state - hidden for exp. methods Near MRO in glass - can be studied by 29Si MAS-NMR - Q-species ? Qn; n = # of BO in SiO4 tetrahedron - The silicate glass structure is non-periodic therefore all scattering methods give only a one-dimensional projection of the particle density which is called, Radial Distribution Function. - The structure is usually divided into three regions according to a scale of observation. The short range order medium and long range order. - The medium range order is specific for the glassy state and is miserably accessible by present-day experimental methods. - The near MRO can be in principle studied by the solid state NMR. NMR is able to distinguish silicon atoms according to the number of bridging oxygens bound to them. Hereafter, such units will be called Q-species.- The silicate glass structure is non-periodic therefore all scattering methods give only a one-dimensional projection of the particle density which is called, Radial Distribution Function. - The structure is usually divided into three regions according to a scale of observation. The short range order medium and long range order. - The medium range order is specific for the glassy state and is miserably accessible by present-day experimental methods. - The near MRO can be in principle studied by the solid state NMR. NMR is able to distinguish silicon atoms according to the number of bridging oxygens bound to them. Hereafter, such units will be called Q-species.

4. Introduction - Q-species reflect: - connectivity of silicate network - distribution of alkalis in silicate structure fractions of Qn units deviates from binary model -supposes only two units are present in a system disproportionation (speciation) of Qn units was observed by 29Si MAS-NMR and Raman: 2Q3 ? Q4 + Q2 distribution of Qn units depends on: - alkali modifier concentration (xM2O? : NBO? Q4? ) - type of alkali modifier (from Cs to Li : Q3? Q4 ? Q2 ? ) - cooling rate of glass ( cooling rate ? : Q3? ) Q-species are very useful because they reflect connectivity of the silicate network and help to understand the distribution of modifiers in silicate glasses. - It was experimentally observed that the fractions of Qn units deviates from simple binary model which supposes only two units present in glass. - It was deduced that Q-species disproportionate according to the equation: 2Q3 = Q2 + Q4 - The Qn distribution changes with alkali modifier concentration. - The speciation increases from Cs to Li. Rapid cooling decreases the fraction of Q3.Q-species are very useful because they reflect connectivity of the silicate network and help to understand the distribution of modifiers in silicate glasses. - It was experimentally observed that the fractions of Qn units deviates from simple binary model which supposes only two units present in glass. - It was deduced that Q-species disproportionate according to the equation: 2Q3 = Q2 + Q4 - The Qn distribution changes with alkali modifier concentration. - The speciation increases from Cs to Li. Rapid cooling decreases the fraction of Q3.

5. Introduction - {Si}5 defects SiO5 detected by 29Si MAS-NMR /Stebbins 1991/ glass prepared under unusual conditions (high pressure, rapid cooling rate) more in tetrasilicates (mainly in K2O?4SiO2) not confirmed by other methods in glass (but found in some crystalline silicates) play a role in migration of oxygen atoms in silicate melts? /Farnan 1998/ - intermediate of reaction, formally charge (-1), trigonal bi-pyramid common in MD simulated silicate glasses (if only pair potentials used, no three-body (bond-angle) potentials) Excepting Q-species, the silicate glass may contain also defects Si5 which were firstly identified by the solid state NMR. - Such defects were found in glass prepared under high pressure and/or rapid cooling rate. - It was suggested that this defects play a role in transport processes at high temperature. - Si5?s are abundant in rapidly cooled MD glass.Excepting Q-species, the silicate glass may contain also defects Si5 which were firstly identified by the solid state NMR. - Such defects were found in glass prepared under high pressure and/or rapid cooling rate. - It was suggested that this defects play a role in transport processes at high temperature. - Si5?s are abundant in rapidly cooled MD glass.

6. Questions addressed by this study Can MD simulation provide Qn-distribution comparable with real experiments? What if different? Can MD simulation provide a deeper insight into the silicate glass structure? Meaning of the speciation? How can {Si}5 defects contribute to the better understanding of the silicate melt processes? {Si}5, the mere drawbacks of a MD simulation? The aim of our work can be expressed in these three questions: - Can be the MD simulated Qn-distribution compared with real experiments? - What new can MD simulation say about the silicate structure? - How can be handled the silicate melt by the {Si}5 defects at all?The aim of our work can be expressed in these three questions: - Can be the MD simulated Qn-distribution compared with real experiments? - What new can MD simulation say about the silicate structure? - How can be handled the silicate melt by the {Si}5 defects at all?

7. Experimental - Molecular Dynamics computer simulation of atoms - classical (non-quantum) interactions, Newtonian eq. of motion, parameters of ionic potentials /Beest 1991/ glassy systems xNa2O?(1-x)SiO2 and M2O?2SiO2 x ? ? 0; 0.8 ?, M ? {Li, Na, K, Rb, Cs} 1500 atoms, density of experimental glasses DL_POLY software, cubic box, , constant volume calc., periodic boundary conditions, SPM Ewald, leap-frog, time step 2 fs, short range cut-off 7.6 ?, ... MD glass prepared by simulated cooling of very hot system of atoms at 5000 K down to 300 K. Cut-off criterion for BO (Qn) determination 2.3 ? - Two glassy systems were investigated: a binary sodium-silicate system and all alkali-disilicates. - Our computer experiment were carried out with help of Molecular Dynamics simulation. - This method allows us to imitate the cooling process on the atomic scale. - Two glassy systems were investigated: a binary sodium-silicate system and all alkali-disilicates. - Our computer experiment were carried out with help of Molecular Dynamics simulation. - This method allows us to imitate the cooling process on the atomic scale.

8. Results - y(Qn) on x(M2O) Distribution of Q-units in xNa2O?(1-x)SiO2 glass: RBM model (solid line); binary model (dashed line); MD simulation (filled symbols); MAS-NMR (open symbols). In this figure, there can be seen the dependence of Q-species fractions on concentration of sodium oxide. - MD simulated and experimental results are shown together with two theoretical models. - The speciation of Q3 units is clearly visible but is very high for MD simulation. It approaches the model of randomly connected Qn units. In this figure, there can be seen the dependence of Q-species fractions on concentration of sodium oxide. - MD simulated and experimental results are shown together with two theoretical models. - The speciation of Q3 units is clearly visible but is very high for MD simulation. It approaches the model of randomly connected Qn units.

9. Results - y(Qn) on x(M2O) Fraction of {Si}5 defects with respect to % of sodium oxide in MD simulated Na2O-SiO2 glass. In this figure, you can see the dependence of Si5 defects on concentration of sodium oxide. - These defects exist only on the silica-rich tail of the concentration interval with the maximum at the tetrasilicate composition. In this figure, you can see the dependence of Si5 defects on concentration of sodium oxide. - These defects exist only on the silica-rich tail of the concentration interval with the maximum at the tetrasilicate composition.

10. Results - y(Qn) on type of alkali Fraction of Q3 units in binary alkali disilicate MD glasses (solid line) and real glasses (dashed line). MD results are shifted by 34 %. Average values from 4 configurations. In this figure, there is the dependence of Q3 units on a type of alkali oxide. - MD results reproduces the trend of experimental data except for a constant value of 34 %. In this figure, there is the dependence of Q3 units on a type of alkali oxide. - MD results reproduces the trend of experimental data except for a constant value of 34 %.

11. Results - y(Qn) on type of alkali Fraction of {Si}5 defects in binary alkali disilicate MD glasses. Average values from 4 configurations. In this figure, you can see the dependence of Si5 defects on a type of alkali oxide. - The fraction of defects is about ten times higher than in real glasses. - These defects show maximum at the potassium disilicate.In this figure, you can see the dependence of Si5 defects on a type of alkali oxide. - The fraction of defects is about ten times higher than in real glasses. - These defects show maximum at the potassium disilicate.

12. Discussion and Conclusion Qn-distribution with respect to alkali concentration - much higher speciation in MD glass ? rapid cooling ? high fictive temperature ? high configurational entropy ? fits random bonding model (RBM) very well Fraction of {Si}5 with respect to alkali concentration - a bit higher than in real glasses ? rapid cooling ? insufficient relaxation of the structure ? frozen intermediates - maximum near tetrasilicate composition ? confirms experimental findings ? {Si}5 vanishes in a depolymer. structure ? strong indication of a transport role in the compact silicate structure at high temperature. - The Qn distribution in MD glass reveals much higher speciation than experiments do. This can be attributed to rapid cooling of a simulated glass. It results in the very high configurational entropy. Therefore, the random bonding model works very well for MD glass. - Rapid cooling also causes the higher fraction of Si5 defects. - The maximum of Si5 is very close to the tetrasilicate composition and it strongly confirms the hypothesis about a transporting role of Si5 defects. - The Qn distribution in MD glass reveals much higher speciation than experiments do. This can be attributed to rapid cooling of a simulated glass. It results in the very high configurational entropy. Therefore, the random bonding model works very well for MD glass. - Rapid cooling also causes the higher fraction of Si5 defects. - The maximum of Si5 is very close to the tetrasilicate composition and it strongly confirms the hypothesis about a transporting role of Si5 defects.

13. Discussion and Conclusion Qn-distribution with respect to type of alkali - very high speciation in all alkali glasses ? rapid cooling - speciation increases from Cs to Li as in glasses ? caused by alkali field strength ? small cations catch NBO ? NBO is mainly on Q2 ? Q2 generated by the speciation 2Q3 ? Q2 + Q4 Fraction of {Si}5 with respect to type of alkali - maximum in potassium glass ? confirms experimental findings ? structural incompatibility of K-cation with the silicate network ? further analyses are needed - A very high speciation was found among all simulated alkali disilicates. Again, this can be attributed to the rapid cooling. - The speciation increases from Cs to Li which is in agreement with experiment. A driving force for the speciation may be seen in high field strength of small alkalis. They prefer bonding to more charged NBO?s. - Maximum number of defects is present in the potassium system in agreement with experiment. It seems that potassium cation demonstrates some kind of structural incompatibility towards silicate network. - A very high speciation was found among all simulated alkali disilicates. Again, this can be attributed to the rapid cooling. - The speciation increases from Cs to Li which is in agreement with experiment. A driving force for the speciation may be seen in high field strength of small alkalis. They prefer bonding to more charged NBO?s. - Maximum number of defects is present in the potassium system in agreement with experiment. It seems that potassium cation demonstrates some kind of structural incompatibility towards silicate network.

14. Outlook Quantum MD simulations are coming + precise atomic forces + electronic structure - more demanding Thank you In near future, our group is going to simulate the glass structure by quantum methods offering more precise calculations of atomic forces and the electronic structure. In the figure, you can see our first result, the oxynitride melt. Thank you for your attention. In near future, our group is going to simulate the glass structure by quantum methods offering more precise calculations of atomic forces and the electronic structure. In the figure, you can see our first result, the oxynitride melt. Thank you for your attention.


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