Expert Meeting on Structure and Vibrations in Oxide Glasses

Structure of Oxide Glasses through the First Principles Simulation of Vibrational Spectra Alfredo Pasquarello CSEA-ITP-SB-EPFL, Lausanne, Switzerland Using a density-functional framework, we investigate the vibrational spectra of disordered oxides to determine to what extent these spectra provide information about their structure. Our approach benefits from a recently developed technique which allows one to apply finite electric fields in periodic density-functional calculations [1]. Through this approach, it becomes possible to calculate the otherwise computationally expensive coupling factors which are required to access infrared, Raman, and hyper-Raman spectra. We illustrate this scheme through applications on disordered oxides, such as SiO2 [2,3], GeO2 [4,5], and B2O3 [6]. We find that, among the vibrational spectra, the Raman spectra are most sensitive to mediumrange structural properties through their strong dependence on the O bond-angle distribution. This dependence allows us to estimate the concentrations of small ring structures in these glasses through comparisons between calculated and measured spectra [2-6]. [1] P. Umari and A. Pasquarello, Ab initio molecular dynamics in a finite homogeneous electric field. Phys. Rev. Lett. 89, 157602 (2002). [2] P. Umari, X. Gonze, and A. Pasquarello, Concentration of small ring structures in vitreous silica from a first-principles analysis of the Raman spectrum, Phys. Rev. Lett. 90, 027401 (2003). [3] L. Giacomazzi, P. Umari, and A. Pasquarello, Medium-range structure of vitreous SiO2 obtained through first-principles invesigation of vibrational spectra, Phys. Rev. B 79, 064202 (2009). [4] L. Giacomazzi, P. Umari, and A. Pasquarello, Medium-range structural properties in vitreous germania through the first-principles analysis of vibrational spectra, Phys. Rev. Lett. 95, 075505 (2005). [5] L. Giacomazzi, P. Umari, and A. Pasquarello, Vibrational spectra of vitreous germania from first principles, Phys. Rev. B 74, 155208 (2006). [6] P. Umari and A. Pasquarello, Fraction of boroxol rings in vitreous boron oxide from a firstprinciples analysis of Raman and NMR spectra, Phys. Rev. Lett. 95, 137401 (2005). Nature of Vibrational Modes in simple Silicate Glasses Simona Ispas Laboratoire Charles Coulomb, University Montpellier II and CNRS UMR 5221, F-34095 Montpellier, France The vibrational properties of silica glass have been intensively studied experimentally and theoretically during the last four decades. However, only few investigations have explored the evolution of the vibrational features of simple alkali glasses or silica under compression. In this contribution, we will present a mode analysis of two simple (Li/Na) alkali glass models obtained by combined classical and Car-Parrinello molecular dynamics. In particular we will discuss how the presence of the network modifier influences the relevant vibrational parameters: positions, shapes and intensities of the main peaks in the vibrational density of states (VDOS) and IR spectra. We will also show the decomposition of the VDOS on symmetry-adapted modes of the basic structural units of our models networks: SiO4 tetrahedra and SiOSi bridges. The same analysis will be carried out in the case of few silica glass models at normal density as well as at higher density generated by Car-Parrinello molecular dynamics simulations. Structure and mixing properties in soda and alkaline-earth silicate glasses and melts. Daniel R. Neuville Géochimie&Cosmochime, CNRS-Institut de Physique du Globe de Paris, Paris Sorbonne Cité 1 rue Jussieu, 75005 Paris Cedex 05, France Configurational entropy has been linked with the structure of Na2O-MO-SiO2 (M=Mg, Ca, Sr, Ba) glasses and melts, based on combined viscosimetry and Raman spectroscopic investigations. From viscosity measurements at low and high temperatures, we have obtained the configurational entropy, Sconf (using log h = Ae + Be/TSconf, where h is the viscosity, T the temperature and Ae, Be two constants). Using Raman spectroscopy, we obtained structural information from the Q speciation and from the variation of the boson peak with chemical composition. A rapid decrease in the viscosity at low temperature was observed in M-silicate melts with addition of Na2O. At high temperature, the viscosity is almost the same for the M-and Nasilicate liquids. The configurational entropy calculated from the viscosity measurements for M/Na mixing shows a non-ideal variation, which can be interpreted in term of non random distribution of Na and M in the silicate network. The addition of Na2O to the M silicate melts produces a decrease of the fragility of the liquid and an increase of the Q 3 /Q 2 ratio observed with Raman spectroscopy. Infrared spectroscopy of borate glasses in bulk and thin film forms E.I. Kamitsos Theoretical and Physical Chemistry Institute National Hellenic Research Foundation, 48 Vass. Constantinou Ave., 116 35 Athens, Greece Infrared spectroscopy is an effective tool for investigating the structure of glasses. This presentation reviews some applications for the characterization of alkali borate glasses in bulk and thin film forms. Infrared reflectance spectra of bulk glasses are analyzed to study the effect of alkali content and type on the short-range order of the borate network and on the interactions of alkali ions with their sites in glass. Infrared spectroscopy of thin films requires proper consideration of optical effects before it can reveal compositional and/or thermal history effects relative to bulk glasses having the same nominal composition with films. Vibrations and Glass Structure Bernard Hehlen Laboratoire Charles Coulomb, University Montpellier II and CNRS UMR 5221, F-34095 Montpellier, France The talk summarizes Hyper-Raman and Raman scattering experiments performed in simple oxide glasses. The first part will be devoted to the description of the vibrations in the glass formers SiO2 and B2O3. Here, the inelastic light scattering results are compared to literature IRabsorption data, and a “normal modes” analysis is performed using a simple structural model which considers a randomly connected network of well defined elementary structural units (SiO4 tetrahedra for silica, and BO3 triangles and B3O3 rings in boron oxide). Simple Ramanspectra analyses relating the vibrations to quantitative structural information will be describe in a second part. The structural quantities that can be accessed are for example, the concentration of small rings (permanently densified silica), the Si-O-Si angle (silica and silicates), and the role (modifier or compensator) in silica-based glasses. The SiO2-GeO2 and alkali-GeO2 binaries at ambient and high T Grant S Henderson Department of Geology, University of Toronto, Canada Raman spectra are extremely sensitive to changes in the structure of glasses, not just at the local short range scale but also at longer length scales such as the median range structure. However, the observed spectra are often unable to be fully interpreted due to a lack of understanding of the origin of vibrational features, as well as, an inability to quantify these features. Here I will discuss the vibrational spectra of glasses along the SiO2-GeO2 binary and the alkali-GeO2 binaries, with respect to what is observed at ambient and high T (including the melt phase), emphasizing the spectral changes we see but don’t fully understand due to our lack of knowledge noted above. Finally, I will briefly discuss some recent findings on Na2O-SiO2 glasses and their relevance to our understanding of glass structure theories such as the modified random network model. Disorder-induced vibrational anomalies and random-matrix statistics Walter Schirmacher Technische Universität München,Physik-Department E13 And Universität Mainz Institut für Physik,Staudinger Weg 7, D-55099 Mainz, Germany The enhancement of the Density of states of disordered solids with respect to the Debye expectation ("boson peak") is discussed in terms of symmetry arguments. It is pointed out that at low frequencies the vibrational spectrum consists of eigenstates, which are plane waves due to the global translational and rotational invariance of the material. Above a certain frequency the vibrational states become more and more affected by the fact that locally these symmetries do not hold. The strong degeneracy of the plane-wave states becomes lifted and the states exhibit level repulsion due to the random-matrix character of the dynamical matrix. The boson peak arises as a result of a crossover from the plane-wave regime to the randommatrix regime. This argument is independent of specific mechanisms and models of disorder. Quasi-Localized Vibrations : Boson Peak and Phonons Herbert Schober Forschungszentrum Juelich Gmbh, 52425 Juelich Quasi-localized modes provide a unified picture for tunneling, hopping diffusion and an excess in low frequency vibrations, leading to the boson peak. The quasi-localized vibrations can be pictured as local vibrations hybridized with the sound waves. The structure of the localized modes depends on the material. They can also be taken as fragments of low frequency optic vibrations. The soft potential model and its later extensions provides a tool to calculate with few experimental parameters the boson peak or the related damping of the phonons. It also makes predictions on the variation under changed external parameters such as pressure which have been verified experimentally. The underlying physical picture assumes a strong disorder as opposed to many q-space discussions where a mere distortion of crystalline-like phonons is assumed. Computer simulations of low-frequency modes in glasses: application to far-IR absorption S.R. Elliott 5Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom Dynamics of silica glass — are glasses more like crystals than we imagine? Martin Dove Department of Earth Sciences, University of Cambridge Downing Street, Cambridge CB2 3EQ We approach glasses and liquids from the perspective of our studies of crystalline materials, with a strong focus on network materials such as phases of silica, and more recently metalorganic framework structures. Our starting point was Mike Thorpe's and Jim Phillips's ideas of network rigidity/flexibility based on constraint counting. In the case of crystalline silica, the standard counting methods suggest that silica is exactly balanced between being overconstrained and under-constrained, yet because of crystalline symmetry the number of independent constraints is reduced and the balance is lost. This additional flexibility is responsible for phenomena such as phase transitions, negative thermal expansion, and zeolite flexibility. When we applied our tools to silica glass, we found more flexibility that the standard counting methods would suggest, even to the extent of some crystalline network structures. Moreover, neutron scattering experiments suggest close similarities in both structure and dynamics between amorphous silica and polycrystalline cristobalite. My talk with review these results, and I will suggest that the approach from crystalline materials can offer useful insights. Evidence from the Dynamic Structure Factor at the Boson Peak