Atomic structure of beryllium boroaluminate glasses: A multi-nuclear NMR spectroscopic study

The short-range structure and network speciation have been studied in a series of beryllium boroaluminate glasses using ¹¹B, ²⁷Al and ⁹Be NMR spectroscopy. All glasses are characterized by a coexistence of BeO₄, BO₃, BO₄, AlO₄, AlO₅ and AlO₆ species. The concentrations of BO₃, AlO₅ and AlO₆ species in these glasses are significantly higher and the geometry of the B-O and Al-O coordination polyhedra are unusually disordered compared to those in other alkali and alkaline-earth boroaluminate glasses reported previously in the literature. These results indicate that Be atoms may not play the typical role of a network-modifying cation in these glasses. This structural scenario is consistent with the highest field strength and electronegativity of Be among all alkali and alkaline-earth metals.     

A MAS NMR structural study of cadmium phosphate glasses

Cadmium phosphate glasses, of general formula xCdO(1−x)P₂O₅ (0.25?x?0.6), have been prepared by melting in alumina crucibles, with resulting dissolution of up to 6.4 mol% Al₂O₃. The local structure in these glasses has been studied using ³¹P, ²⁷Al and ¹¹³Cd nuclear magnetic resonance. The distribution of [PO₄]Qⁿ species as a function of composition has been shown to follow the simple binary model. The rate of change of the chemical shift of the ³¹P species in the Q² environment depends on the bond order, which in turn reflects the extent of double bonding between phosphorus and oxygen.     

Raman spectroscopic investigation of the structure of silicate glasses (II). Soda-alkaline earth-alumina ternary and quaternary glasses

Raman spectra of some ternary and quaternary glasses in the system Na₂OCaOMgOAl₂O₃SiO₂ are presented. The spectra are interpreted in terms of the structural alteration of the glass as the composition is altered from the binary end members to more complicated glasses. Addition of CaO and MgO to soda-silica glasses act only to increase the disorder of the network slightly. Addition of Al₂O₃ greatly modifies the network. In some soda-lime-aluminosiliscate compositions an estimate can be made of the amount of aluminum in four- and six-fold coordination. It is shown that the amounts of four- and sixfold coordinated aluminum depend on the glass composition.     

Deep-UV Raman spectroscopic analysis of structure and dissolution rates of silica-rich sodium borosilicate glasses

As part of ongoing studies to evaluate relationships between structure and rates of dissolution of silicate glasses in aqueous media, sodium borosilicate glasses of composition Na₂O·xB₂O₃·(3 − x)SiO₂, with x ≤ 1 (Na₂O/B₂O₃ ratio ≥ 1), were analyzed using deep-UV Raman spectroscopy. Results were quantified in terms of the fraction of SiO₄ tetrahedra with one non-bridging oxygen (Q³) and then correlated with Na₂O and B₂O₃ content. The Q³ fraction was found to increase with increasing Na₂O content, in agreement with studies on related glasses, and, as long as the value of x was not too high, this contributed to higher rates of dissolution in single pass flow-through testing. In contrast, dissolution rates were less strongly determined by the Q³ fraction when the value of x was near unity, and appeared to grow larger upon further reduction of the Q³ fraction. Results were interpreted to indicate the increasingly important role of network hydrolysis in the glass dissolution mechanism as the BO₄ tetrahedron replaces the Q³ unit as the charge-compensating structure for Na⁺ ions. Finally, the use of deep-UV Raman spectroscopy was found to be advantageous in studying finely powdered glasses in cases where visible Raman spectroscopy suffered from weak Raman scattering and fluorescence interference.     

Raman study of the structure of alkali germanosilicate glasses (I): Sodium and potassium metagermanosilicate glasses

Polarized and depolarized Raman spectra of sodium and potassium metagermanosilicate glasses are given. It is concluded from the Raman spectra that the structure of these glasses consists of metagermanosilicate chains in which both SiO₄ and GeO₄ tetrahedra are present, probably mixed in a random way.     

Raman spectroscopic investigations of the effect of the doping metal on the structure of binary tellurium-oxide glasses

Raman studies over the range 10 to 1000 cm⁻¹ have been performed on binary tellurium-oxide glasses (1-x)TeO₂-xMO (M = Pb,Zn or Mg) prepared using a conventional melt technique. The intensities and positions of Raman bands observed in the range above 250 cm⁻¹ were found to depend both on the compound oxide and on the amount of doping. Indeed, the ratio of the intensity of the band around 680 cm⁻¹ (assigned to vibrations of TeO₄ trigonal bipyramids) with respect to that of the component around 750 cm⁻¹ (related to stretching-vibrations in TeO₃₊₁, TeO₃ and MO groups) are affected in different ways for the glass-modifier MgO and for the intermediate glass-formers PbO and ZnO. Concurrently, an explicit dependence on the compound oxide and amount of doping was also observed on the maximum of the boson peak (BP) in the low-frequency region around 40 cm⁻¹. The structural correlation lengths in the glasses, calculated using the model described by Shuker and Gammon, were found to be about 0.50 nm (1-x)TeO₂-xMgO glasses and around 0.65 nm for (1-x)TeO₂-xMO (M = Pb or Zn) glasses. All these results are interpreted in terms of the effect of the metal oxide on the changes induced in the structural arrangements of ¹_χ[TeO₄-TeO₃] chains.     

Influence of zirconium on the structure of pristine and leached soda-lime borosilicate glasses: Towards a quantitative approach by O MQMAS NMR

¹⁷O MQMAS NMR was used to characterize the influence of zirconium on the structural organization of soda-lime borosilicate glasses. A new method of quantitative analysis of the ¹⁷O MQMAS spectra is presented, by a direct fit of the two-dimensional MQMAS spectrum which provides the resolution of all the structural groups in glasses containing up to five oxides. Additional data were also obtained from the quantitative deconvolution of the ¹¹B MAS NMR spectra, with the help of the direct fit of MQMAS data as well. Excess of non-bridging oxygen is clearly identified in these glasses. Sixfolded zirconium is preferentially compensated rather than the tetrahedral boron and calcium only partially compensate the tetrahedral boron. Alteration gels arising from glass leaching were probed by oxygen-17 supplied by the alteration solution. Most of the zirconium is inserted in the silicate network forming Si-O-Zr bonds with the same configuration in the glass and in the gel. During leaching, calcium clearly remains in the alteration gel, either near non-bridging oxygen or as a zirconium charge compensator. This quantitative approach applied to ¹⁷O MQMAS spectra demonstrates its potential for investigating the structure of increasingly complex glass and gel compositions.     

Raman study of the structure of alkali germanosilicate glasses II. Lithium,sodium and potassium digermanosilicate glasses

Polarized and depolarized Raman spectra of lithium, sodium and potassium digermanosilicate glasses of the composition 33 A₂O 67[(1 ? y)SiO₂, y GeO₂] and powder Raman spectra of crystalline Li₂O · 2 GeO₂ and K₂O · 2 GeO₂ are given. From the Raman spectra it is concluded that the structure of the glasses mainly consists of networks of SiO₄ and GeO₄ tetrahedra with one non-bridging oxygen atom, probably mixed in a random way. The structure is comparable to that of crystalline disilicates and digermanates.     

The structure of SiO2-GeO2 glasses: A spectroscopic study

Four glasses of the SiO₂-GeO₂ binary system have been synthesized via a sol-gel route followed by a heat treatment and a quench. Glass structure has been determined by Ge K-edge X-ray absorption spectroscopy (XAS) at low temperature and Raman spectroscopy. These mixed glasses present a continuous random network of interconnected GeO₄ and SiO₄ tetrahedra, with GeO₄ tetrahedra similar to the GeO₄ units in GeO₂ glass and continuous compositional variations from GeO₂-rich regions to SiO₂-rich regions. Such a random mixture is consistent with physical properties of these binary glasses as well as with the chemical dependence of their polyamorphism at high pressure. This EXAFS-derived mean Ge-O-Si angles are close to the Ge-O-Ge mean angle in GeO₂ glass, 134° and 130°, respectively. This misfit with the Si-O-Si angles might explain the ease of formation of isolated and pair defects centers, which are suspected to be at the origin of photo-induced modifications of optical properties in Ge-bearing SiO₂ glasses.     

Octahedral fluoride glasses: Raman spectra and structure of niobium pentafluoride

Raman spectroscopy is used to characterize the NbF₅ phases in the temperature range 80–500 K. A new clear glass is formed by quenching the melt to liquid nitrogen temperatures having a glass transition at ～206 K and crystallization at ～233 K. For all phases including the melt, the glass, the supercooled liquid, the crystalline solid and the gas, the Raman spectra show a rather common high frequency band at ～760 cm^?1 which is attributed to the Nb–F terminal frequency of partially bridged ‘NbF₆’ octahedra. Based on the systematics of the Raman spectra for all phases and the literature physicochemical data a model is proposed for the glass and the liquid phases where ‘NbF₆’ octahedral bridged in cis and/or trans configurations form a variety of cyclic and/or chain structures which intermix building up the overall structure. At exceptionally low energies (<11 cm^?1) a rather weak in intensity Boson peak is observed in the glass which shifts to even lower energies with increasing temperature. Librational and/or tortional motions of the bridged octahedra participating in the glass structure are possible candidates for the origin of this peak.     

Ageing and crystallization of non-crystalline Se thin films: A tentative structural model

The crystallization of various types of amorphous selenium films prepared under rigorous control was studied by DTA, optical microscopy, X-ray diffraction and magnetic measurements. The results, particularly the observation of ageing during several years, made it possible to correlate the preparation of the films, the nature of the condensed species and the evolution below and above T_g: all the results are consistent with the assumption that the constituent species are essentially cyclic $\text{Se}{}_{\mathrm{n}}\text{(n ? 8)}$ with a distribution of n depending on the preparation.     

A new model for the structure of chalcogenide glasses: The closed cluster model

A new model is proposed for the structure of low atomic coordination chalcogenide glasses. The closed clusters model is based on the various types of clusters, packed by van der Waals forces in a molecular-type packing. Arguments are given to support the new model, taken as example the typical chalcogenide glass As₂S₃.     

The structure of GeO2-SiO2 glasses and melts: A Raman spectroscopy study

We have investigated a series of glasses and melts along the GeO₂-SiO₂ join using insitu Raman spectroscopy. The results for both the glasses and melts are consistent with a continuous random network in which there are ‘regions’ that are SiO₂-like, GeO₂-like and mixed GeO₂-SiO₂-like. Incorporation of GeO₂ into the SiO₂ network is initially accommodated via the 3- and 4-membered SiO₄ rings which are lost as they convert to larger mixed Ge/Si rings. The LO-TO mode behavior is also consistent with a network that is composed of different ‘regions’ and is similar to that expected from the Bruggeman effective media model. At the highest temperatures there are indications that the mixed Ge/Si rings convert back to small 3-membered GeO₄ rings and large SiO₄ rings; the small 3- and 4-membered SiO₄ rings are not reformed.     

A structural study of sol–gel and melt-quenched phosphate-based glasses

Phosphate-based glasses have recently attracted much interest as a new generation of biomaterials because of their ability to react and dissolve in the physiological environment and eventually to be replaced by regenerated hard or soft tissue. A series of phosphate-based glasses containing 45 mol% P₂O₅ and various amounts of CaO and Na₂O were synthesized by sol–gel and melt-quenching techniques. A comparison between the structure of the sol–gel glass and the structure of the analogous melt-quenched glasses has been undertaken. A broad-based characterization approach combining different techniques has been used to investigate the short-range structure of the glasses and the effect of adding modifier oxides to the network structure (conventional and high energy X-ray diffraction, infra-red spectroscopy, ³¹P solid state magic angle spinning NMR spectroscopy). Sol–gel and melt-quenched glasses appear to have a similar structure, showing similar Qⁿ distributions and atomic correlations.     

High resolution 29Si NMR study of the structure and devitrification of lead-silicate glasses

High speed magic-angle rotation of glass samples in the strong polarizing field of a superconducting magnet yields high-resolution ²⁹Si NMR spectra. Using this technique glasses of various composition, PbO·SiO₂, (PbO)₂·SiO₂, and (PbO)₄·SiO₂ were studied and the influence of thermal treatment followed. Crystallization of PbO·SiO₂ glass has been found to be a complex process leading to a structure identical with that of the mineral alamosite. The ²⁹Si NMR spectrum of crystalline alamosite consists of three lines in agreement with the structure determined by X-ray diffraction.     

Multinuclear NMR spectroscopic studies of structure and dynamics in hydrous NaAlSi3O8 and Ca0.5AlSi3O8 glasses

We studied the local structure and dynamics of dissolved water in hydrous aluminosilicate glasses with the compositions NaAlSi₃O₈·0.3H₂O, NaAlSi₃O₈·1.3H₂O and Ca_0.5AlSi₃O₈·1.3H₂O by using multinuclear NMR spectroscopy. Glasses were produced by melting glass powders with deuterium oxide under high pressure and the resulting ²H/(¹H + ²H) ratio of the hydrous glasses was larger than 80%. NMR spectra reveal clear evidence for the presence of OH groups as well as of H₂O molecules. The motion of these water species in the time range from some μs to some ms was studied by ¹H NMR relaxation rates and the temperature dependence of the ²H NMR spectra. The motion is faster in the Ca-bearing glass than in the Na-bearing glass. The results are compared with those from quasi-elastic neutron scattering.     

Surface structure and properties of NiZr model metallic glasses: A molecular dynamics simulation

A systematic study of the surface structure and properties of NiZr model metallic glasses is reported using atomistic simulations. It is found that at low temperatures below the glass transition temperatures, the surface retains the amorphous structure and the surface energy γ is significantly lower (∼50%) than that of the corresponding crystalline alloy constituents. The variation of alloy concentration has little effect on γ; but increase in cooling rate and annealing temperature can lead to large decrease in γ. At elevated temperatures, γ increases with temperature and surface melting occurs at a temperature about 30% below T_g. At all temperatures up to T_g, the surface remains atomically smooth.