Evidence of Ni-containing ordered domains in low-alkali borate glasses

A noteworthy inhomogeneous distribution of Ni in low-alkali borate glasses has been shown by Ni K-edge EXAFS (Extended X-ray Absorption Fine Structure). The first evidence of a multiple scattering feature in a glassy matrix indicates an extended intermediate range order extending up to 6 Å. The Ni-enriched domains consist of corner- and edge-sharing $\rm NiO_6$ octahedra, with three-dimensional connections. The presence of this unusual order in glasses is likely associated with the rigid borate groups present in low-alkali borate glasses

The environment of transition elements in oxide glasses

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Rediscovering ancient glass technologies through the examination of opacifier crystals

International audienceThe aim of the study is to understand how antimonate opacifying crystals were obtained throughout history. Two archaeological glass productions opacified with calcium and lead antimonates are studied in this paper, in order to rediscover ancient opaque glass technologies: Roman mosaic tesserae (1st cent. B.C.-4th cent. A.D.) and Nevers lampworking glass (18th cent. A.D.). The fine examination of crystalline phases and of the vitreous matrix is undertaken using various and complementary techniques. Results are compared with a modern reference production, for which the technological process is well known. We demonstrate that Ca-antimonate opacifiers in Roman mosaic tesserae, as well as in Nevers lampworking glass, were obtained by in situ crystallization. Nevertheless, Roman and Nevers glass would have undergone different firing processes. We propose that the addition of previously synthesized crystals or the use of "anime" could be the process used to obtain Pb-antimonate opacified glass, for both productions studied.We demonstrate that CaO, PbO and Sb2O3 concentrations in the bulk compositions and in the matrices, and their evolution with the crystallinity ratio, offer robust criteria for the distinction of the opacification process used. Also, the different crystalline structures help to provide information on the experimental condition

Thestructureof SiO2–GeO2glasses: Aspectroscopicstudy

International audienceFourglassesoftheSiO2–GeO2binarysystemhavebeensynthesizedviaasol–gel routefollowedbyaheattreatmentandaquench. GlassstructurehasbeendeterminedbyGeK-edgeX-rayabsorptionspectroscopy(XAS)atlowtemperatureandRamanspectroscopy. ThesemixedglassespresentacontinuousrandomnetworkofinterconnectedGeO4andSiO4tetrahedra,withGeO4tetrahedrasimilarto theGeO4unitsinGeO2glassandcontinuouscompositional variationsfromGeO2-richregionstoSiO2-richregions. Sucharandom mixtureisconsistent withphysical propertiesof thesebinaryglassesaswell aswiththechemical dependenceof theirpolyamorphism athighpressure.ThisEXAFS-derivedmeanGe–O–SianglesareclosetotheGe–O–GemeanangleinGeO2glass,13

Intermediate-range order in the silicate network glasses NaFexAl1−xSi2O6 (x=0, 0.5, 0.8,1): A neutron diffraction and empirical potential structure refinement modeling investigation

International audienceThe local structural environment, and the spatial distribution of iron and aluminum ions in sodosilicate glasses with composition NaFexAl1-xSi2O6 x=1, 0.8, 0.5, and 0 were studied by high-resolution neutron diffraction combined with structural modeling using the empirical potential structure refinement code. This work gives evidence of differences in the structural behavior of Al3+ and Fe3+, which are both often considered to act as network formers in charge-balanced compositions. The short-range environment and the structural role of the two cations are not composition dependent; hence, the structure of intermediate glasses can then be seen as a mixture of the structures of the two end members. All Al3+ is four coordinated for a distance d~4Al3+-O=1.760.01 Å. The high-resolution neutron data allow deciphering between two populations of Fe. The majority of Fe3+ is four coordinated d~4Fe3+-O=1.870.01 Å while the remaining Fe3+ and all Fe2+ 12% of total Fe are five coordinated d~5Fe-O=2.010.01 Å. Both AlO4 and FeO4 are randomly distributed and connected with the silicate network in which they share corners with SiO4 tetrahedra, in agreement with a network-forming role of those species. On the contrary FeO5 tends to form clusters and to share edges with each other. Five-coordinated Fe is interpreted as network modifier and it turns out that, even if this coordination number is rare in crystals, it is more common in glasses in which it can have a key role on physical properties

Structural role of molybdenum in nuclear glasses: an EXAFS study.

The Mo environment has been investigated in inactive nuclear glasses using extended X-ray absorption spectroscopy (XAS). Mo is present in a tetrahedron coordinated to oxygen in the form of molybdate groups [MoO4]2− (d(Mo–O)=1.78 Å). This surrounding is not affected by the presence of noble metal phases in the nuclear glass. Relying on the XAS results, on the bond-valence model and on molecular dynamics simulations of a simplified borosilicate model glass, we show that these groups are not directly linked to the borosilicate network but rather located within alkali and alkaline-earth rich domains in the glass. This specific location in the glass network is a way to understand the low solubility of Mo in glasses melted under oxidizing conditions. It also explains the possible phase separation of a yellow phase enriched in alkali molybdates in molten nuclear glasses or the nucleation of calcium molybdates during thermal aging of these glasses. Boron coordination changes in the molten and the glassy states may explain the difference in the composition of the crystalline molybdates, as they exert a direct influence on the activity of alkalis in borosilicate glasses and melts

Comparative effects of thermal quenching and ballistic collisions in SiO2-B2O3-Na2O glass

International audienceThe differences between the effects of irradiation and thermal quenching were studied in SiO2-B2O3- Na2O glass by molecular dynamics. Classical molecular dynamics simulations reproduced quenching rates between 2 1012 K/s and instantaneous quenching. Each configuration was then subjected to a series of displacement cascades. An acceleration of the quenching rate and the accumulation of ballistic collisions result in qualitatively similar effects: swelling, increasing disorder, depolymerization of the glassy network, a decrease in the mean coordination number of B atoms. Nevertheless, ballistic processes induce specific effects that are not reproduced by accelerated quenching: an even larger degree of disorder, higher potential energy, greater depolymerization around B sites and different free volume distribution. The memory of the initial quenching rate is largely lost following an accumulation of displacement cascades, and the structure of the irradiated glasses adopts a final configuration weakly dependant on the initial state. This study shows that comparing the structure of the initial glass prepared at different quenching rates with the structure of irradiated glasses makes it possible to discriminate between thermal quenching and ballistic effects