Bernard Piriou

A Raman spectroscopic study of glasses along the joins silica-calcium aluminate, silica-sodium aluminate, and silica-potassium aluminate

Aluminosilicate glasses with compositions along the joins silica-calcium aluminate, silica sodium aluminate and silica-potassium aluminate have been prepared by conventional and solar melting techniques and studied by Raman spectroscopy. The Raman spectra of crystalline calcium aluminate, anorthite and silica polymorphs are discussed in relation to their crystal structures, and compared with the spectra of the corresponding glasses. The glass and crystal spectra are generally comparable, suggesting similar vibrational structures. These crystals have structures based on tetrahedral aluminosilicate frameworks, and a similar molecular structure is suggested for the glasses, although it is noted that the Raman spectra do not directly characterize the aluminate polyhedra. Within the three glass series, our interpretation of the unresolved high-frequency bands shows the appearance of discrete bands near 1120, 1000, 930 and 890 cm−1 as the silica content is decreased. This is compared with the behaviour of high-frequency bands in simple silicate systems, and used to suggest that the four bands in the aluminosilicate systems are due to stretching vibrations of silicate tetrahedra bound to one, two, three and four aluminium atoms. The spectra of calcium, sodium, potassium and lithium aluminosilicate glasses with similar silica contents are compared, and interpreted by the above model. This is used to construct a simple model for the effect of metal cation on aluminosilicate molecular groups in the glass structure, consistent with the results of calorimetric studies on similar systems.

The structures and vibrational spectra of crystals and glasses in the silica-alumina system

Solar furnace melting and fast-quench techniques have been used to prepare SiO2ue5f8Al2O3 glasses to high alumina content (near 60 mol% Al2O3), which have been studied by Raman spectroscopy. These spectra may not be simply interpreted. The structures of crystalline compounds in the SiO2ue5f8Al2O3 system are discussed in relation to their vibrational spectra. On the basis of this discussion and other considerations, a structural model for the silica-alumina glass system is proposed, which is consistent with the stable or metastable immiscibility suggested along this join. The essential features of this model include a modified silica structure at low alumina content, and “structure-broken” regions at high alumina compositions, with silicon in tetrahedral coordination, but aluminium assuming a variety of bonding geometries. These are proposed to include aluminate tetrahedra with higher polymerization than simple corner-sharing, and less well-defined polyhedra of higher average coordination number.

Raman spectroscopy of calcium aluminate glasses and crystals

Solar furnace melting and fast-quench techniques have been used to prepare calcium aluminate glasses from 75 mol% CaO to 82 mol% Al2O3, which have been studied by Raman spectroscopy. The CaAl2O4 glass spectrum may be interpreted in terms of a fully-polymerized network of tetrahedral aluminate units, which is depolymerized on addition of CaO component analogous to binary silicate systems. The spectra of glasses with higher alumina content than CaAl2O4 may not be simply interpreted and a structural model is proposed which would be consistent with the glass spectra and with observed crystal structures along the CaAl2O4ue5f8Al2O3 join. This model suggests formation of highly condensed aluminate tetrahedral on initial addition of alumina, with the appearance of aluminate polyhedra of higher average coordination at higher alumina content. Similar high coordination polyhedral are also suggested for a limited composition range along the CaOue5f8CaAl2O4 join. These interpretations are compared with the results of a previous study in the SiO2ue5f8Al2O3 glass system.

A Raman study of pressure‐densified vitreous silica

We have obtained polarized Raman spectra for vitreous silica densified at 3.95 GPa and 530u2009°C. The glass spectra show considerable changes on densification, which are discussed in terms of the observed reduction in average intertetrahedral bond angle and the central force model for connected networks. The results of this densification experiment are compared with previous work at high pressure but room temperature. This comparison suggests a model for the densification mechanism, and indirectly supports recent structural interpretations of the Raman spectrum of fused silica.

DSC and Raman studies of lead borate and lead silicate glasses

Several compositions in the xPbO·(1−x)B2O3 (0.20 ≤ x ≤ 0.65) and xPbO·(1−x)SiO2 (0.39 ≤ x ≤ 0.66) vitreous systems were examined by differential scanning calorimetry and by Raman scattering for the former system. Borate glasses with low PbO concentrations are built up in two and three dimensions with three- and four-coordinated boron atoms; Pb2+ acts as a charge balance in the covalent network. Glasses with high PbO content contain ring- and chain-metaborates and PbO4 entities. Differences in Raman scattering of alkali borate glasses are observed; in particular the boroxol rings still exist for x = 0.27. The heat capacities of the glasses at 200°C are an additive function of composition within the accuracy of the differential scanning calorimetry measurements. The heat capacity differences between the vitreous and supercooled liquid states are higher for lead borates than for lead silicates, indicating more profound changes in the lead borates during the glass transition. In the former glasses, the fictive temperature attains a maximum around 0.27PbO corresponding to the highest degree of polymerization. In lead silicates, the fictive temperature decreases continually upon adding PbO, and parallels the depolymerization of the silicate network.

Yttrium SiAlON glasses: structure and mechanical properties — elasticity and viscosity

Abstract Two series of oxynitride glasses in the YSiAlON system, with varying ratios of Al Si and Al Y , were studied from both structural and mechanical point of view. A Raman scattering study showed that by changing the Al; content, at constant Y cationic equivalent concentration (e/o), aluminum substitutes for silicon without affecting the degree of polymerisation (cross-linking) of the glass network, whereas at constant Si e/o, replacement of yttrium by aluminum results in a higher polymerisation degree. The effect of glass composition on the hardness, Youngs modulus, the thermal expansion coefficient and the glass transition temperature range are discussed in light of the corresponding structural changes.

Raman spectroscopic studies of fluorophosphate glasses

Some fluorophosphate glasses obtained by fluorination of NaPO3, xNaPO3−yAlF3 or tNaPO3−wAlF3−vCaF2 have been studied by Raman spectroscopy in VV and VH polarisations. The observed bands have been assigned by comparison with those vitreous NaPO3 and Na4P2O7, as well as crystallised K2PO3F and KPO2F2, and molten cryolite. Structural models are proposed for the various compositions. They are built up with arrangements of following species: (PO3)2n−n chains; (P2O7)4− dimers, (AlF6)3− octahedra, P(O, F)4, (AlF4)− and (AlOF3)2− tetrahedral groups. For glasses rich in phosphate −OPO2F− and −OPOF2 terminal groups are displayed; the latter being favoured for a higher fluorine concentration.

Structural studies in lead germanate glasses: EXAFS and vibrational spectroscopy

The Raman, IR absorption and EXAFS spectra at the Ge K-edge and Pb LIII-edge of eight lead germanate glasses, with general formula xPbO(1−x)GeO2 with x = 0.20, 0.25, 0.33, 0.40, 0.50, 0.53, 0.56 and 0.60, have been measured. The occurrence of [GeO6] units besides [GeO4] could not be deduced unambiguously from the data. The vibrational and EXAFS data agree with a progressive depolymerization of the network. Starting from all Ge atoms linked to four bridging oxygens in GeO2 (x = 0), the number of tetrahedral units with one or two non-bridging oxygens increases with x. At low content, Pb2+ ions act as modifiers in the germanate structure, but to a lesser extent than an equivalent number of alkaline ions.

Study of sodium silicate melt and glass by infrared reflectance spectroscopy

Abstract Infrared reflectance spectra and the derived absorption spectra of a Na 2 O-1.41 SiO 2 glass and of the parent melt are reported. Polymers with 2 bridging oxygens per silicon (chains) and 3 bridging oxygens (sheets) are identified. Spectra of glass and melt are qualitatively similar. Melt absorption bands show a 10–30 cm −1 shift to lower frequencies. They are broader and less intense than those of the glass which does not agree with Sweet and Whites reflectance measurements on similar glass and melt. The small differences between glass and melt can be attributed to the thermal expansion of the medium.

Investigation by raman scattering of the [TeO2−RMO0.5] (M = Ag or Tl) glasses and of the related ionic conductors [TeO2−RMO0.5]1−x)[AgI]x

Abstract The structure of binary glasses [TeO 2 − R MO 0.5 ], with M = Tl (or Ag) and 0.22 ⪯ R ⪯ 1 (0.28 ⪯ R ⪯ 1) has been investigated by Raman scattering under VV and VH polarizations. The two systems behave in nearly the same way: in the tellurite-rich part, the structure is built up of [TeO 4 ] trigonal pyramids, a disordered form of the paratellurite α-TeO 2 . With increasing R between 0.40 (0.35) and 1 (the upper R value limiting the vitreous domain in each system), an increasing proportion of tri-coordinated tellurium [TeO 3 ] is formed in the glasses. The ternary [TeO 2 − R MO 0.5 ] (1− x ) [ AgI ] x glasses have been reported as ionic conductors. The structure of the tellurite network is not modified by the addition of silver iodide. A band appearing in the low-energy part of the Raman spectra is assigned to vibrations of [AgI 4 ] species.