D. de Ligny

Environments around Al, Si, and Ca in aluminate and aluminosilicate melts by X-ray absorption spectroscopy at high temperature

Abstract Structural data on silicate, aluminate, and aluminosilicate melts are difficult to measure and understand at high temperature. X-ray absorption spectroscopy (XAS) performed in situ at high temperature has been used to probe the local environment of low-Z elements (Al, Si, and Ca). For fully tetrahedral network glasses, CaAl2Si2O8 (anorthite) and CaAl2O4, the modifications in the Al K-edge spectra with increasing temperature can be attributed to a structural rearrangement of the network or to an increase of fivefold-coordinated Al. For the Ca3Al2O6 composition, where Al is localized in a depolymerized tetrahedral site associated with non-bridging O atoms, XAS spectra at the Al K-edge are barely affected by temperature. Depending on the composition, Ca K-edge spectra investigated in these experiments allow us to follow changes in the distortion of the Ca sites in melts at high temperature. The structural modifications at both short and intermediate range upon melting are well shown by these XAS measurements.

Permanent densification of compressed silica glass: a Raman-density calibration curve

Raman scattering experiments have been carried out to study persistent densification in SiO(2) glass following hydrostatic compression at room temperature. A new relationship linking selective Raman parameters to the degree of densification in the glass has been developed here. This approach will allow quantification of the residual densification in silica following microindentation experiments, with the goal being the development of a constitutive law for amorphous silica.

Progressive transformations of silica glass upon densification.

The elastic and plastic behaviors of silica glasses densified at various maximum pressure reached (12 GPa, 15 GPa, 19 GPa, and 22 GPa), were analyzed using in situ Raman and Brillouin spectroscopies. The elastic anomaly was observed to progressively vanish up to a maximum pressure reached of 12 GPa, beyond which it is completely suppressed. Above the elastic anomaly the mechanical behavior of silica glass, as derived from Brillouin measurements, is interpreted in terms of pressure induced transformation of low density amorphous silica into high density amorphous silica.

Structural heterogeneity and pressure-relaxation in compressed borosilicate glasses by in situ small angle X-ray scattering

We report on Brillouin and in situ small angle X-ray scattering (SAXS) analyses of topological heterogeneity in compressed sodium borosilicate glasses. SAXS intensity extrapolated to very low angular regimes, I(q = 0), is related to compressibility. From Brillouin scattering and analyses of the elastic properties of the glass, the Landau-Placzek ratio is determined and taken as a direct reflection of the amplitude of frozen-in density fluctuations. It is demonstrated that with increasing fictive pressure, topological (mid- and long-range) homogeneity of the glass increases significantly. Heating and cooling as well as isothermal scans were performed to follow the evolution of density fluctuations upon pressure recovery. For a sample with a fictive pressure p(f) of 470 MPa, complete recovery to p(f) = 0.1 MPa was observed to occur close to the glass transition temperature. The values of fictive and apparent fictive temperature, respectively, as obtained via the intersection method from plots of I(q = 0) vs. temperature were found in good agreement with previous calorimetric analyses. Isothermal scans suggest that mid- and long-range recovery govern macroscopic density relaxation.

Correlation between boson peak and anomalous elastic behavior in GeO2 glass: An in situ Raman scattering study under high-pressure

We present low-frequency Raman scattering of pure GeO(2) glass under pressure up to 4 GPa, corresponding to an elastic transformation. Intensity variation and frequency shift of the boson peak are analysed and compared to the Debye model. The decrease of the boson peak intensity scaled by the Debye energy is correlated to the elastic anomalous properties under pressure up to 1.5 GPa, and interpreted as an elastic homogenisation process at the local scale. We emphasize similarities between a-GeO(2) and a-SiO(2) behavior under pressure, and compare our results to other experiments, numerical studies, and predictions of several models concerning amorphous systems.

In situ Brillouin study of sodium alumino silicate glasses under pressure.

The in situ elastic and plastic behaviors of sodium aluminosilicate glasses with different degrees of depolymerization were analyzed using Brillouin spectroscopy. The observed elastic anomaly progressively vanished with depolymerization. The densification process appears to be different from that observed in pure silica glass. In the plastic regime of densified glasses hysteresis loops were observed and related to modification of the local silicon environment facilitated by the addition of sodium.

Kinetics of iron redox reaction in silicate melts: A high temperature Xanes study on an alkali basalt

In Earth and Materials sciences, iron is the most important transition element. Glass and melt properties are strongly affected by iron content and redox state with the consequence that some properties (i.e. viscosity, heat capacity, crystallization...) depend not only on the amounts of Fe2+ and Fe3+, but also on the coordination state of these ions. In this work we investigate iron redox reactions through XANES experiments at the K-edge of iron. Using a high-temperature heating device, pre-edge of XANES spectra exhibits definite advantages to make in-situ measurements and to determine the evolution of redox state with time, temperature and composition of synthetic silicate melts. In this study, new kinetics measurements are presented for a basalt melt from the 31,000-BC eruption of the Puy de Lemptegy Volcano in France. These measurements have been made between 773 K and at superliquidus temperatures up to 1923 K.

Structure of spinel at high temperature using in-situ XANES study at the Al and Mg K-edge

We present structural information obtained on spinel at high temperature (298–2400 K) using in situ XANES at the Mg and Al K-edge. Spinel, [4](Alx,Mg1-x)[6](Al2-x,Mgx)O4, with increasing temperature, show a substitution of Mg by Al and Al by Mg in their respective sites. This substitution corresponds to an inversion of the Mg and Al sites. Furthermore, both experiments at the Al and Mg K-edges are in good agreement with XANES calculation made using FDMNES code.

The Silicon Environment in Silica Polymorphs, Aluminosilicate Crystals and Melts: An In Situ High Temperature XAS Study

High temperature X‐ray absorption spectroscopy at the Si K‐edge has been used to obtain in situ information on SiO2 phase transitions upon heating. Important modifications are observed for the XANES spectra of the high temperature polymorphs, in relation to disordering of the SiO4 tetrahedra beyond the short‐range correlations. This paper also presents the XANES spectra of anorthite (CaAl2Si2O8) from room temperature up to the melt (1900 K). This study shows the possibilities for determining the Si environment in crystals and glasses up to the liquid state using in situ XANES measurements.