Laurent Cormier

The structure of amorphous, crystalline and liquid?GeO2

Germanium dioxide (GeO2) is a chemical analogue of SiO2. Furthermore, it is also to some extent a structural analogue, as the low- and high-pressure short-range order (tetrahedral and octahedral) is the same. However, a number of differences exist. For example, the GeO2 phase diagram exhibits a smaller number of polymorphs, and all three GeO2 phases (crystalline, glass, liquid) have an increased sensitivity to pressure, undergoing pressure-induced changes at much lower pressures than their equivalent SiO2 analogues. In addition, differences exist in GeO2 glass in the medium-range order, resulting in the glass transition temperature of germania being much lower than for silica. This review highlights the structure of amorphous GeO2 by different experimental (e.g., Raman and NMR spectroscopy, neutron and x-ray diffraction) and theoretical methods (e.g., classical molecular dynamics, ab initio calculations). It also addresses the structures of liquid and crystalline GeO2, that have received much less attention. Furthermore, we compare and contrast the structures of GeO2 and SiO2, as well as along the GeO2–SiO2 join. It is probably a very timely review, as interest in this compound, that can be investigated in the liquid state at relatively low temperatures and pressures, continues to increase.

Amorphous materials: Properties, structure, and durability† Structure of Mg- and Mg/Ca aluminosilicate glasses: 27Al NMR and Raman spectroscopy investigations

Abstract The structure and properties of glasses and melts in the MgO-Al2O3-SiO2 (MAS) and CaO-MgOAl2O3- SiO2 (CMAS) systems play an important role in Earth and material sciences. Aluminum has a crucial influence in these systems, and its environment is still questioned. In this paper, we present new results using Raman spectroscopy and 27Al nuclear magnetic resonance on MAS and CMAS glasses. We propose an Al/Si tetrahedral distribution in the glass network in different Qn species for silicon and essentially in Q4 and VAl for aluminum. For the CMAS glasses, an increase of VAl and VIAl is clearly visible as a function of the increase of Mg/Ca ratio in the (Ca,Mg)3Al2Si3O12 (garnet) and (Ca,Mg)AlSi2O8 (anorthite) glass compositions. In the MAS system, the proportion of VAl and VIAl increases with decreasing SiO2 and, similarly with calcium aluminosilicate glasses, the maximum of VAl is located in the center of the ternary system.

Structure-property relationships in multicomponent oxide glasses

Cations play a complex structural role in oxide glasses, as they occur in different kinds of environments, which allow them to exert a contrasted influence on physical and chemical properties of these glasses. The combination of structural information given by a wide range of spectroscopic methods and by radiation scattering, combined with numerical modelling, has given insight on the structural organisation around these cations. Among these characteristic properties are unusually low-coordination numbers, such as 5-fold coordination, and the presence of extended ordered domains, in which cation polyhedra are edge- or corner-sharing. This review presents evidence for a structural control of several physical and chemical properties in oxide multicomponent glasses. The use of zinc as a stabilising glass component arises from its network-forming position, which implies the presence of low-charge cations in its surrounding and as a consequence decreases the concentration of modifier components. The compositional dependence of glass coloration by transition elements has been investigated thoroughly through the example of nickel in silicate and borate glasses. The wide range of coloration observed may be explained by the existence of three kinds of environments, with nickel occurring in 4-, 5-, and 6- coordination. The relationships of these sites with the medium-range organisation of the glasses have been understood by a combined use of EXAFS spectroscopy and neutron scattering with isotopic substitution. The two other examples that are presented to illustrate structure–property relationships concern the physical solubility of gazes in glasses and the alteration processes of glasses used as analogues of nuclear waste matrices. In this last example, the use of structural probes as zirconium illustrates the influence of the alteration solution on the process of glass corrosion and further development of a gel at the glass–solution interface. A comparison with the evolution of the surrounding of iron shows that the two major processes, hydrolysis/condensation and dissolution/precipitation, depend on the element considered.

Structure and properties of low-silica calcium aluminosilicate glasses

Abstract The structure of three calcium aluminosilicate glasses with low-silica content (61CaO · 39Al 2 O 3 (Ca0.39), 55CaO · 35Al 2 O 3 · 10SiO 2 (Ca10.35) and 49CaO · 31Al 2 O 3 · 20SiO 2 (Ca20.31)), synthesized by a standard quenching technique, were studied by X-ray and neutron diffraction. The data are consistent with the hypothesis that Si and Al are present in tetrahedral sites and Ca in octahedral sites for all compositions. Si–T (T=Si or Al) and Si–O2 contributions (where O2 means the second nearest oxygen) appear at 10 mol% silica content, which indicate that silicon enters the aluminate network.

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.

Cationic environment in silicate glasses studied by neutron diffraction with isotopic substitution

The method of neutron diffraction coupled with isotopic substitution is presented and recent investigations on the environment around cations in silicate (Ti in K2O·TiO2·2SiO2, Ca and Ni in 2CaO·NiO·3SiO2) and aluminosilicate (Li in Li2O·Al2O3·2SiO2) glasses are reviewed. The examination of the cation-centered pairs obtained from the first difference function presents striking similarities for all investigated cations. These functions indicate a well-defined short- and medium-range environment around cations. The local site generally presents a lower coordination number than that found in the crystals of similar composition. The environment around Ti in vitreous K2O·TiO2·2SiO2 corresponds to a square-based pyramid and direct TiO5–TiO5 linkages were observed experimentally in the second difference function, contrary to crystals. A detailed description of the cation site distortion for Li and Ca may be given by this method. The distribution of cations at medium range, which can be extracted by the double difference method, reveals the presence of cation-rich regions in silicate glasses. The cation–cation distances often indicate a two-dimensional character in the cationic organization. On the contrary, Li-aluminosilicate glass shows a more homogeneous cation distribution, in relation with the charge-compensating role of Li in this glass. This non-homogeneous distribution of cations may be related to the nano-inhomogeneities proposed in the models of supercooled liquids.

Environment of Ni, Co and Zn in low alkali borate glasses: information from EXAFS and XANES spectra.

XANES spectroscopy confirms that transition elements such as nickel, cobalt and zinc are octahedrally co-ordinated in low-alkali borate glasses, a co-ordination state which is unusual in most oxide glasses. EXAFS spectroscopy indicates that, despite their diluted character, transition elements are inhomogeneously distributed, with a medium range order extending up to 6 A with multiple scattering features characteristic of the presence of collinear cations. This peculiar structure is attributed to the presence of rigid units in these low-alkali borate glasses. The presence of these ordered domains in 0.1Li2O–0.9B2O3 glasses with NiO contents ranging from 0.5 to 2 wt% shows their independence relative to the concentration of the transition element.

Cationic ordering in oxide glasses: the example of transition elements

Abstract Structural data have been obtained on the cation surroundings in multi-component silicate and borosilicate glasses using chemically selective spectroscopic and scattering methods, such as extended X-ray absorption and neutron scattering with isotope substitution (NSIS). Transition elements such as Ni or Ti may occur in unusual 5-coordinated sites which coexist with other coordination numbers, depending on glass composition. Distribution of cationic sites in the glassy structure is responsible for unusual spectroscopic properties, as shown by Fe2+ Mössbauer spectroscopy. The environment of cations such as Zn, Zr or Mo, has been determined by EXAFS and discussed using the bond valence theory, which predicts the way to charge compensate the oxygen neighbours and which indicates the linkage of cationic sites with the silicate framework. Cation-cation correlations are given by NSIS up to ~8 Å , indicating an extensive Medium Range Ordering (MRO) with corner- and edge-linked cationic polyhedra, for Ti and Ni-bearing glasses, respectively. This heterogeneous cationic distribution in glasses is consistent with the presence of two-dimensional domains in which cation mixing may occur, as shown in a Ca-Ni metasilicate glass. Three-dimensional domains have also been found by Ni-K edge EXAFS in the case of low alkali borate glasses, with a local structure which mimics some aspects of crystalline NiO. The presence of ordered cationic domains, clearly illustrated by Reverse Monte Carlo simulations helps to rationalize the physical properties of multi-component silicate glasses.

High-resolution Al L2,3-edge x-ray absorption near edge structure spectra of Al-containing crystals and glasses: coordination number and bonding information from edge components

High-resolution Al L2,3-edge x-ray absorption near edge structure (XANES) spectra have been measured in selected materials containing aluminium in 4-, 5-?and 6-coordination. A shift of 1.5?eV is observed between the onset of [4]Al and [6]Al?L2,3-edge XANES, in agreement with the magnitude of the shift observed at the Al K-edge. The differences in the position and shape of low-energy components of Al L2,3-edge XANES spectra provide a unique fingerprint of the geometry of the Al site and of the nature of Al?O chemical bond. The high resolution allows the calculation of electronic parameters such as the spin?orbit coupling and exchange energy using intermediate coupling theory. The electron?hole exchange energy decreases in tetrahedral as compared to octahedral symmetry, in relation with the increased screening of the core hole in the former. Al L2,3-edge XANES spectra confirm a major structural difference between glassy and crystalline NaAlSi2O6, with Al in 4-?and 6-coordination, respectively, Al coordination remaining unchanged in NaAl1?xFexSi2O6 glasses, as Fe is substituted for Al.

The structure of crystals, glasses, and melts along the CaO-Al2O3 join: Results from Raman, Al L- and K-edge X-ray absorption, and 27Al NMR spectroscopy

Abstract Calcium aluminate glasses are important materials where AlO-4/2 is the only network former. Aluminum in crystals or glasses between CaO and Al2O3 can have different environments as a function of the CaO/Al2O3 ratio. Using X-ray absorption at the Al K- and L-edges, Raman and 27Al NMR spectroscopies, we have determined the structural surroundings of Al in glasses, crystals, and melts in this binary system. Aluminum is in octahedral coordination at high-Al2O3 content (>80 mol%) and essentially in fourfold coordination with 4 bridging O atoms (BOs) at Al2O3 contents between 30 and 75 mol%. At around 25 mol% Al2O3, Al is in tetrahedral coordination with two BOs. The presence of higher-coordinated species at high-Al2O3 contents and their absence at low Al2O3 imply different viscous flow mechanisms for high- and low-concentration Al2O3 networks.