Grant S. Henderson

The structure of glasses along the Na2OGeO2 join

Raman spectra of glasses with compositions along the Na2OGeO2 join and corresponding crystalline phases have been investigated using micro-Raman techniques. The main vibrational band in the Raman spectrum of GeO2, which is associated with symmetric stretch of the GeOGe bonds, is observed to contain fine structure. These weak vibrational features may be indicative of several distinct 4-membered ring populations within the network. Changes in the intensity of the fine structure with addition of Na2O indicate that the ring statistics change systematically with composition. No evidence is observed along the join for the formation of 6-fold coordinated germanium atoms. The ‘germanate anomaly’ exhibited by these glasses appears to result from the formation of 3-membered rings of GeO4 tetrahedra. The maximum (or minimum) in the anomaly occurs when the network becomes saturated in 3-membered GeO4 rings. Beyond this point, the continued formation of Q3 tetrahedra (tetrahedra containing 1 NBO) is responsible for the decline (increase) in some properties. The glass structure begins to resemble the crystalline digermanate structure at ≥ 30–35 mol% added Na2O. Crystallization occurs at ∼ 40 mol% Na2O compositions at which large numbers of Q2 tetrahedra form and there is total breakdown of the glass network.

The structure of alkali germanophosphate glasses by Raman spectroscopy

Abstract A series of alkali germanophosphate glasses ((R2O)x(GeO2:P2O5)1−x where R=Na, K and Rb) with variable GeO2:P2O5 ratios (8:1, 6:1, 4:1 and 2:1) have been investigated using Raman spectroscopy. The glass network may be treated as being made up of separate germanate and phosphate components. Addition of alkali cations indicates that the alkalis preferentially modify the phosphate part of the network. Network depolymerization occurs by formation of Q2 and Q1 PO4 tetrahedra. At high Ge:P ratios the glasses exhibit a density anomaly. The anomaly is attributed to the formation of small three-membered GeO4 rings. However, alkali cation size and mass are contributing factors to the shape of the density anomaly in the K2O and Rb2O containing glasses. Depolymerization of the 2:1 alkali germanophosphate glasses is predominantly by formation of Q2 PO4 tetrahedra. At low Ge:P ratios (2:1) density trends are linear. No ring transition is observed in these glasses. Furthermore, the presence of [5]Ge cannot be ruled out, but if present, does not play a major role in generating a ‘germanate anomaly’.

A Si K-edge EXAFS/XANES study of sodium silicate glasses

Abstract Si K-edge X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS) have been obtained on a series of sodium silicate glasses containing 15–40 mol% Na2O. The XANES reveals that, with the addition of Na2O, the SiSi bond distance distribution decreases and there are increased contributions from multiple scattering beyond the second coordination sphere. These responses imply that some degree of network ordering occurs with alkali addition. The EXAFS data indicate that the SiO bond distance increases from 1.61 ± 0.02 A for amorphous SiO2 to 1.66 ± 0.02 A for 30 mol% added Na2O. For Na2O > 30 mol%, the SiO bond distance decreases. The SiO bond distance changes indicate that, for ≤ 30 mol% Na2O, network depolymerisation effects on the SiO bond dominate any effect from increased non-bridging oxygen (NBO) formation. For compositions > 30 mol% Na2O, increased NBO formation has the dominant effect on the SiO bond distance. This may indicate that the microsegregation of network modifiers from network formers, as predicted by molecular dynamics studies, is significant at ≥ 30 mol% added Na2O.

Germanium coordination and the germanate anomaly

The structural mechanism responsible for the germanate anomaly along the X 2 O-GeO 2 join, where X = Li, Na, K, Rb, and Cs, was investigated using Raman spectroscopy. Density maxima were measured at ∼17.5–20 mol%, 15 mol%, and 10 mol% for Li 2 O, Na 2 O and K 2 O, respectively. A maximum at 15 mol% and a minimum at 32.5 mol% were measured for Rb 2 O while the Cs 2 O glasses exhibited a maximum at 17.5 mol%. A coordination change of IV Ge to VI Ge was not observed. It is proposed that the germanate anomaly is a consequence of formation of small 3-membered GeO 4 rings as alkali oxide is added to the glass. The small rings cause a density increase. The anomaly maximum is reached when continued small ring formation cannot occur without straining the glass network. At this point the network generates large numbers of Q 3 non-bridging oxygens (NBOs) and at higher alkali oxide contents, Q 2 NBOs. The formation of the NBOs causes a decline in the density. The Rb 2 O- and Cs 2 O-containing glasses do not exhibit similar density trends to the lighter alkali-containing glasses. This is because of competing density effects from smallring formation and the increased mass of the alkalis. The overall effect on the density trends of these glasses is to skew the anomaly maxima to higher alkali oxide compositions.

ZrTiO4 crystallisation in nanosized liquid–liquid phase-separation droplets in glass—a quantitative XANES study

The crystallisation of the nucleation agent ZrTiO4 in a low thermal-expansion lithium aluminosilicate glass-ceramics is monitored as a function of time by combining transmission electron microscopy with Ti-L2,3 X-ray absorption near-edge structure spectroscopy. The formation of liquid–liquid phase-separation droplets is shown to precede ZrTiO4 crystallisation within the latter nanosized droplets. Quantitative data on crystalline fractions enable conclusions on the self-limited growth of ZrTiO4 nanocrystals in low thermal-expansion glass-ceramics and based on Avramis equation, the growth is shown to be restricted by a barrier (the outer border of the phase-separation droplet). It is shown that liquid–liquid phase separation and crystallisation are temporally decoupled. The size of ZrTiO4 crystallites is determined by the restricted volume of the phase-separation droplets they crystallise in. The volume of the droplets in turn is restricted by the formation of a diffusion barrier in the surrounding residual glass.

In situ high‐resolution atomic force microscope imaging of biological surfaces

In situ high‐resolution atomic force microscope imaging of biological surfaces was performed on cells with relatively rigid surfaces (e.g., bacteria). The surface of Lactobacillus helveticus (a rod‐shaped bacterium) was investigated before and after exposure to LiCl, a denaturant. Image details were stable both at variant force loads and under different scan directions. From images of the oblique lattice structure (i.e., S layer of L. helveticus), it was estimated that the lateral resolution of the images was up to 2 nm. This resolution can be explained by assuming that there is an apex with a curvature of radius of ∼10 nm near the end of the tip. Modelling of this geometry indicates that such a tip configuration is particularly suitable for in situ high‐resolution imaging of relatively soft objects covered by a rigid shell (membrane).

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.

Improved atomic force microscopy resolution using an electric double layer

High resolution (“atomic”) images of clinochlore and muscovite have been obtained in aqueous solution by inducing an electric double layer between the atomic force microscope tip and the sample surface. The electric double layer is created by the addition of a surfactant to water and greatly improves image resolution. A theoretical model is proposed to explain the improved resolution.

Epsilon sodium disilicate : a high pressure layer structure [Na2Si2O5]

Abstract Epsilon sodium disilicate (e-Na 2 Si 2 O 5 ) synthesized at 7 GPa, 1100°C, for a 12-hr run time (M6/8 superpress, Edmonton) is orthorhombic with a = 5.580(1), b = 9.441(4), c = 8.356(3) A, Pbc 2 1 , Z = 4, D x = 2.749 g · cm −3 . The structure ( R = 4.0%) is based on a disilicate sheet of alternating six-membered rings of UUUUDD and DDDDUU SiO 4 tetrahedra in the (100) plane, with linking Na(1) five-fold coordinated to 2.57 A and Na(2) four-fold coordinated to 2.55 A: 〈Si(1)O〉 = 1.633 A, Si(1) O nbr = 1.580 A; 〈Si(2)O〉 = 1.623 A, Si(2)O nbr = 1.571 A. The structure is similar to that of β-Na 2 Si 2 O 5 , the 1 bar, 610–700°C polymorph, that has rings of UDUDUD tetrahedra. Densification is accommodated largely by a decrease in dihedral (SiOSi) bond angles that range from 127.0 to 129.3°, compared with 135.1° to 137.1° for β-Na 2 Si 2 O 5 ( D x = 2.57 g · cm −3 ) and 138.9° to 160.0° for α-Na 2 Si 2 O 5 ( D x = 2.50 g · cm −3 ). The e phase structure is consistent with 29 Si MAS-NMR and Raman spectra, and supports our earlier suggestion that densification of silicate melts to moderate pressures is accommodated predominantly by a decrease in dihedral bond angle through crimping of ring structures and a decrease in the number of SiO 4 tetrahedra per ring.