For the first time a detailed structural model has been determined which shows how the lone-pairs of electrons are arranged relative to each other in a glass network containing lone-pair cations. High energy X-ray and neutron diffraction patterns of a very high lead content silicate glass (80PbO·20SiO2) have been used to build three-dimensional models using empirical potential structure refinement. Coordination number and bond angle distributions reveal structural similarity to crystalline Pb11Si3O17 and α- and β-PbO, and therefore strong evidence for a plumbite glass network built from pyramidal [PbO(m)] polyhedra (m ~ 3-4), with stereochemically active lone-pairs, although with greater disorder in the first coordination shell of lead compared to the first coordination shell of silicon. The oxygen atoms are coordinated predominantly to four cations. Explicit introduction of lone-pair entities into some models leads to modification of the local Pb environment, whilst still allowing for reproduction of the measured diffraction patterns, thus demonstrating the non-uniqueness of the solutions. Nonetheless, the models share many features with crystalline Pb11Si3O17, including the O-Pb-O bond angle distribution, which is more highly structured than reported for lower Pb content glasses using reverse Monte Carlo techniques. The lone-pair separation of 2.85 Å in the model glasses compares favourably with that estimated in α-PbO as 2.88 Å, and these lone-pairs organise to create voids in the glass, just as they create channels in Pb11Si3O17 and interlayer spaces in the PbO polymorphs.

Understanding the structure of single-component glasses is essential for developing structural models of more complex multicomponent glasses. Currently, such models for tellurite systems are purely qualitative. This study presents neutron diffraction and Raman spectroscopy measurements of the structure of pure amorphous TeO 2 , showing that it is formed from a combination of two-thirds [TeO 4 ] pseudo-trigonal bipyramids and onethird [TeO 3 ] trigonal pyramids with a terminal oxygen. This is in contrast to all crystalline polymorphs of TeO 2 , which are formed solely from the four-coordinated units. Using this result, a quantitative model has been developed that successfully predicts the average Te−O coordination number, n TeO , for a series of potassium tellurite glasses, xK 2 O•(100 − x)TeO 2 . The observed n TeO is constant up to 15 mol % K 2 O due to the presence of terminal oxygen atoms in the tellurite network.

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Although the interactions among glass formers and modifiers, for example, connectivity and charge distribution, have been studied extensively in oxide glasses, the impact of a particular modifier species on the mechanical performance of aluminoborosilicate (ABS) glasses is not well understood. This work compares the indentation properties of six ABS glasses, each of which contains a different network modifier (NWM) with varying field strength (FS). Three alkali and three alkaline earth ABS glasses were designed with low NWM content and [NWM] ≈ [Al2O3], to test the modifier FS effect at low concentrations and to maximize three‐coordinated boron. It has been found that both hardness and crack resistance increase with increasing FS in these ABS systems, which is surprising in the context of historical reports. Using 11B, 27Al, and 29Si solid‐state nuclear magnetic resonance, this work provides evidence of how charge distributions differ as a function of NWM species, and how this relates to the observed indentation behaviors.

Structures of binary PbO‐SiO2 glasses have been studied in detail over the compositional range 35 to 80 mol% PbO using high‐resolution neutron diffraction, high‐energy X‐ray diffraction, static 207Pb NMR, and structural modeling. The changes in the local environment of Pb(II) are subtle; it has a low coordination to oxygen (∼3 to 4) plus a stereochemically active electron lone pair and, thus, behaves as a glass network forming (or intermediate) cation over the entire composition range. This conclusion contradicts previous reports that Pb(II) is a network modifier at low concentrations, and is supported by an analysis of lead and alkaline earth silicate glass molar volumes. The Pb‐O peak bond length shortens by 0.04 Å with increasing PbO content, indicating stronger, more covalent bonding, and consistent with an increase in the number of short (≤ 2.70 Å) Pb‐O bonds, from 3.3 to 3.6. This is accompanied by increased axial symmetry of the Pb(II) sites, and is interpreted as a gradual transition toward square pyramidal [PbO4] sites such as those found in crystalline PbO polymorphs. An attendant decrease in the periodicity associated with the first sharp diffraction peak (FSDP) toward that of β‐PbO, accompanied by increases in the correlation lengths associated with the plumbite network (FSDP) and silicate anions (neutron prepeak), provides evidence of increased intermediate‐range order and has implications for the glass forming limit imposed by crystallization. Pb(II) electron lone pairs occupy the natural voids within the silicate network at low PbO contents, while at high PbO contents they aggregate to create voids that form part of the plumbite network, analogous to the open channels in Pb11Si3O17 and the layered structures of α‐ and β‐PbO. Si‐O and Pb‐O bond lengths have been correlated with 29Si and 207Pb NMR chemical shifts, respectively. This is the first time that such correlations have been demonstrated for glasses and attests to the accuracy with which pulsed neutron total scattering can measure average bond lengths.

High-resolution X-ray pair distribution functions for molten and glassy TeO2 reveal coordination numbers n TeO ≈ 4. However, distinct from the known α-, β-, and γ-TeO2 polymorphs, there is considerable short-range disorder such that no clear cutoff distance between bonded and nonbonded interactions exists. We suggest that this is similar to disorder in δ-TeO2 and arises from a broad distribution of asymmetric Te–O–Te bridges, something that we observe becomes increasingly asymmetric with increasing liquid temperature. Such behavior is qualitatively consistent with existing interpretations of Raman scattering spectra, and equivalent to temperature-induced coordination number reduction, for sufficiently large cutoff radii. Therefore, TeO2 contains a distribution of local environments that are, furthermore, temperature dependent, making it distinct from the canonical single-oxide glass formers. Our results are in good agreement with high-level ab initio cluster calculations.

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A report is given of the first detailed investigation of the relation of structure to the germanate anomaly in alkaline earth germanate glasses. Pulsed neutron diffraction and high-energy X-ray diffraction measurements on calcium germanate glasses (22 to 41 mol % CaO, roller-quenched) show that the average Ge–O coordination number, n GeO, and average bond length, r GeO, exhibit broad maxima at ∼28 mol % CaO. Maxima in the mass density, atom number density, and glass-transition temperature also occur at a similar composition and are characteristic of the germanate anomaly. The anomalies occur at a higher modifier oxide content for alkaline earth (Ae) germanate glasses than for alkali (A) germanate glasses, attributed to the higher bond strengths within Ae-O polyhedra compared to those in A-O polyhedra. Consequently, the stabilization of [GeO6] octahedra and/or bridges between higher-coordinated [GeO m ] units (m = 5, 6) is a feature of AeO–GeO2 glasses, in contrast to the existence of only [GeO4] tetrahedra and [GeO5] units proposed in alkali (Cs) germanate glasses. At high CaO content, n GeO behaves as in thallium germanate glasses because of the very similar metal–oxygen bond strengths within [CaO7] polyhedra and [TlO3] units. The composition dependence of the atom number density at low modifier content follows a common trend for Ca, Sr, and Ba germanate glasses because modifier cations fill germanate network cages. Our results show that the germanate anomaly for calcium germanate glasses is related to both a packing effect and a structural change in the germanate network; there is not a simple dichotomy whereby only one of these factors is of significance for the germanate anomaly.