Probing Medium-Range Order in Oxide Glasses at High Pressure

Densification in glassy networks has traditionally been described in terms of short-range structures, such as how atoms are coordinated and how the coordination polyhedron is linked in the second coordination environment. While changes in medium-range structures beyond the second coordination shells may play an important role, experimental verification of the densification beyond short-range structures is among the remaining challenges in the physical sciences. Here, a correlation NMR experiment for prototypical borate glasses under compression up to 9 GPa offers insights into the pressure-induced evolution of proximity among cations on a medium-range scale. Whereas amorphous networks at ambient pressure may favor the formation of medium-range clusters consisting primarily of similar coordination species, such segregation between distinct coordination environments tends to decrease with increasing pressure, promoting a more homogeneous distribution of dissimilar structural units. Together with an increase in the average coordination number, densification of glass accompanies a preferential rearrangement toward a random distribution, which may increase the configurational entropy. The results highlight the direct link between the pressure-induced increase in medium-range disorder and the densification of glasses under extreme compression.

Mechanisms of Pressure-Induced Structural Transformation in Confined Sodium Borate Glasses

In this paper, density functional theory simulations were conducted to investigate the structural adaptation of sodium borates xNa₂O·(100-x)B₂O₃ (x = 25, 33, 50, and 60 mol %) during the compression/decompression between 0 and 10 GPa. The sodium borates are confined between two Fe₂O₃ substrates and undergo the compression by reducing the gap between the two surfaces. The results reveal the borate response to the load through a two-stage transformation: rearrangement at low pressure and polymerization at high pressure. The pressure required to initiate the polymerization depends directly on the portion of fourfold-coordinated (^[4]B) boron in the sodium borates. We found that the polymerization occurs through three different mechanisms to form BO₄ tetrahedra with surface oxygen and nonbridging and bridging oxygen. The electronic structure was analyzed to understand the nature of these mechanisms. The conversions from BO₃ to BO₄ are mostly irreversible as a large number of newly formed BO₄ remain unchanged under the decompression. In addition, the formation of a sodium-rich layer can be observed when the systems were compressed to high pressure. Our simulation provides insight into sodium borate glass responses to extreme condition and the underlying electronic mechanisms that can account for these behaviors.

Dynamic characterization of structural relaxation in V2O5-P2O5 bulk oxide glass

In oxide glasses, the microscopic hidden flow and the structural origin of the glass-to-liquid transition (GLT) are unclear due to the lack of detailed structural information. Herein, we investigate the evolution of the microscopic localized flow during GLT in a V2O5-P2O5 bulk oxide glass (BOG) by combining differential scanning calorimetry, temperature- and frequency-dependent bending experiments and stress relaxation spectra. The characteristic changes, their intrinsic correlations with the GLT process and the complete relaxation process are discussed in detail. We have observed three relaxation stages in the V2O5-P2O5 bulk oxide glass. Stage (I) corresponds to the nano-scale liquid-like movement with reversible activation of flow units. Stage (II) refers to the cooperative interaction of α and β relaxation, whereas stage (III) represents the glass transition process. In the frequency spectra, we have obtained a different result with metallic glasses by using a quasi-point defect model. When T < 480 K (Tβ), the correlation factor χ related to the quasi-point defect concentration is low and nearly constant, whereas, for T > 480 K (Tβ), χ shows a linear relationship with temperature. The present study provides useful insights to describe the relationship between the architecture of local atomic arrangements and mechanical properties of oxide glass.

Vibrational spectroscopy and density of K2O-B2O3-GeO2 glasses with variable B/Ge ratio

Glasses of the K₂O-B₂O₃-GeO₂ system were studied by means of Raman and IR spectroscopy. The density of the samples was measured and the dependence of the molar volume and atomic density on composition was calculated. Curve-fitting of Raman spectra was applied to obtain a definition of the main structural units formed in the system. The conditions for highly-coordinated boron and germanium atoms were obtained. It was shown that potassium cations remain connected to germanate structural units at a B/Ge ratio of up to 1, whereas the explicit redistribution of borate and germanate structural groupings becomes most noticeable only at a B/Ge ratio > 2.

Structural Compromise between High Hardness and Crack Resistance in Aluminoborate Glasses

Alkali aluminoborate glasses have recently been shown to exhibit a high threshold for indentation cracking compared to other bulk oxide glasses. However, to enable the use of these materials in engineering applications, there is a need to improve their hardness by tuning the chemical composition. In this study, we substitute alkaline earth for alkali network-modifying species at fixed aluminoborate base glass composition and correlate it with changes in the structure, mechanical properties, and densification behavior. We find that the increase in field strength (i.e., the charge-to-size ratio) achieved by substituting alkaline earth oxide from BaO to MgO manifests itself in a monotonic increase in several properties, such as atomic packing density, glass-transition temperature, densification ability, indentation hardness, and crack resistance. Although the use of alkaline earth oxides as modifier enables higher hardness values (increasing from 2.0 GPa for Cs to 5.8 GPa for Mg), their crack resistance is generally lower than that of the corresponding alkali aluminoborate glasses. We discuss the origin of this compromise between hardness and crack resistance in terms of the ability of the glass networks to undergo structural transformations and self-adapt under stress. We show that the extent of volume densification scales linearly with the number of pressure-induced coordination number changes of B and Al.

NMR Spectroscopy in Glass Science: A Review of the Elements

The study of inorganic glass structure is critically important for basic glass science and especially the commercial development of glasses for a variety of technological uses. One of the best means by which to achieve this understanding is through application of solid-state nuclear magnetic resonance (NMR) spectroscopy, which has a long and interesting history. This technique is element specific, but highly complex, and thus, one of the many inquiries made by non-NMR specialists working in glass science is what type of information and which elements can be studied by this method. This review presents a summary of the different elements that are amenable to the study of glasses by NMR spectroscopy and provides examples of the type of atomic level structural information that can be achieved. It serves to inform the non-specialist working in glass science and technology about some of the benefits and challenges involved in the study of inorganic glass structure using modern, readily-available NMR methods.

Accessing Forbidden Glass Regimes through High-Pressure Sub-Tg Annealing

Density and hardness of glasses are known to increase upon both compression at the glass transition temperature (Tg) and ambient pressure sub-Tg annealing. However, a serial combination of the two methods does not result in higher density and hardness, since the effect of compression is countered by subsequent annealing and vice versa. In this study, we circumvent this by introducing a novel treatment protocol that enables the preparation of high-density, high-hardness bulk aluminosilicate glasses. This is done by first compressing a sodium-magnesium aluminosilicate glass at 1 GPa at Tg, followed by sub-Tg annealing in-situ at 1 GPa. Through density, hardness, and heat capacity measurements, we demonstrate that the effects of hot compression and sub-Tg annealing can be combined to access a "forbidden glass" regime that is inaccessible through thermal history or pressure history variation alone. We also study the relaxation behavior of the densified samples during subsequent ambient pressure sub-Tg annealing. Density and hardness are found to relax and approach their ambient condition values upon annealing, but the difference in relaxation time of density and hardness, which is usually observed for hot compressed glasses, vanishes for samples previously subjected to high-pressure sub-Tg annealing. This confirms the unique configurational state of these glasses.