Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов

2.4 Short‐range Structure and Order in Oxide Glasses

       Jonathan F. Stebbins

       Department of Geological Sciences, Stanford University, Stanford, CA, USA

1 Introduction

      Information about short‐range structure is crucial for understanding, predicting, and optimizing the physical and chemical properties of oxide glasses. Glass structures also provide first approximations for the high‐temperature molten precursors of these materials, whether in a glass melting tank or a natural magma system, and thus give fundamental information for understanding and predicting liquid properties [1, 2]. Glasses are often simply categorized as “disordered” solids, lacking the long‐range order and the accompanying Bragg diffraction peaks of their crystalline counterparts. The nature and extent of this disorder, however, and how these vary with composition, temperature, and pressure, are of key importance. Much is now qualitatively known about such questions, but quantitative understanding is in many cases just beginning.

Because the same short‐range bonding interactions are present in crystals, glasses, and liquids, a sensible starting point for discussions of oxide glass structure is often that of known crystalline compounds. However, this approach has limitations and can be downright misleading. An example is shown in Figure 1, which compares the oxygen ion environments of crystalline and glassy CaTiSiO₅ (titanite), a compound with a particularly high entropy of fusion [3]. Some of the same short‐range structures are present in both, but the glass (and thus liquid) has not only quantitatively greater ranges in local structural variables but major qualitative differences in the type and variety of atomic‐scale environments. Furthermore, these types of data typically do not reveal the degree of mixing among the various structural groups, which thus leaves the interesting problem of possible medium‐range heterogeneities unanswered.

      In this chapter, we begin with a brief summary of the short‐range structures of well‐known single‐component network oxide glasses, and compare and contrast the different mechanisms by which addition of network modifiers cause rearrangement of these structures. We then continue with the type and degrees of order/disorder that result from mixing of multiple modifier and/or network cations in multicomponent glasses and liquids. Silicates, aluminosilicates, borates, germanates, and phosphates are emphasized as these are most common in nature and in technology. However, it is important to note that other glasses, based on oxides that also have small, highly charged cations and that thus can behave as “network formers” in some ranges of compositions, can provide intriguing clues as to important principles of glass structure/property relationships. These include (at least) TeO₂, As₂O₅, V₂O₅, Sb₂O₅, CrO₃, and MoO₃.

2 One‐component Oxide Glass Formers

      Pure SiO₂, GeO₂, and B₂O₃ readily form glasses on cooling from the melt, and epitomize the “network‐forming” oxides. The Si⁴⁺, Ge⁴⁺, and B³⁺ cations all have high valences, relatively high electronegativities, and are small enough to be stable in four‐ and/or three‐coordination with oxygen. The strongly bonded and interconnected structure that results contributes to high liquid viscosities, slow diffusion, and crystal growth rates, and thus good glass‐forming ability.

Of the three pure‐oxide glass formers, silica is by far the best studied because of its many important technological applications, although many of its properties are anomalous with respect to multicomponent silicate glasses. The structures of the multiple (low pressure) crystalline forms of silica are all comprised of three‐dimensional networks of corner‐shared tetrahedra linked by Si─O─Si