A Crystallographic Guide to the Structure of Borate Glasses
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ABSTRACT Borate glasses are an enigma in that there is now increasing evidence that their structures are dominated by superstructuralunits, which comprise well defined arrangements of the basic B0 3 and B0 4 structural units, with no internal degrees of freedom in the form of variable bond or torsion angles. In the present paper, it is shown that considerable insight into the structure of borate glasses can be gained from a study of the corresponding crystalline polymorphs. A simple, model is proposed to predict the fraction, x4, of 4-fold co-ordinated boron atoms in vitreous borate networks and the topological criteria for the formation of such networks are discussed, taking into account the degrees of freedom necessary for conventional glass formation. 1. INTRODUCTION An extremely controversial current issue in the field of glasses and glass formers concerns the role of superstructural units in the formation of borate glasses, as indicated by the lively discussion at a recent international conference [1]. Borate glasses are fascinating materials that have not been studied to anything like the extent they deserve, especially taking into account the challenges they present to conventional ideas concerning the structure of network glasses, as embodied in the traditional random network theory. This was first proposed by Zachariasen [23,
using structural principles elucidated from crystallography, taking due regard of the additional degrees of topological fireedom required by a disordered network appropriate to the vitreous state. The purpose of the present paper is to demonstrate that a similar consideration of the crystal structures of anhydrous borates, mM,,O.nB 20 3 (abbreviated mM.nB), leads to a structural theory/model for borate glasses which is consistent with a wide range of experimental data. 2. GLASS-FORMING REGIONS Vitreous B20 3 itself is a glass former par excellence and forms binary glasses with a wide range of other oxides. The single-phase glass-forming regions obtained for binary borate systems using conventional quenching techniques [3-5] are shown as a function of the mole fraction, xM, of the second oxide in Fig. 1, together with the crystalline polymorphs occurring in each system [5,6]. For all of the M20-B2 0 3 binary systems in Fig. I(A), the single-phase glass-forming region commences at zero xM, whereas, for all of the MO-B 20 3 systems {Fig. 1(B) }, single phase glasses are not formed at low modifier contents. Any glass formation in this region results in phaseseparated glasses. For example, Shelby [7] records two glass transition temperatures for PbOB20 3 glasses in the region 0.005
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