Boroxol Rings in Vitreous

I hereafter briefly summarise the structural works on V-B2O3 taking advantage of several good reviews which appeared at various times [28, 30-32]. A special focus to atomistic simulations is then given in the next section. 

In pure boron trioxide, the basic building unit is the planar BO3 triangle [33] (see Fig. 14.1). The first proposed structural model of V-B2O3 is that of a continuous random network of such corner-linked triangles [27]. This view was however questioned, as soon as 1953 [34], by the observation of an extremely sharp peak at 

NMR derived values range from / = 65-85 %. Hereafter, structures in this range of / will be refered to as the bowxol-rich model. 

The decrease of / with pressure, which is supported by other types of experiments [72, 73, 76-78] and simulations [78, 79], can be understood by simple steric arguments (Fig. 14.1) because of their poor packing efficiency, boroxols tend to be replaced with increasing density by more compacted units, first the BO3 triangle 

The temperature dependence of the Raman peak (808 cm ) has been measured [67-69] from 77 up to 1867 K, i.e. from the glass to the liquid state (see Fig. 14.2). The rings concentration is essentially constant from room temperature up to the glass transition. Then at Tg ( 470-540K), boroxols start to open, and above Tg the number of rings steeply decreases [69]. Neudon diffraction measurements in the liquid state are consistent with this view [80]. 

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Tossell, J. A., and P. Lazzeretti (1990). Calculation of the structure, vibrational spectra, and polarizability of boroxine, H3B3O3, a model for boroxol rings in vitreous BjO,. J. Phys. Chem. 94, 1723-24. 

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