The Structure of Tellurium Borate Glasses.

B₂O₃ and TeO₂ are both glass formers which have been shown to change coordination with the addition of a modifier.  This results in the tellurium borate glass system having a complex relationship between glass composition and structure, which has been little studied.  ¹¹B MAS-NMR, and Raman studies have shown that the average coordination of boron increases from 3- to 4-coordination with the addition of B₂O₃ , while tellurium coordination decreases from 4- to 3-coordination.  Studies have been carried out, which indicate that the lower boundary for glass is imposed by a liquid-liquid phase separation at ~ 74 mol.% TeO₂.  The tendency of tellurium borates to phase separate, resulting in changes in composition, can explain the conflicting data published in the literature (e.g. Raman studies).  
This study has placed strong emphasis on accurately determining the compositions of the samples studied and quantitative ¹¹B NMR has been used.  Measurements of density and N₄ values disagree with previously reported relationships between these properties and composition.  Fitting the total correlation function, T(r), measured by neutron diffraction, has allowed the boron and tellurium environments to be studied together.  The boron coordination number was in good agreement with that measured by ¹¹B NMR; Sekiya et al. have suggested that the coordination changes are a charge balancing mechanism; to balance the [BO₄]- unit, there is a corresponding increase in [TeO₃]+ trigonal pyramids.  Although overlapping correlations in T(r) make the total coordination for the Te–O environment difficult to determine, comparisons between the measured Te coordination numbers and calculations of the Te coordination numbers necessary to balance the N₄ values have shown that the argument of Sekiya et al. is reasonable.