Transition and post-transition metal ions in borate glasses: Borate ligand speciation, cluster formation, and their effect on glass transition and mechanical properties

A series of transition and post-transition metal ion (Mn, Cu, Zn, Pb, Bi) binary borate glasses was studied with special consideration of the cations impact on the borate structure, the cations cross-linking capacity, and more generally, structure-property correlations. Infrared (IR) and Raman spectroscopies were used for the structural characterization. These complementary techniques are sensitive to the short-range order as in the differentiation of tetrahedral and trigonal borate units or regarding the number of non-bridging oxygen ions per unit. Moreover, vibrational spectroscopy is also sensitive to the intermediate-range order and to the presence of superstructural units, such as rings and chains, or the combination of rings. In order to clarify band assignments for the various borate entities, examples are given from pure vitreous B₂O₃ to meta-, pyro-, ortho-, and even overmodified borate glass compositions. For binary metaborate glasses, the impact of the modifier cation on the borate speciation is shown. High field strength cations such as Zn²⁺ enhance the disproportionation of metaborate to polyborate and pyroborate units. Pb²⁺ and Bi³⁺ induce cluster formation, resulting in PbO_n- and BiO_n-pseudophases. Both lead and bismuth borate glasses show also a tendency to stabilize very large superstructural units in the form of diborate polyanions. Far-IR spectra reflect on the bonding states of modifier cations in glasses. The frequency of the measured cation-site vibration band was used to obtain the average force constant for the metal-oxygen bonding, F_M-O. A linear correlation between glass transition temperature (T_g) and F_M-O was shown for the metaborate glass series. The mechanical properties of the glasses also correlate with the force constant F_M-O, though for cations of similar force constant the fraction of tetrahedral borate units (N₄) strongly affects the thermal and mechanical properties. For paramagnetic Cu- and Mn-borate glasses, N₄ was determined from the IR spectra after deducing the relative absorption coefficient of boron tetrahedral versus boron trigonal units, α = α₄/α₃, using NMR literature data of the diamagnetic glasses.