Structural description of the Na2B4O7-Na3AlF6-TiO2 system.: 2.: A multinuclear NMR approach of melts and solids

A quantitative structural investigation of pseudo-binary Na2B4O7-[Na3AlF6-TiO2](11) was carried out by NMR spectroscopy. We applied high-resolution solid-state NMR on quenched samples and in situ high-temperature NMR spectroscopy on molten compositions to obtain a more accurate description of the melts. Consistent with our previous findings, two compositional ranges can be evidenced. Below 50 mol % Na2B4O7, the Ti-IV -> Ti-VI valence conversion prevails, leading to the formation of B(O,F)(3) species at the expense of B(O,F)(4) ones and a low average coordination number of aluminum; above 50 mol %, the system behaves mainly like the binary Na2B4O7-Na3AlF6 where Na3AlF6 is playing a network former role. The dominant oxyfluoro-species evidenced by double-resonance NMR in the quenched samples are BO2F2, BO3F, BO2F, BOF2, NaO7F, NaO6F2, and AlO4F with sodium in various coordination states, Na(O,F)(8), Na(O,F)(7), and Na(O,F)(6). F-Ti bonds were found to be almost negligible whereas F-Al-2, F-B, F-Na, and F-Al-Na bonds were clearly observed in the solid state.