Nuclear Magnetic Resonance studies of the Na₂B₄O₇-Na₃AlF₆-TiO₂ system

 

 

C.Bessada¹, E.M.Anghel²

¹CNRS, Centre de Recherche sur les Materiaux a Hautes Temperatures, 1D, Avenue de la Recherche Scientifique, 45071 Orleans Cedex  2, France

 

²Institute of Physical Chemistry “I.G.Murgulescu” of Romanian Academy of Science, Spl. Independentei 202, 77208 Bucharest, Romania

 

 

Abstract:

Structural investigation of the the Na₂B₄O₇-[Na₃AlF₆-TiO₂]_x systems (where x stands for £ 11 % TiO₂), carried out by X-ray diffraction, IR, UV-Vis spectroscopy, were supplemented with high temperature and MAS NMR spectroscopy.

Given the fact that the electroactive components of the NaCl-Na₂B₄O₇-Na₃AlF₆-TiO₂ molten electrolyte for TiB₂ electrosynthesis, both Na₂B₄O₇ and TiO₂ (in a lower coordination, i.e. four-fold), act as glass formers, it is obvious that very complex changes are expected when mixing all three components of the ternary systems under discussion. Thus, according to our preliminary work, concerning structural aspects of the Na₂B₄O₇-Na₃AlF₆ system, Na₃AlF₆ acts as a glassy network modifier on the vitreous structure of Na₂B₄O₇ by forming new BF₄^- and /or BO₃F^4- anions as a result of mutual substitution of O^2- and F^-. Moreover, comparative study of the ternary Na₂B₄O₇-[Na₃AlF₆-TiO₂]_x systems with x=3.5 and respectively x=11 % TiO₂ revealed the different behavior of the two. Accordingly, in the Na₂B₄O₇-[Na₃AlF₆-TiO₂]_{x=11% TiO2} not only Na₃AlF₆ but also TiO₂ (in an octahedral coordination Ti(O, F)₆) play the role of modifier as the TiO₂ concentration exceeds 5 %.

Regarding aluminium bearing species, the conversion of the AlF_x^(x-3)-anions (resulted from Na₃AlF₆ dissociation) into the AlO_x-yF_y (x+y£5) anions was pointed out on the  ²⁷Al NMR spectra of the Na₂B₄O₇-[Na₃AlF₆-TiO₂]_{x=11% TiO2} melts.