Nuclear Magnetic Resonance studies of the Na_2B_4O_7-Na_3AlF_6-TiO_2 system

摘要

Structural investigation of the Na_2B_4O_7-[Na_3AlF_6-TiO_2]_x systems (where x stands for 11% TiO_2), carried out by X-ray diffraction, IR, UV-Vis spectroscopy, were supplemented with high temperature and MAS NMR spectroscopy. A comparative study of the ternary Na_2B_4O_7-[Na_3AlF_6-TiO_2]_x systems with x=3.5 and respectively x=11% TiO_2 revealed a different behaviour. Accordingly, in the Na_2B_4O_7-[Na_3AlF_6-TiO_2]_(x = 11% TiO2) mixtures not only Na_3AlF_6 but also TiO_2 (in an octahedral coordination Ti(O, F)_6) play the role of modifier as the TiO_2 concentration exceeds 5%. Regarding aluminium bearing species, the conversion of the AlF_x~((x-3)-) anions (resulted from Na_3AlF_6 dissociation) into the AlO_(x-y)_y (where x+y<6) anions was pointed out on the ~(27)Al NMR spectra for Na_2B_4O_7-[Na_3AlF_6-TiQ_2]_(x = 11% TiO_2) melts. The devitrification mechanism of borate glass under cryolithe and anatase influence was established and temperature structural evolution of aluminium and boron bearing species was also discussed.