Kinetic fragility and structure of lithium borophosphate glasses analysed by 1D/2D NMR

The macroscopic and high temperature properties of lithium borophosphate glasses were determined in this contribution. Our data, obtained on 50Li₂O-xB₂O₃-(50-x)P₂O₅ glasses, confirm a continuous and linear increase of the glass transition temperature with the B/P substitution but show a two-domain evolution of the kinetic fragility with a steep decrease in the low B₂O₃ region (0 ≤ x ≤ 10) followed by a moderate increase for higher B₂O₃ contents. In order to understand this different behaviour, the glass structure was investigated in detail using 1D and 2D ¹¹B/³¹P correlation solid state nuclear magnetic resonance. The local and medium orders of borate units were determined by 1D MAS-NMR, 2D ¹¹B DQSQ- and ¹¹B(³¹P) D-HMQC NMR experiments. The latter NMR technique was also used to deeply interpret the 1D ³¹P MAS-NMR spectra. Altogether the data allow (i) highlighting of the presence of four borate and seven phosphate units, (ii) evaluation of the number of homopolar POP and mixed POB linkages, and (iii) contribute to a better understanding of the T_g and kinetic fragility evolution.