Revisiting the 'mixed glass former effect' in ultra-fast quenched borophosphate glasses by advanced 1D/2D solid state NMR

In this paper, the concept of mixed glass former effect (MGFE), an intriguing phenomenon leading to glasses with high conductivity and increased thermal properties, has been revisited thanks to the combination of (i) an efficient elaboration technique that helped in producing for the first time a series of Li-rich borophosphate glasses from pure phosphate to pure borate and (ii) a structural investigation based on advanced 1D/2D solid state NMR performed at standard and very high magnetic fields (9.4 and 18.8 T) using very recently developed methods (B-11{P-31} D-HMQC, B-11 DQ-SQ). Homogeneous glasses, completely free of crystallization and exhibiting unreported and high values of conductivity when compared to other oxide-based materials, were prepared all along the 45Li(2)O-55[xB(2)O(3)-(1-x) P2O5] line, thanks to an ultra-fast quenching method. It allowed investigation of the relation between electrical/thermal properties and structure over the full range of composition. The advanced 1D/2D solid state NMR investigation helped in producing unreported and deep insights into the glass structure about (i) the multiple BO4 chemical environments, (ii) the BO3-BO4 connectivity, (iii) the wide range of phosphate speciation (described with the Q(m,BOx)(n) notation) and (iv) unexpected and unreported correlations between trigonal boron and phosphate species suggesting an original structure where both tetragonal and trigonal boron interact with the phosphate species. While confirming that the MGFE has a direct impact on the vitreous transition temperature, this investigation helped in giving the first clear evidence of the link that exists between structure and conductivity changes in these solid electrolytes.