Short Range Structural Models of the Glass Transition Temperatures and Densities of 0.5Na2S + 0.5[xGeS2 + (1 – x)PS5/2] Mixed Glass Former Glasses
The 0.5Na2S + 0.5[xGeS2 + (1 – x)PS5/2] mixed glass former (MGF) glass system exhibits a nonlinear and nonadditive negative change in the Na+ ion conductivity as one glass former, PS5/2, is exchanged for the other, GeS2. This behavior, known as the mixed glass former effect (MGFE), is also manifest in a negative deviation from the linear interpolation of the glass transition temperatures (Tg) of the binary end-member glasses, x = 0 and x = 1. Interestingly, the composition dependence of the densities of these ternary MGF glasses reveals a slightly positive MGFE deviation from a linear interpolation of the densities of the binary end-member glasses, x = 0 and x = 1. From our previous studies of the structures of these glasses using IR, Raman, and NMR spectroscopies, we find that a disproportionation reaction occurs between PS7/24- and GeS32- units into PS43- and GeS5/21- units. This disproportionation combined with the formation of Ge4S104- anions from GeS5/21- groups leads to the negative MGFE in Tg. A best-fit model of the Tgs of these glasses was developed to quantify the amount of GeS5/21- units that form Ge4S104- molecular anions in the ternary glasses (∼5–10%). This refined structural model was used to develop a short-range structural model of the molar volumes, which shows that the slight densification of the ternary glasses is due to the improved packing efficiency of the germanium sulfide species.
Reprinted with permission from Journal of Physical Chemistry B 118 (2014): 3710, doi: 10.1021/jp411942t. Copyright 2014 American Chemical Society.