Short Range Structural Models of the Glass Transition Temperatures and Densities of 0.5Na sub(2)S + 0.5[xGeS sub(2) + (1 - x)PS sub(5/2)] Mixed Glass Former Glasses

The 0.5Na sub(2)S + 0.5[xGeS sub(2) + (1 - x)PS sub(5/2)] mixed glass former (MGF) glass system exhibits a nonlinear and nonadditive negative change in the Na super(+) ion conductivity as one glass former, PS sub(5/2), is exchanged for the other, GeS sub(2). 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 (T sub(g)) 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 PS sub(7/2) super(4-) and GeS sub(3) super(2-) units into PS sub(4) super(3-) and GeS sub(5/2) super(1-) units. This disproportionation combined with the formation of Ge sub(4)S sub(10) super(4-) anions from GeS sub(5/2) super(1-) groups leads to the negative MGFE in T sub(g). A best-fit model of the T sub(g)s of these glasses was developed to quantify the amount of GeS sub(5/2) super(1-) units that form Ge sub(4)S sub(10) super(4-) 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.