IR, Raman, and NMR Studies of the Short-Range Structures of 0.5Na sub(2)S + 0.5[xGeS sub(2) + (1-x)PS sub(5/2)] Mixed Glass-Former Glasses

A nonlinear and nonadditive composition-dependent change of the ionic conductivity in mixed glass-former (MGF) glasses when one glass former, such as PS sub(5/2), is replaced by a second glass former, such as GeS sub(2), at constant alkali modifier concentrations, such as Na sub(2)S, is known as the mixed glass-former effect (MGFE). Alkali ion conducting glasses are of particular interest for use as solid electrolytes in alkali-based all-solid-state batteries because sulfide amorphous materials have significantly higher alkali ion conductivities than their oxide glass counterparts. In this study of the ternary MGF system Na sub(2)S + GeS sub(2) + PS sub(5/2), we report the careful structural characterization of these glasses using a combination of vibrational, infrared (IR), Raman, and nuclear magnetic resonance (NMR) spectroscopies. Our measurements of the 0.5Na sub(2)S + 0.5[xGeS sub(2) + (1-x)PS sub(5/2)] MGF system show that this glass system exhibits a strongly negative MGFE and non-Arrhenius ionic conductivities. While this negative MGFE in the Na super(+) ion conductivity makes these glasses less attractive for use in solid-state Na batteries, the structural origin of this effect is important to better understand the mechanisms of ion conduction in the glassy state. For these reasons, we have examined the structures of ternary 0.5Na sub(2)S + 0.5[xGeS sub(2) + (1-x)PS sub(5/2)] glasses using Raman, IR, and super(31)P MAS NMR spectroscopies. In these studies, it is found that the substitution of PS sub(5/2) by GeS sub(2), that is, increasing x, leads to unequal sharing of the Na super(+) in these glasses. Thus, in all MGF compositions, phosphorus groups are associated with a disproportionately larger fraction, f sub(Na(P)) > 0.5(1 - x), of the Na super(+) ions while the germanium groups are found to be Na super(+)-deficient relative to the total amount of Na super(+) present in the glass, that is, f sub(Na(Ge)) < 0.5x. From the spectroscopic study of these glasses, a short-range order (SRO) structural model is developed for these glasses and is based on the germanium and phosphorus SRO groups in these glasses as a first step in understanding the unique negative MGFE and non-Arrhenius behavior in the Na super(+) ion conductivity in these glasses.