1. Na2S Materials World Network: An International Collaborative Educational and Research Program in the Study of Mixed Glass Former PhenomenaSteve W. Martin, Iowa State University, DMR-0710564 Background: In the Mixed Glass Former Effect (MGFE), mixing two glass formers at constant alkali content produces logarithmic changes in the ionic conductivity and in the thermal and mechanical properties. Such improved glasses are attractive solid electrolytes in future sodium batteries. Objectives: In an international collaboration between the US and Germany, the atomic-level short range structure (SRS) origins of the MGFE are being examined. In this particular system, Na Ge P S, the dc Na+ ion conductivity reaches a minimum as the two glass formers are mixed. In this study we seek an atomic level understanding of the origins of this first-ever observed negative MGFE. 0.50 Fig. 1 Fig. 2 Results:The Na+ ion conductivity decreases and the activation energy increases as Ge is added to the series 0.5Na2S + 0.5[xGeS2 + (1-x)PS5/2], Fig. 1. Raman and 31P NMR spectroscopies, Figs. 2 and 3, were used to determine the SRS of these glasses, Fig. 4. As Ge is added to the glass, P takes charge away from Ge to form more highly charged units, P1 P0 units and Ge forms more polymerized units Ge2 Ge3, Fig. 5. Hence, a fraction of the total charge (x) that would be expected to reside on Ge is preferentially taken by the P, red symbols, Fig. 4. This extra charge causes the P SRS units to become depolymerized and more negative charge resides on these smaller SRS P units. These more negative P SRS units create tighter binding sites for the Na+ cations and the conductivity decreases with Ge additions Conclusions: The Na+ ion conductivity minimum in these Na Ge P S glasses may be associated with the formation of higher trapping energy, highly charged, small P SRS units such as P0. PS5/2 GeS2 Fig. 4 Fig. 3 Fig. 4