Fast-Ion Conduction and Flexibility of Glassy Networks

We observe two thresholds in the variations of electrical conductivity of dry solid electrolyte (AgI) x(AgPO3)1-x glasses, when the AgI additive concentration x increases to 9.5% and to 37.8%. Raman scattering complemented by calorimetric measurements confirm that these thresholds are signatures of the rigidity phase transitions; at x = 9.5% from a stressed rigid to an isostatically (stress free) rigid phase, and at x = 37.8% from isostatically rigid to a flexible phase. In the flexible phase, the electrical conductivity seems to increase as a power of x, this is in good agreement with the theoretical prediction based on 3d percolation.

The solid electrolytes, AgI, Ag2S, Ag2Se, exist in a non-crystalline or glassy phase, usually not as stoichiometric solids but as additives in base network glasses [1]. These additives can either segregate [1, 2] as separate phases, or uniformly mix [1] with the base glass to form homogeneous solid electrolyte glasses. Understanding of ion-transport in these systems is a basic scientific challenge, with important technological consequences. These materials find use in batteries, sensors, non-volatile memories for portable devices [3] and electro-chromic displays [4].

Phase Diagrams of Disordered Solids

Phase diagrams of disordered solids based on connectedness of their backbones have their origin in the simple and elegant ideas of mechanical constraints. Notion of constraints in mechanics were introduced by J.L.Lagrange [10], and applied to understand mechanical stability of macroscopic structures by J.C. Maxwell [11], and to model elastic behaviour of covalent glassy networks by J.C. Phillips [12] and M.F. Thorpe [13]. That there are actually three (Flexible, Intermediate and Stressed-Rigid) and not two (Flexible and rigid) elastic phases of disordered solids is a more recent development [14-17]. This new understanding has been useful in hanlding problems such as the unfolding process of proteins [13], design of thin-film gate dielectrics for transistors [15], and Satisfiability problems in Computational science [16].

Vibrational Anomalies in Glassy Electrolytes

The thermal and electrical results above lead to the obvious question- are there vibrational anomalies associated with structure of these electrolyte glasses as were observed earlier in covalent systems [17, 24]? This, indeed, is the case as revealed by our Raman scattering results (Fig.2). The base glass is widely believed [20, 25, 26] to consist of chains of quasi-tetrahedral PO4 units with each P atoms having two bridging (Ob) and two terminal (Ot) oxygen near-neighbors. In the base glass (x = 0), modes near 1140 cm-1 and 684 cm

The observed Raman lineshapes when analyzed in terms of a superposition of Gaussians provide variations in frequency of the P-Ot symmetric mode near 1140 cm-1 (Fig. 2 and 3a). The mode is found to steadily red-shift displaying two vibrational thresholds, one near x = 9.5%, and the other near x = 37.8% which correlate rather well with the walls of the reversibility window (fig.1b) and the steps in electrical conductivity. Red-shift of the mode in question occurs as the inter-chain spacings increase due to insertion of AgI lowering the global connectivity of the backbone.

Conclusions and Future Implications

The thermal, optical and electrical results presented above lend themselves to the following interpretation. The reversibility window, 9.5% < x < 37.8%, in analogy to the case of covalent glasses [17, 24], we identify with the Intermediate phase of the present glass network. As such, we can conclude that understanding the varying elasticity phases of these solid electrolyte glasses is the key to unlocking its potential application in various fields ranging from battery technology to sensor systems and computational science.