Evidence for Intermediate Phase in Solid Electrolyte Glasses

Here we examine a dry solid electrolyte (AgI)_x(AgPO₃)_1-x glasses in the 0 < x < 0.54 composition range, and provide evidence for existence of the three generic elastic phases (floppy, intermediate (IP) and stressed Rigid). IPs are marginally rigid elastic phases with unusual functionalities and excited much interest in the field. Our experiments have included Raman scattering, Infrared reflectance, modulated differential scanning Calorimetry (MDSC), AC electrical conductivity and molar volume experiments. Our experiments reveal that the physical properties of these glasses including the glass transition temperatures (T_gs) and the elastic phases display substantial dependence on handling of precursors in synthesis of samples.

Previous reports on T_gs of AgPO₃ base glass lie in the 163 °C < T_g < 192 °C range when precursors are processed at laboratory ambient (relative humidity (RH) ~ 50 percent). However, we found that as RH decreases, T_gs steadily increase in the 181(2)°C < T_g < 254(2)°C range. T_g of dry AgPO₃ is found to be 254(2)°C. Infrared reflectance shows the concentration of both free and bonded water increases in the low-T_g (181(2)°C) compared with the high-T_g (254(2) °C) samples. In Raman scattering, the Boson peak scattering strength increases, and the scattering strength ratio, R, of the P-O_t (terminal) to P-O_b (bridging) mode of the P-O-P- chains decreases as sample T_gs increase.

Moving next to (AgI)_x(AgPO₃)_1-x glasses, calorimetric measurements have permitted establishing glass compositions across which T_g become thermally reversing, the reversibility window (RW), also identified with IPs which fixes the three elastic phases. Results on driest glasses (set A) show RW to be in the 0.095 < x < 0.378 range, and for the less dry samples (set B) in the 0.22 < x < 0.35 range. Variations in Raman mode frequency of the P-O_t stretch vibration of chains, υ(x), has permitted a measurement of optical elasticity power-laws; in the stressed-rigid phase the power-law is found to be p₁ = 1.25(2), and in the flexible phase p₂ = 0.98(3). These values corroborate the elastic phase identifications. Ionic conductivities reveal a step-like increase when glasses become stress-free at x > x_c(1) = 0.095, and a logarithmic increase in conductivity (σ ~ (x-x_c(2)^μ) once they become flexible at x > x_c(2) = 0.378 with a power-law μ = 1.78 (10). The power-law is consistent with percolation of 3D filamentary conduction pathways. Ideas on network flexibility promoting ion-conduction are in harmony with the unified approach of Ingram et al. who have emphasized the similarity of process compliance or elasticity relating to ion-transport and structural relaxation in decoupled systems. Boson mode frequency and scattering strength display thresholds that coincide with the two elastic phase boundaries. In particular, the scattering strength of the boson mode increases almost linearly with glass composition x, with a slope that tracks the floppy mode fraction as a function of mean coordination number r predicted by mean-field rigidity theory.These data suggest that the excess low frequency vibrations contributing to boson mode in flexible glasses come largely from floppy modes.