Correlation Between I-Ag Distance and Ionic Conductivity in AgI Fast-Ion-Conducting Glasses

A large number of AgI-based fast-ion-conducting glasses have been investigated by K-iodine extended x-ray absorption fine structure spectroscopy (EXAFS) measurements at liquid nitrogen temperature. A general correlation between the I-Ag distance measured by EXAFS and the glass activation energy for dc ionic conductivity has been found out: glasses with longer I-Ag distances display higher ionic conductivity, independently from the chemical composition of their host glassy matrix. This behavior can be related to the progressive increase of the “pathway volume” for ionic conduction.

Figure 1

EXAFS signals $k\chi (k)$ (top panels) and corresponding Fourier transforms (bottom panels) in crystalline $\beta$-AgI at 300 K (panels a) and in three selected glasses at LN temperature: $(\mathrm{AgI}{)}_{0.50}({\mathrm{Ag}}_2\mathrm{O}:4·{\mathrm{B}}_2{\mathrm{O}}_3{)}_{0.50}$ (panels b), $(\mathrm{AgI}{)}_{0.50}({\mathrm{AgPO}}_3{)}_{0.50}$ (panels c), $(\mathrm{AgI}{)}_{0.70}({\mathrm{Ag}}_2{\mathrm{WO}}_4{)}_{0.30}$ (panels d). Dashed and continuous lines in the Fourier transform are the imaginary part and the modulus, respectively.

Figure 2

Mean I-Ag distance measured by EXAFS in different families of AgI-based glasses at LN temperature, plotted against their activation energy for dc ionic conductivity. The symbols refer to the glass family: $\otimes =\mathrm{AgI}\mathrm{\text{−}}{\mathrm{Ag}}_2\mathrm{O}\mathrm{\text{−}}1·{\mathrm{B}}_2{\mathrm{O}}_3$, $○=\mathrm{AgI}\mathrm{\text{−}}{\mathrm{Ag}}_2\mathrm{O}\mathrm{\text{−}}2·{\mathrm{B}}_2{\mathrm{O}}_3$, $\oplus =\mathrm{AgI}\mathrm{\text{−}}{\mathrm{Ag}}_2\mathrm{O}\mathrm{\text{−}}3·{\mathrm{B}}_2{\mathrm{O}}_3$, $●=\mathrm{AgI}\mathrm{\text{−}}{\mathrm{Ag}}_2\mathrm{O}\mathrm{\text{−}}4·{\mathrm{B}}_2{\mathrm{O}}_3$, $■=\mathrm{AgI}\mathrm{\text{−}}{\mathrm{AgPO}}_3$, $◊=\mathrm{AgI}\mathrm{\text{−}}{\mathrm{Ag}}_2{\mathrm{MoO}}_4$, $⧫=\mathrm{AgI}\mathrm{\text{−}}{\mathrm{Ag}}_2{\mathrm{WO}}_4$. Dashed line is the reference value of crystalline $\beta$-AgI at LN temperature.

Figure 3

The linear relationship between activation energy for dc ionic conductivity and cube root of the volume expansion $V-V_0$ between Ag and I ions. Symbols are the same as in Fig. 2.