Effect of pressure on ionic conductivity in rubidium silver iodide and silver iodide

The effect of pressure on the ionic conductivity of Rb${\mathrm{Ag}}_4$ ${\mathrm{I}}_5$ and AgI has been measured, using single crystals and polycrystalline samples, up to pressures of 6 kbar. The activation volumes for motion in $\alpha -\mathrm{Rb}{\mathrm{Ag}}_4{\mathrm{I}}_5$ and $\beta -\mathrm{Rb}{\mathrm{Ag}}_4{\mathrm{I}}_5$, respectively, are -0.4 ± 0.2 and -0.2 ± 0.1 ${\mathrm{cm}}^3$/mole. In $\alpha -\mathrm{AgI}$, the motion volume increases from 0.56 ± 0.1 ${\mathrm{cm}}^3$/mole at 435 K to 0.8 ± 0.1 ${\mathrm{cm}}^3$/mole at 623 K. These values are unusually small in relation to the activation energies and are not consistent with the strain-energy model or a domain-diffusion mechanism. The logarithms of the ionic conductivities of $\alpha$- and $\beta -\mathrm{Rb}{\mathrm{Ag}}_4{\mathrm{I}}_5$ increase linearly at first and then decrease quadratically with pressure. This is related to the large quadratic pressure dependence of the second-order transition temperature $\Delta T_c\left(\mathrm{K}\right)=0.141\mathrm{}P\left(\mathrm{kbar}\right)+0.111\mathrm{}{P}^2\left({\mathrm{kbar}}^2\right)$. The variation of the 122-K transition temperature with pressure is $\Delta T_c\left(\mathrm{K}\right)=5.65\mathrm{}P\left(\mathrm{kbar}\right)-0.53\mathrm{}{P}^2\left({\mathrm{kbar}}^2\right)$, implying a molar volume change of $V_{\beta }-V_{\gamma }=0.37\mathrm{}\pm 0.01$ ${\mathrm{cm}}^3$/mole and a change in compressibility $K_{\beta }-K_{\gamma }=(0.033\mathrm{}\pm 0.001)\times {10}^{-11\mathrm{}}$ ${\mathrm{cm}}^2$/dyn across the transition. The ionic conductivity of $\gamma -\mathrm{Rb}{\mathrm{Ag}}_4{\mathrm{I}}_5$ initially decreases with an activation volume of 9 ± 1 ${\mathrm{cm}}^3$/mole, and then levels off with increasing pressure. The negative activation volume for conduction along the $c$ axis in $\beta -\mathrm{AgI}$ has been confirmed. Both low-temperature phases have large formation volumes consistent with the theory of Rice et al. of transitions to the superionic phase.