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

The effect of pressure on the ionic conductivity of RbAg₄{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 ${α}{-}Rb{Ag}₄{I}₅$ and ${β}{-}Rb{Ag}₄{I}₅, respectively, are -0.4 ± 0.2 and -0.2 ± 0.1{cm}³$/mole. In ${α}{-}AgI, the motion volume increases from 0.56 ± 0.1{cm}³/mole at 435 K to 0.8 ± 0.1{cm}³$/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 α- and β-RbAg₄I₅ 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 ΔTc(K)=0.141P(kbar)+0.111P²(kbar²). The variation of the 122-K transition temperature with pressure is ΔTc(K)=5.65P(kbar)-0.53P²(kbar²), implying a molar volume change of V_β-V_γ=0.37±0.01 cm³/mole and a change in compressibility K_β-K_γ=(0.033±0.001)×10^-11 cm²/dyn across the transition. The ionic conductivity of γ-RbAg₄I₅ initially decreases with an activation volume of 9 ±{} 1 cm³/mole, and then levels off with increasing pressure. The negative activation volume for conduction along the c axis in β-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.