Structural and nonstructural factors in fast ion conduction inAg2SO4at high pressure

The ac electrical conductivity of ${\mathrm{Ag}}_2{\mathrm{SO}}_4$ has been measured as a function of pressure up to 52 kbar and in the temperature range 300–1000 K. At low $P,$ the orthorhombic to hexagonal transition, with positive $\Delta V_{\mathrm{ortho}\to \mathrm{hex}},$ is accompanied by a large increase in conductivity suggesting dominant control of lattice volume on fast ion conduction. The high-$P$ conductivity data confirm the maximum in the orthorhombic-hexagonal $P-T$ phase boundary at ∼16 kbar as reported in an earlier differential thermal analysis study. However, at high $P,$ where $\Delta V_{\mathrm{ortho}\to \mathrm{hex}}$ is negative, the persistent but muted increase in conductivity across the orthorhombic to hexagonal boundary suggests that $P$ induced changes in nonstructural factors dominate in controlling conduction. The $\sigma -P$ data show a pronounced trough between 15 and 20 kbar which virtually disappears at the orthorhombic to hexagonal transition at ∼720 K. Above 720 K, the decrease in σ becomes more gradual with $P.\mathrm{}$ The activation energy, $Q_c,$ for ionic conductivity in the hexagonal phase is effectively independent of $P$ up to ∼10 kbar followed by a precipitous drop at 16 kbar with complete recovery at ∼30 kbar. The coincidence of the minimum in $Q_c$ and the maximum in the orthorhombic-hexagonal $P-T$ phase boundary is discussed in terms of the deformability of the ${\mathrm{Ag}}^{+}$ ion and its effects on bond strength and the thermodynamic constraints of Clapeyron slopes, respectively. The activation volume for both the orthorhombic and hexagonal phases is strongly $T$ dependent for $P<\mathrm{}16\mathrm{}$ kbar but $T$ independent for $20<P<\mathrm{}52\mathrm{}$ kbar. The results are compared with previously reported results of parallel studies on ${\mathrm{Na}}_2{\mathrm{SO}}_4$ and ${\mathrm{AgTlSO}}_4.$