Impedance measurements on purified silver chloride crystals using ionic vs. electronic contacts

Impedance measurements are made on purified silver chloride crystals covering thicknesses from 0.064 cm to 0.400 cm at temperatures from 293 K to 310 K using constant ionic strength solution contacts with varying silver ion concentrations. Results characterize the macroscopic transport properties from 10 kHz to 0.01 Hz and at d.c. Electrical relaxation times, τ_el, are independent of crystal thickness and bathing solution composition, but dependent on temperature. The resistive component of the impedance, R_∞, is linear with crystal thickness, δ, and the geometric capacitance, C_g, linear with δ^?1. In agreement with Macdonald's theory, R_∞ is equal to the d.c. resistance, R₀. This indicates rapid ion exchange at the surface and no Warburg diffusion. The dimensionless dielectric constant, calculated from C_g and the crystal dimensions, is 15.4. An average activation energy of 0.298 eV, indicative of vacancy transport is found. The parameter α in the equivalent non-ideal Cole-Cole representation is 0.96 indicating a narrow distribution of time constants. These results are compared with the electronic contact measurements typical to solid state physics. Higher values of resistivity and lower values of C_g and conductivity are found with graphite and vapor deposited silver. These differences were as large as 10%. Calculated dielectric constants at 10 kHz were 14.8 and 13.5, respectively, while the most recent published value is 11.14. These differences suggest that ionic solution contact measurements may give more reliable values because surface coverage is more nearly uniform on the molecular level.