Impedance measurements using ionic contacts on purified and doped silver bromide crystals

Abstract

Impedance measurements are made on purified, vacancy-doped and interstitial-doped silver bromide crystals of thicknesses from 0.061 cm to 0.260 cm using constant ionic strength solution contacts at temperatures from 20°C to 35°C. Results characterize the macroscopic transport properties from 316 kHz to 0.01 Hz and at d.c. In all cases the high frequency limit resistive component of the impedance, R ∞ , is equal to the d.c. resistance R 0 . This result indicates rapid ion exchange at the surfaces and neither Warburg diffusion nor surface kinetic-limitation of transport. As expected, R ∞ is a function of charge carrier concentration, which depends on extrinsic dopant concentration over the temperature range measured. For pure crystals the resistance, R ∞ , is linearly dependent on crystal thickness, δ, while thickness correlation could not be tested for the doped crystals as dopant level was not uniform. The geometric capacitance for all crystals, C g , is linear with δ −1 . The dimensionless dielectric constant, κ, calculated from C g and crystal dimentsions, is 13.9±0.7. The electric relaxation time, τ el , defined as the product R ∞ C g , at 25°C is found to be 50±3 μs for pure crystals, 1 to 4 μs for the Cd 2+ -doped crystals and 12 to 66 μs for the S 2− -doped crystals. Temperature dependance of R ∞ allows determination of crystal transport activation energies. For pure and Cd 2+ -doped crystals a value of 0.33±0.02 eV is found. For the S 2+ -doped crystals values from 0.48 to 0.28 eV are found. The parameter α, in the non-ideal Cole-Cole representation of impedance plane arcs, is 0.96±0.01 for purified crystals, 0.93±0.03 for Cd 2+ -doped, and 0.92±0.02 for S 2− -doped crystals.