Ionic conductivity and space charge region in silver bromide (111) microcrystals

In order to determine the ionic conductivity of AgBr (111) microcrystals of photographic emulsions, measurements of the dielectric loss are made between 250 and 330 K. Undoped as well as emulsions doped with PbBr2 are investigated. The results are interpreted on the basis of a space charge model. It is shown that the conductivity at 307 K for a AgBr (111) microcrystal in an undoped emulsion is 2.6 × 10−6 ω−1 cm−1 which is two orders of magnitude higher than that of large single crystals. The free energy of formation at room temperature is found to be 0·23 eV for an interstitial silver ion and 0·65 eV for a vacancy, respectively. For the surface potential a value of -0.35 eV is obtained. This means that the surface of a (111) AgBr grain is charged negatively while a compensating positive space charge is built up by interstitial silver ions. It is further shown that Pb2+ ions located on the surface of an emulsion grain do not alter the surface potential nor the space charge characteristics.