Bonding, structure and compositional distribution in sheet crystal silver iodobromide (111)

Abstract

The addition of small amounts of iodide to the silver bromide is known to have a significant effect on the photographic properties of silver halides. The change in the photographic sensitivity depends on both the concentration and distribution of iodide ions. Hence, it is important to obtain detailed information on the chemical bonding and compositional distribution of iodide in silver halide alloys. The depth distribution of iodide ions was determined with the ISS (ion scattering spectrometry) technique at both the top and bottom (111) surface layers of AgBr 0.97I 0.03, AgBr 0.94I 0.06 and AgBr 0.90I 0.10 sheet crystals grown on a flat Vycor® (quartz) substrate. ISS data show that iodide ions are more heavily segregated at the last-solidified surface. For the first-solidified surface, there is a smaller enhancement of iodide ions within the top 100 Å region relative to the rest of the near-surface region. Systematic fluorescence EXAFS (extended X-ray absorption fine structure) and SEXAFS (surface EXAFS) measurements were made at 35 ± 5 K on both the Br and I absorption edges to study the local iodide distribution, bonding and structure of the top surface and bulk layers. EXAFS and SEXAFS data show that there are significant differences in the local iodide distribution and silver ion vacancy concentration between the bulk and surface layers. These differences vary with the concentration of iodide ions in the samples. For the bulk layers, there are no changes in the local iodide distribution and silver vacancy concentration as a function of iodide concentration, in contrast to the surface layers where significant differences were observed. Changes in the iodide concentration, local iodide distribution and silver vacancy concentration have negligible effects on the nearest-neighbor distances for both the surface and bulk layers. The nearest-neighbor I-Ag and I-Br distances are found to be slightly larger that the corresponding Br-Ag and Br-Br distances.