Silver bromide octahedral

Spittler and Willig and co-workers measured the temperature dependence of k for a cyanine dye adsorbed onto silver bromide octahedral crystals [196]. Nonexponential experimental decays were observed and a curve subtraction technique... 

Muhr, H., R. David, J. Villennaux, and P. H. Jezequel (1995). Crystallization and precipitation engineering—V. Simulation of the precipitation of silver bromide octahedral crystals in a double jet semi-batch reactor, Chem. Eng. Sci., 50(2), 345-355. 

Silver bromide crystals, formed from stoichiometric amounts of silver nitrate and potassium bromide, are characterized by a cubic stmcture having interionic distances of 0.29 nm. If, however, an excess of either ion is present, octahedral crystals tend to form. The yellow color of silver bromide has been attributed to ionic deformation, an indication of its partially covalent character. Silver bromide melts at 434°C and dissociates when heated above 500°C. 

The crystal stmcture of photographic silver bromide is often octahedral. 

When Berthier treated a specimen of this ore from the San Onofe Mine with an excess of hot ammonium hydroxide, he observed, mixed with the metallic silver, a green powder which had been only incompletely attacked. This was the circumstance, said he, which drew my attention to the ore from Plateros and which led me to realize that the substance which had been taken for silver chloride is pure bromide, without admixture of chloride or iodide, a substance which had not yet been met within the mineral realm and which therefore constitutes a new species (151). Berthier learned that this mineral is not rare in Mexico but is often found in beautiful cubic and octahedral crystals. He also found the same mineral at Huelgoeth, Department of Finistere, France, and discovered some of it among the Chilean silver minerals which Ignaz Domeyko, professor of chemistry at the College of Coquimbo, had sent to the School of Mines at Paris (151, 152). The mineral which Berthier analyzed was evidently bromyrite (silver bromide). 

Point defects. Point defects (Fig. 5.1) are limited to a single point in the lattice, although the lattice will buckle locally so that the influence of point defects may spread quite far. A Frenkel defect consists of a misplaced interstitial atom and a lattice vacancy (the site the atom should have occupied). For example, silver bromide, which has the NaCl structure, has substantial numbers of Ag+ ions in tetrahedral holes in the ccp Br array, instead of in the expected octahedral holes. Frenkel defects are especially common in salts containing large, polarizable anions like bromide or iodide. 

Fig. 1. Using the carbon replica technique, this is an electron micrograph of octahedral silver bromide grains. (Photo by Dr. Donald L Black, Eastman Kodak Company)...

The combined (S + Au)-sensitization usually is applied during the manufacture of the emulsion, but sensitization can be achieved also by bathing a coated S-sensitized emulsion in an aurous thiocyanate solution before exposure (161). Reduction sensitization can be combined with (S + Au)-sensitization under some conditions. Collier (162) found that reduction sensitization either before or after (S + Au)-sensitization increased both sensitivity and fog in a l-ym octahedral grain silver bromide emulsion. The largest increase in sensitivity and lowest increase in fog were achieved when reduction sensitization was applied after the S+Au. Hydrogen hypersensitization likewide is effective (108) and can produce large increases in sensitivity for some emulsions. 

Photographic emulsions composed of octahedral silver bromide grains were used in these experiments. The average grain size was 0.7 pm, or 0.2 pm. Methanolic solutions of various dyes were added to the emulsions, which were then coated at about 40 mg silver bromide/dm and at about 30 mg gelatin/dm on cellulose triacetate film base. The coated films were subjected to both ESR and sensitometric measurements at room temperature. 

Fig. 5.27 Silver bromide adopts an NaCl lattice, (a) An ideal lattice can be described in terms of Ag ions occupying octahedral holes in a cubic close-packed array of bromide ions, (b) A Frenkel defect in AgBr involves the migration of Ag ions into tetrahedral holes in the diagram, one Ag+ ion occupies a tetrahedral hole which was originally vacant in (a), leaving the central octahedral hole empty. Colour code Ag, pale grey Br, gold.

Evans FJ, Gilman P (1975) Comparison of the spectral sensitization of surface and internally sensitized core/shell octahedral silver bromide emulsions. Photogr Sci Eng 19 344-351... 

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