Silver bromide cubic

Early work showed that low light intensity excitation sources had to be used to avoid excited-state annihilation processes [193, 194]. Using a low-intensity mode-locked laser source and single photon counting, Muenter measured the rates of fluorescence decay, k, for some typical monomeric cyanine dyes on silver chloride and silver bromide cubic crystals [195] k was obtained from Eq. (108) for k, and assuming that kf + A nr) could be determined from measurements of dye lifetimes in... 

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. 

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). 

Fig. 2. Using the carbon replica technique, this is an electron micrograph of cubic silver bromide grains in which the comers have been slightly rounded due to the presence of a silver complexing agent. (Photo by Dr. Donald h Black, Eastman Kodak Company)...

In our experiments with the low-temperature sensitivity of monodisperse silver bromide emulsions having 0.5 ym cubic grains, the sensitivity of the (S+Au)-sensitized emulsion differed from that of the S-sensitized emulsion both with respect to the dependence on the degree of sensitization and the correlation with microwave photoconductivity measurements (16). 

Figure 50. Dependence of Orel on sensitizer reduction potential [187], The solid curve represents the expected dependence based on Marcus theory using a value for I of 0,05 eV. The energy obtained for the CB of the cubic silver bromide crystals used in this experiment, F red, is -1.37 V, vs. SCE. Figure adapted from [187].

Iron (II) sulphide never has the precise composition FeS—the sulphur is always present in excess. This could be due either to the inclusion in the lattice of extra, interstitial S atoms or to the omission from it of some of the Fe atoms. The second explanation is correct (Hagg and Sucksdorff, 1933), the phenomcon being an example of lattice defect (p. 152). There are two types of lattice defect. In Schottky defects, found in iron(Il) sulphide, holes are left at random through the crystal because of migration of ions to the surface. In Frenkel defects, holes are left at random by atoms which have moved to interstitial positions. Silver bromide has a perfect face-centred cubic arrangement of Br ions but the Ag+ ions are partly in interstitial positions. The effect is even more marked in silver iodide (p. 153). 

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.

The surface area associated with a given mass of material subdivided into equal-size particles increases in inverse proportion to the linear dimensions of the particles. Thus the area exposed by unit mass (the specific surface area, as) is given by 6/pd, where p is the density of the material and d is the edge length in the case of cubic particles or the diameter in the case of spheres. If the material is made up of molecules of linear dimension h and molecular volume h3, then the fraction of molecules in the surface layer is given approximately by 6 h/d). Thus for a substance of molar volume 30cm3moP or of molecular volume 0.05 nm3 (e.g. silver bromide) h = 0.37 nm. For a 1 cm cube only... 

Silver bromide, AgBr, has a cubic unit cell with an edge of 0.576 nm. There are four silver atoms in the unit cell assume that there are four interstitial positions available for silver atoms. Calculate the absolute number of interstitial defects present per cubic metre at 300 K. 

AgBr SILVER BROMIDE Molecular weight 187.80 System cubic a- 0.5755 Density 6470 Z 4 ... 

Light yellow, odorless powder slowly darkened by light. Crystals are hexagonal Or cubic, d 5.67 mp 552. Practically insol in water (0.03 mg/l) in acid (except coned HI in which it dissolves readily on heating) in ammonium carbonate. Freely sol in solns of alkali cyanides or iedides 35 mg dissolve in a liter of 10% ammouia appreciably sol in coned solns of alkali bromides, chlorides, thiocyanates, thiosulfates, mercuric and silver nitrates. It is slowly attacked by boiling coned acids, but not affected by hot solns of alkali hydroxides. 

Silver chloride and bromide have been synthesized by Shah and group [317] by mixing reverse microemulsions I and II containing NaAOT/alkane (e.g. n-heptane)/aqueous solution of silver nitrate (I) and NaAOT/alkane/aqueous solution of sodium chloride or bromide (II). This led to the precipitation of AgCl or AgBr. The particles were spherical in shape and had a diameter of 5-10 nm the synthesized AgCl crystallized with a cubic (fee) structure. 

Embolite [Named from the Greek for intermediate, alluding to the l l ratio of chloride and bromide] (ICSD 9252 and PDF 26-964) Ag(Br,Cl) M = 165.55 65.16 wt.%Ag 24.13 wt.% Br 10.71 wt.% G (Halides) Cubic Isotropic 1 p=2.15 Habit massive. Color yellowish green or grayish yellow. Luster adamantine, greasy. Diaphaneity transparent to translucent. tr white. Occurrence oxidized portions of silver deposits. 

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