Interstitial silver

Fig. 9. Schematic of a two-dimensional cross section of an AgBr emulsion grain showing the surface and formation of various point defects A, processes forming negative kink sites and interstitial silver ions B, positive kink site and C, process forming a silver ion vacancy at a lattice position and positive kink...

The liberated electron is free to move in the structure and migrates to an interstitial silver ion, Ag, which is part of a Frenkel defect, to form a neutral silver atom Ag ... 

The silver ion produced in this reaction is then neutralized by association with another interstitial silver atom, thus ... 

The influence of light causes changes similar to that occurring in a photographic emulsion. The photons liberate electrons and these are trapped by interstitial silver ions, which exist as Frenkel defects, to form neutral silver atoms. Unlike the photographic process, the electrons are liberated by the Cu+ ions, which are converted to Cu2+ ions (CuAg) in the process ... 

The silver halide crystals show ionic conductivity by Frenkel defects (interstitial silver ions, Ag"t). 

The electron, if not lost by recombination or some secondary process, can combine with an interstitial silver ion or possibly a surface silver ion to form a silver atom. The photolytic silver is formed as, or aggregates to form, nuclei rather than existing as individual atoms distributed throughout the crystal. Only one nucleus consisting of a few silver atoms is needed to produce a detectable photographic effect. 

Levi and associates (26) showed that the decay kinetics in the Dember effect measured on a (S+Au)-sensitized iodobromide emulsion could be expressed by the sum of first- and second-order reactions. They attribute the first-order process to the reaction of electrons and interstitial silver ions... 

In 1938, Gurney and Mott (212) proposed a mechanism that depends on the movement and trapping of photoelectrons and the movement of interstitial silver ions to the trapped electrons. They wrote ... 

In the Gurney-Mott mechanism, the trapped electron exerts a coulombic attraction for the interstitial silver ion. This attraction would be limited to a short distance by the high dielectric constant of the silver bromide. Slifkin (1) estimated that the electrostatic potential of a unit point charge in silver bromide falls to within the thermal noise level at a distance of "some 15 interatomic spacings." The maximum charge on the sulfide nucleus would be 1 e. The charge on a positive kink or jog site after capture of an electron would not exceed e/2. An AgJ would have to diffuse to within the attraction range before coulombic forces could become a factor. 

Fig. 1. Defect chemical phenomena (a) formation of a silver vacancy and an interstitial silver ion in AgCl (b) formation of a silver vacancy in AgCl by doping with CdCl2 (c) non-stoichiometry in an oxide caused by the excorporation of oxygen.

The latent image catalyzes the reduction of silver ion either from the solid silver halide phase, as in chemical development, or from a soluble source of silver ion, as in physical development (Figure 21). One view of chemical development is that interstitial silver ions move through the silver halide crystal and are reduced on the underside of the latent image speck. In purely physical development complexed silver ion moves through the solution and is reduced on the nucleus. In this sense physical development and the early stages of chemical development are similar. 

Matejec and Meyer [101] suggest that the concentration of interstitial silver ions is insufficient to account for the rates of development observed in practice. Calcu-... 

Certain acid dyes [67] stabilize silver oxalate by forming surface compounds, while other dyestufis accelerate the decomposition because their redox properties enhance the ease of electron transfer from the oxalate ion to the silver. The influences of incorporated cadmium, copper and other ions on the rate of thermal decomposition, and on the concentration and mobility of interstitial silver ions, have been reviewed [46,68]. 

The layer is thought to be doped with interstitial silver atoms which act as electron donors and endow the layer with n-type semiconducting properties. The work function of such a catalyst falls progressively, up to 7 mol % addition of barium. The authors have argued that this effect will favour the adsorption of the more strongly polarized oxygen species, i.e. atomic rather than molecular oxygen, which results in the observed loss of selectivity and increase in activity. 

Figure 1. Mechanism of latent-image growth in silver halide. Key e, electrons Ag, interstitial silver ion h , holes T, traps and S, special sites.

Frenkel defects and impurity ions can diffuse through the silver halide lattice by a number of mechanisms. Silver ions can diffuse by a vacancy mechanism or by replacement processes such as collinear or noncollinear interstitialcy jump mechanisms [18]. The collinear interstitial mechanism is one in which an interstitial silver ion moves in a [111] direction, forcing an adjacent lattice silver ion into an interstitial position and replacing it The enthalpies and entropies derived from temperature-dependent ionic conductivity measurements for these processes are included in Table 4. The collinear interstitial mechanism is the most facile process at room temperature, but the other mechanisms are thought to contribute at higher temperatures. 

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