Silver bromide (AgBr), a soft, pale-yellow, water-insoluble salt known (along with other silver halides) for its unusual sensitivity to light . This property has allowed silver halides to become the basis of modern photographic materials. [2] AgBR is widely used in photographic films and some are believed to be used to make the Shroud of Turin . [3] Salt can be found naturally as the mineral bromarzyrite .
preparation
Although the compound can be found in mineral form, AgBr is usually prepared by the reaction of silver nitrate with an alkali bromide, especially potassium bromide:
Agno 3 (AQ) + KBR (AQ) → AgBr (Ron) + KNO 3 (AQ)
Although less convenient, salt can also be prepared directly from its ingredients.
Modern preparation of a simple, light-sensitive surface involves making an emulsion of silver halide crystals in gelatin, which is then coated onto a film or other support. Crystals are formed by precipitation in a controlled environment to produce small, uniform crystals (typically <1 µm in diameter and containing ~10 12 Ag atoms) called grains.
reactions
Silver bromide readily reacts with liquid ammonia to generate a variety of amine complexes, such as Ag(NHAg).3)2Br and Ag (NH3)2BR2, In general:
AgBr + mNH 3 + (n – 1) Br–→ Ag (NH3)IBR1-not n
Silver bromide reacts with triphenylphosphine to give a Tris(triphenylphosphine) product:
physical properties
crystal structure
AgF, AgCl, and AgBr all have a face-centered cubic (fcc) rock-salt (NaCl) lattice structure with the following lattice parameters:
| compound | crystal | structure | lattice, a /Å |
| AGF | FCC | rock salt, NaCl | 4.936 |
| AgCl, Chlorargyrite | FCC | rock salt, NaCl | 5.5491 |
| AgBr, Bromergyrite | FCC | rock salt, NaCl | 5.7745 |
unit cell structure
The larger halide ions are arranged in cubic close-packing, while the smaller silver ions fill the octahedral gap between them, giving a 6-coordinate structure where a silver ion is surrounded by Ag + 6 Br ions, and its Adverse. The coordination geometry for AgBr in the NaCl structure is unexpected for Ag(I) which typically form linear, triangular (3-coordinate Ag) or tetrahedral (4-coordinate Ag) complexes.
Unlike other silver halides, iodargyrite (AgI) has a hexagonal zincite lattice structure.
solubility
Silver halide has a wide range of solubility. The solubility of AgF is about 6 × 10 7 times the solubility of AgI. These differences are attributed to the relative solvation enthalpies of the halide ions; The solvation enthalpy of fluoride is unusually large. [7]
| compound | Solubility (g/100g H2O ) |
| AGF | १७२ |
| AgCl | 0.00019 |
| AgBr | 0.000014 |
| Movement | 0.000003 |
-synthesis
Although photographic processes have been in development since the mid-1800s, there was no suitable theoretical explanation until 1938 with the publication of a paper by RW Gurney and NF Mott. [8] This paper triggered a large amount of research in the fields of solid-state chemistry and physics, as well as particularly in the silver halide photosensitivity phenomenon. [2]
Further research into this mechanism revealed that the photographic properties of silver halides (particularly AgBr) were the result of a deviation from an ideal crystal structure. Factors such as crystal growth, impurities and surface defects all affect the concentration of point ionic defects and electronic traps, which affect the sensitivity to light and allow the formation of a latent image. [3]Frenkel defect and quadrilateral deformity
The major defect in silver halides is the Frenkel defect, where silver ions are located in high concentrations with their negatively charged silver-ion vacancies (Ag v – ) interspersed (Ag i + ). What is unique about the AgBr Frenkel pairs is that the interstitial Ag i + is exceptionally mobile, and that its concentration in the layer below the grain surface (called the space-charge layer) is far beyond the inner bulk. is more. [3] [9]The formation energy of the Frenkel pair is low at 1.16 eV, and the migration activation energy is unusually low at 0.05 eV (compared to NaCl:2.18 eV for the formation of a Schottky pair and 0 for cation migration). .75 eV) ) These low energies result in large defect concentrations, which can reach close to 1% of the melting point.
The low activation energy in silver bromide can be attributed to the high quaternary polarization of the silver ions; That is, it can be easily deformed from a sphere to an ellipse. This property, a result of the d 9 electronic configuration of the silver ion, facilitates migration into both the silver ion and the silver-ion vacancies, thus giving an unusually low migration energy (for Ag v : 0.29–0.33 eV). , compared to 0.65 eV for NaCl). [9]
Studies have shown that the defect concentration (up to several powers of 10) is highly influenced by crystal size. Most defects, such as interstitial silver ion concentration and surface kinks, are inversely proportional to crystal size, although vacancy defects are directly proportional. This phenomenon is attributed to changes in the surface chemistry equilibrium, and thus affects each defect concentration differently. [3]
The impurity concentration can be controlled by crystal growth or by direct addition of impurities to crystal solutions. Although impurities in the silver bromide lattice are necessary to stimulate Frenkel defect formation, studies by Hamilton have shown that above a specific concentration of impurities, the number of interstitial silver ions and positive kink defects is several orders of magnitude. rapidly diminishes. After this point, only silver-ion vacancy defects, which actually increase by several orders of magnitude, are prominent. [3]electron trap and hole trap
When light is incident on a silver halide grain surface, a photoelectron is generated when a halide loses its electron in the conduction band:
X − + Hν → X + E −
After the electron is released, it will combine with an intermediate Ag i + to form a silver metal atom Ag i 0 :
E – + Ag I + → Ag I 0
Through defects in the crystal, the electron is able to lose its energy and become trapped in the atom. [2] The grain boundaries and the extent of defects in crystals affect the lifetime of the photoelectron, where a crystal with a large concentration of defects will trap an electron much faster than a pure crystal. [10]
When a photoelectron is mobilized, a photohole h• is also formed, which also needs to be neutralized. However, the lifetime of a photohole does not correlate with that of a photoelectron. This description suggests a different trapping mechanism; Malinowski suggests that hole traps may be related to defects resulting from impurities. [10] Once trapped, the holes attract mobile, negatively charged defects in the lattice: the spacing of the interstitial silver Ag v :
]H• + Ag V – H.Ag V
Formation of h.Ag v lowers its energy sufficiently to stabilize the complex and reduce the probability of ejection of the hole back into the valence band (equilibrium constant for the hole-complex in the interior of the crystal is 10 −). 4 is estimated). [10]
Additional investigations on electron- and hole-trapping revealed that impurities may also be an important trapping system. As a result, the interstitial silver ions cannot be reduced. Therefore, these traps are actually loss mechanisms, and are thought to trap inefficiencies. For example, atmospheric oxygen can interact with photoelectrons to form an O – species, which can interact with a hole to reverse the complex and undergo recombination. Metal ion impurities such as copper (I), iron (II), and cadmium (II) have exhibited hole-trapping in silver bromide. [3]crystal surface chemistry;
Once the hole-complexes are formed, they diffuse over the grain surface as a result of the concentration gradient formed. Studies have shown that pores near the grain surface have a longer lifetime than those in the bulk, and these holes are in equilibrium with the adsorbed bromine. The net effect is a balance push to create more holes on the surface. Therefore, as the hole-complexes approach the surface, they separate:
h.Ag v – → h• + Ag v – → Br → FRACTION Br 2
From this reaction equilibrium, the hole-complexes on the surface are continuously consumed, which act as sinks until removed from the crystal. This mechanism provides an equivalent for the reduction of the interstitial Ag i to Ag i 0 , which gives an overall equation:
AgBr → Ag + fraction Br 2
latent image formation and photography
Now that some theories have been presented, the actual mechanism of the photographic process can be discussed. In short, as a photographic film is subjected to an image, the incident of photons on the grain generates electrons that interact to generate silver metal. More photons hitting a particular grain will produce a larger concentration of silver atoms, with between 5 and 50 silver atoms (~10 out of 12 atoms) depending on the sensitivity of the emulsion. The film now has a concentration gradient of silver atomic particles depending on the light of varying intensity in their region, producing an invisible “latent image”. [2] [10]
While this process is taking place, bromine atoms are being generated on the surface of the crystal. To collect the bromine, a layer on top of the emulsion, called a sensitizer, acts as the bromine acceptor. [10]
During film development the latent image is intensified by the addition of a chemical, specifically hydroquinone, which selectively reduces grains that contain silver atoms. The process, which is sensitive to temperature and concentration, will completely reduce the grain to silver metal, sharpening the latent image on the order of 10 to 10 . This step demonstrates the advantage and superiority of silver halide over other systems: the latent image, which takes only milliseconds to form and is invisible, is sufficient to form a complete image from it. [2]
After development, the film is “fixed”, during which the remaining silver salt is removed to prevent further reduction, leaving a “negative” image on the film. The agent used is sodium thiosulfate, and reacts according to the following equation:
AgX (s) + 2 Na 2 S 2 O 3 (aq) → Na 3 [Ag (S 2 O 3 ) 2 ] (aq) + NaX (aq)
An indefinite number of positive prints can be produced from a negative by passing light through it and following the same steps outlined above. [2]
semiconductor properties
As silver bromide is heated to within 100 °C of its melting point, an Arrhenius plot of the ionic conductivity shows an increase in value and an “upward turning”. Other physical properties such as elastic moduli, specific heat and electronic energy gaps also increase, suggesting that the crystal is approaching instability. [9] This behavior, typical of a quasi-conductor, is attributed to the temperature-dependence of Frenkel defect formation, and, when normalized against the concentration of Frenkel defects, the Arrhenius plot becomes linear.
