Silver bromide (AgBr), a soft, pale-yellow, water-insoluble salt that is well known (along with other silver parts) for its unusual sensitivity to light . This property has allowed silver skins to become the basis of modern photographic materials. [2] AgBr is widely used in photographic films and is believed to have been used for some of the Shroud of Turin . [3] Salt can be found naturally as the mineral Bromargyrite .
preparation
Although the compound can be found in mineral form, AgBr is usually prepared by the reaction of silver nitrate with alkali bromide, usually potassium bromide :
AgNO 3 (aq) + KBr (aq) → AgBr (s) + KNO 3 (aq)
Although less convenient, salt can also be prepared directly from its ingredients.
Modern preparation of a simple, light-sensitive surface involves creating 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) which are called grains.
reactions
Silver bromide readily reacts with liquid ammonia to generate a variety of amine complexes Ag(NH)3)2Br and Ag (NH)3)2BR−2, In general:
AgBr + m NH 3 + (n – 1) Br−→ Ag (NH)3)IBR1-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 / / |
| AGF | FCC | rock-salt, NaCl | 4.936 |
| AgCl, Chlorargyrite | FCC | rock-salt, NaCl | 5.5491 |
| AgBr, Bromargyrite | FCC | rock-salt, NaCl | 5.7745 |
The larger halide ions are arranged in a cubic-packing, while the smaller silver ions fill the octahedral gap between them, giving a 6-coordinate structure where a silver ion Ag. + 6 Br is surrounded by − ion, and vice versa. 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, iodersite (AgI) has a hexagonal zincite lattice structure.
solubility
Silver halides have a wide range of solubility. The solubility of AgF is approximately 6 × 10 times of AgF . These differences are attributed to the relative solvation enthalpies of the halide ions; The discrepancy in the enthalpy of fluoride concentration is large.
| compound | Solubility (g/100g H2O ) |
| AGF | 172 |
| 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. Mot. [8] This paper triggered a great amount of research in the fields of solid-state chemistry and physics, as well as in particular in silver halide optics information. [2]
Further research into this mechanism showed that the photographic properties of silver haldi (especially 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 .
Frenkel defect and quadrilateral deformity
The major defect in silver halides is the Frenkel defect , where silver ions are located interstitial (Ag I + ) at high concentrations because of their negatively charged silver-ion vacancies (Ag v− ) . What is unique about AgBr Frenkel pairs is that the interstitial AgBr. I + are exceptionally mobile, and that its concentration inside the layer below the grain surface (the space-charged layer) exceeds that of the inner bulk. [3] [9] The formation energy of the Frenkel pair is less than 1.16 e.v. , and the migration activation energy is unusually low at 0.05 eV (compared to the NaCl: Schottky pair for the formation of 2.18 eV(0.75 eV for cationic migration). These low energies result in large defect concentrations, which can reach as high as 1% near the melting point. [9]
The low activation energy in silver bromide can be attributed to the high quaternary polarization of the silver ions; That is, it can deform easily from a sphere to an ellipse. This property is a result of the d 9 electronic configuration of the silver ion, facilitating migration in 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 defect concentrations (up to several powers of 10) are affected 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 the crystal solutions. Although impurities in the bromide lattice are necessary to stimulate Frenkel defect formation, studies by Hamilton have shown that above a specific concentration of impurities, the number of defects with interstitial silver ions and positive kinks by several orders of magnitude. Intensifies. After this point, only silver-ion vacancy defects, which actually increase by several orders of magnitude, are prominent.
electron trap and hole trap
When light is incident on the surface of a silver halide grain, a photoelectron is generated when a halide loses its electron to conduction:
x − + hpo → x + e −
After the electron is released, it will combine with an interstitial Ag I + I to form a silver metal atom Ag I 0 :
e − + if i + → if i 0
Through defects in the crystal, the electron becomes able to lose its energy and become trapped in the atom. [2] The grain boundaries and extent of defects in crystals affect the lifetime of the photoelectron, where crystals 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 has to be neutralized. The lifetime of a photohole, however, does not correlate with that of a photoelectron. This description suggests a different trapping mechanism; Malinowski suggests that the pores may be related to defects resulting from mesh impurities. [10] Once trapped, the holes attract mobile, negatively charged defects in the lattice: the interstitial silver vacancy Ag v− :
h • + ag v − G h. a g v
H. The formation of a g reduces its energy sufficiently to stabilize the v complex and reduces the probability of hole ejection back into the hole band (the equilibrium constant for the hole-complex in the interior of the crystal is at 10 Approximate −4 . [10]
Additional investigations on electron- and hole-trapping have shown that impurities may also be an important trap system. As a result, the interstitial silver ions cannot be reduced. Therefore, these traps are actually loss mechanisms, and are considered trapping inefficiencies. For example, atmospheric oxygen can interact with photoelectrons to form O 2 − 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.
crystal surface chemistry;
Once the hole-complexes are formed, they extend to the grain surface as a result of the concentration gradient formed. Studies have shown that the lifetimes of holes near the grain surface are longer than those in the bulk, and that these holes are in equilibrium with the adsorbed bromine. The net effect is a balancing push on the surface to create more holes. Therefore, as the hole-complex approaches the surface, they separate:
H. Ag v − → H • + Ag v − → Br → Fraction Br 2
By balancing this reaction, the hole-complexes are continuously consumed at the surface, which acts as a sink, until removed from the crystal. This mechanism provides the counterpart for the reduction of interstitial Ag I + Ag to I 0 , giving the overall equation of:
AgBr → Ag + FRACTION Br 2
Latent Image Formation and Photography
Now that some of the principles 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 produces electrons that interact to yield silver metal. More photons hitting a particular grain will produce a greater concentration of silver atoms, between 5 and 50 silver atoms (out of ~10). 12 atoms), depending on the sensitivity of the emulsion. The film now has a concentration gradient of silver atomic speckles depending on the light of varying intensity in its region, producing an invisible ” latent image “.
While this process is taking place, bromine atoms are being produced on the surface of the crystal. To collect the bromine, a layer on top of the emulsion, called a sensitizer, acts as a bromine acceptor.
The latent image is usually sharpened by the addition of a chemical during film development, such as hydroquinone , that the selectivity reduces to grains that contain silver atoms. This process, sensitive to temperature and concentration, will completely reduce the grain to silver metal, sharpening the latent image on the order of 10. 10 10 to 11 . This step demonstrates the advantage and superiority of silver pudding over other systems: the latent image, which takes only milliseconds to form and is invisible, is sufficient to produce the full image.
After development, the film is “fixed”, during which the rest of the silver salts are removed to form, 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 Ron 2 O 3 (aq) → Na 3 [Ag(s) 2 O 3 ) 2 ] (aq) + NaX (aq)
An indefinite number of negative numbers can be generated from negatives by passing light through it and performing the same steps described above.
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 difference also increase, suggesting the crystal is coming from instability. This behavior of a semi-conductor is attributed to a temperature-dependent Frenkel defect formation, and, when normalized against the concentration of Frenkel defects, the Arrhenius plot becomes linear.
