Silver(I) fluoride

Silver(I) fluoride is the inorganic compound with the formula AgF. It is one of the three main fluorides of silver, the others being silver subfluoride and silver(II) fluoride. AgF has relatively few niche applications; it has been employed as a fluorination and desilylation reagent in organic synthesis and in aqueous solution as a topical caries treatment in dentistry. Silver(I) fluoride is the inorganic compound with the formula AgF. It is one of the three main fluorides of silver, the others being silver subfluoride and silver(II) fluoride. AgF has relatively few niche applications; it has been employed as a fluorination and desilylation reagent in organic synthesis and in aqueous solution as a topical caries treatment in dentistry. The hydrates of AgF present as colourless, while pure anhydrous samples are yellow.:150 High-purity silver(I) fluoride can be produced by the heating of silver carbonate to 310 °C under a hydrogen fluoride environment, in a platinum tube::9 Laboratory routes to the compound typically avoid the use of gaseous hydrogen fluoride. One method is the thermal decomposition of silver tetrafluoroborate: In an alternative route, silver(I) oxide is dissolved in concentrated aqueous hydrofluoric acid, and the silver fluoride is precipitated out of the resulting solution by acetone.:10 The structure of AgF has been determined by X-ray diffraction.:3735 At ambient temperature and pressure, silver(I) fluoride exists as the polymorph AgF-I, which adopts a cubic crystal system with space group Fm3m in the Hermann–Mauguin notation. The rock salt structure is adopted by the other silver monohalides. The lattice parameter is 4.936(1) Å, significantly lower than those of AgCl and AgBr.:562 Neutron and X-ray diffraction studies have further shown that at 2.70(2) GPa, a structural transition occurs to a second polymorph (AgF-II) with the caesium chloride structure, and lattice parameter 2.945 Å.:7945:770 The associated decrease in volume is approximately ten percent.:7946 A third polymorph, AgF-III, forms on reducing the pressure to 2.59(2) GPa, and has an inverse nickel arsenide structure. The lattice parameters are a = 3.244(2) Å and c = 6.24(1) Å; the rock salt structure is regained only on reduction of the pressure to 0.9(1) GPa. Non-stochiometric behaviour is exhibited by all three polymorphs under extreme pressures.:939:7947 Silver(I) fluoride exhibits unusual optical properties. Simple electronic band theory predicts that the fundamental exciton absorption for AgF would lie higher than that of AgCl (5.10 eV) and would correspond to a transition from an anionic valence band as for the other silver halides. Experimentally, the fundamental exciton for AgF lies at 4.63 eV.:2604 This discrepancy can be explained by positing transition from a valence band with largely silver 4d-orbital character.:563 The high frequency refractive index is 1.73(2).:3737 In contrast with the other silver halides, anhydrous silver(I) fluoride is not appreciably photosensitive, although the dihydrate is.:286:150 With this and the material's solubility in water considered, it is unsurprising that it has found little application in photography but may have been one of the salts used by Levi Hill in his 'heliochromy'., although a US patent for an experimental AgF-based method was granted in 1970. Unlike the other silver halides, AgF is highly soluble in water (1800 g/L), and it even has some solubility in acetonitrile. It is also unique among silver(I) compounds and the silver halides in that it forms the hydrates AgF.(H2O)2 and AgF.(H2O)4 on precipitation from aqueous solution.:1185 Like the alkali metal fluorides, it dissolves in hydrogen fluoride to give a conducting solution.