Ammonium fluoride is the inorganic compound with the formula NH4F. It crystallizes as small colourless prisms, having a sharp saline taste, and is highly soluble in water. Like all fluoride salts, it is moderately toxic in both acute and chronic overdose.
Structure Ammonium fluoride adopts the wurtzite crystal structure, in which both the ammonium cations and the fluoride anions are stacked in ABABAB... layers, each being tetrahedrally surrounded by four of the other. There are N−H···F hydrogen bonds between the cations and anions.A. F. Wells, Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984. This structure is very similar to ice, and ammonium fluoride is the only substance which can form mixed crystals with water.
In terms of liquid and gaseous forms, ammonium fluoride sublimes when heated—a property common among ammonium salts. In the sublimation, the salt decomposes to ammonia and hydrogen fluoride; the two gases can still recombine, i.e. the reaction is reversible: [NH4]F ⇌ NH3 + HF At elevated pressures, ammonium fluoride melts near 230 °C.
Reactions On passing hydrogen fluoride gas (in excess) through the salt, ammonium fluoride absorbs the gas to form the addition compound ammonium bifluoride. The reaction occurring is: NH4F + HF → NH4HF2
Uses This substance is commonly called "commercial ammonium fluoride". The word "neutral" is sometimes added to "ammonium fluoride" to represent the neutral salt [NH4]F as opposed to the "acid salt" (NH4HF2). The acid salt is usually used in preference to the neutral salt in the etching of glass and related silicates. This property is shared among all soluble fluorides. For this reason it cannot be handled in glass test tubes or apparatus during laboratory work.
Ammonium fluoride is a critical component of buffered oxide etch (BOE), a wet etchant used in microfabrication. It acts as the buffering agent in a solution of concentrated HF, creating an etchant with a more controllable rate of etching (than that of simple concentrated HF solutions).
It is also used for preserving wood, as a mothproofing agent, in printing and dyeing textiles, and as an antiseptic in breweries.
Polymorphism under non-standard conditions Ammonium fluoride (NH4F) exists in several crystalline phases depending on temperature and pressure. At ambient conditions, its structure is very similar to that of ice, unlike the other ammonium halides (NH4Cl, NH4Br, NH4I), which adopt CsCl-type or NaCl-type structures. Under high pressure, NH4F increasingly resembles the other ammonium halides. It adopts a cubic CsCl-type phase (phase III), followed at very high pressure by a tetragonal phase (phase VIII), analogous to phases seen in the other halides at much lower pressures. The transition to phase VIII occurs around 115 GPa in NH4F, roughly an order of magnitude higher than in NH4Cl, NH4Br, or NH4I. An intermediate tetragonal phase (phase IIIt) appears above about 40 GPa as a precursor to phase VIII.
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