Iron(III) phosphate or ferric phosphate is an

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

with the formula Fe PO₄. Four polymorphs of anhydrous FePO₄ are known. Additionally, two polymorphs of the dihydrate FePO₄·(H₂O)₂ are known. These polymorphs have attracted interest as potential cathode materials in batteries.

Structure

The most common form of FePO₄ adopts the structure of α-

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

. As such the material consists of tetrahedral Fe(III) and phosphate sites. As such the P and Fe have

tetrahedral molecular geometry In a tetrahedral molecular geometry, a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are arccos(−) = 109.4712206...° ≈ 109.5° when all four substituents are ...

. At high pressures, a phase change occurs to a more dense structure with

octahedral In geometry, an octahedron (: octahedra or octahedrons) is any polyhedron with eight faces. One special case is the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. Many types of i ...

Fe centres. Two orthorhombic structures and a monoclinic phase are also known. In the two polymorphs of the dihydrate, the Fe centre is octahedral with two mutually cis water

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

Uses

Iron(III) phosphate can be used in steel and metal manufacturing processes. When bonded to a metal surface, iron phosphate prevents further oxidation of the metal. Its presence is partially responsible for the corrosion resistance of the

iron pillar of Delhi The iron pillar of Delhi is a metal structure high with a diameter that was constructed by Chandragupta II (reigned c. 375–415 Common Era, CE), and now stands in the Qutb complex at Mehrauli in Delhi, India.Finbarr Barry Flood, 2003"Pillar, ...

. Iron phosphate coatings are commonly used in preparation for painting or powder coating, in order to increase

adhesion Adhesion is the tendency of dissimilar particles or interface (matter), surfaces to cling to one another. (Cohesion (chemistry), Cohesion refers to the tendency of similar or identical particles and surfaces to cling to one another.) The ...

to the iron or

steel Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...

substrate, and prevent corrosion, which can cause premature failure of subsequent coating processes. It can also be used for bonding

fabrics Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, and different types of fabric. At first, the word "textiles" only referred to woven fabrics. However, weaving is not ...

wood Wood is a structural tissue/material found as xylem in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulosic fibers that are strong in tension and embedded in a matrix of lignin t ...

, and other materials to iron or steel surfaces. Iron phosphate is used to make

lithium iron phosphate Lithium iron phosphate or lithium ferro-phosphate (LFP) is an inorganic compound with the formula . It is a gray, red-grey, brown or black solid that is insoluble in water. The material has attracted attention as a component of lithium iron phosp ...

, the cathode in

lithium iron phosphate batteries The lithium iron phosphate battery ( battery) or LFP battery (''lithium ferrophosphate'') is a type of lithium-ion battery using lithium iron phosphate () as the cathode material, and a graphitic carbon electrode with a metallic backing as th ...

Pesticide

Iron phosphate is one of the few

molluscicide Molluscicides () are chemicals that kill molluscs. They are also known as snail baits, snail pellets, or slug pellets. These pesticides against molluscs are usually used in agriculture or gardening, in order to control Pest (organism)#Gastropods, ...

s approved for use in the practice of

organic farming Organic farming, also known as organic agriculture or ecological farming or biological farming,Labelling, article 30 o''Regulation (EU) 2018/848 of the European Parliament and of the Council of 30 May 2024 on organic production and labelling of ...

. Pesticide pellets contain iron phosphate plus a chelating agent, such as

EDTA Ethylenediaminetetraacetic acid (EDTA), also called EDTA acid, is an aminopolycarboxylic acid with the formula . This white, slightly water-soluble solid is widely used to bind to iron (Fe2+/Fe3+) and calcium ions (Ca2+), forming water-solubl ...

Mineral

Strengite Strengite is a relatively rare iron phosphate mineral with the formula: FePO4·2H2O. The mineral is named after the German mineralogist (1830–1897). Lavender, pink or purple in hue, it is similar to variscite and is partially soluble, part ...

is the mineral form of hydrated ferric phosphate.

Legislation

Iron(III) phosphate is not allowed as food additive in the European Union. It was withdrawn from the list of allowed substances in the directive 2002/46/EC in 2007.

References

External links

{{Phosphates Phosphates Iron(III) compounds