Iron(III) phosphate

Iron(III) phosphate or ferric phosphate^[4][5] is an inorganic compound with the formula FePO₄. 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.

Iron(III) phosphate

Names
IUPAC name Iron(III) phosphate
Other names Ferric orthophosphate, Ferric phosphate
Identifiers
CAS Number	10045-86-0 (anhydrous) 13463-10-0 (dihydrate)
3D model (JSmol)	Interactive image
ChEBI	CHEBI:131371 Y
ChemSpider	23244 Y
ECHA InfoCard	100.030.123
PubChem CID	24861
UNII	N6BAA189V1 Y
CompTox Dashboard (EPA)	DTXSID9035672
InChI InChI=1S/Fe.H3O4P/c;1-5(2,3)4/h;(H3,1,2,3,4)/q+3;/p-3 Y Key: WBJZTOZJJYAKHQ-UHFFFAOYSA-K Y
SMILES [O-]P(=O)([O-])[O-].[Fe+3]
Properties
Chemical formula	FePO4
Molar mass	150.815 g/mol (anhydrous)
Appearance	yellow-brown solid
Density	3.056 g/cm3 (anhydrous) 2.87 g/cm3 (20 °C, dihydrate)
Melting point	250 °C (482 °F; 523 K) (dihydrate) decomposes[1]
Solubility in water	anhydrous: insoluble dihydrate: 0.642 g/100 mL (100 °C)[1]
Solubility product (Ksp)	9.91×10−16[2]
Magnetic susceptibility (χ)	+11,500.0·10−6 cm3/mol
Thermochemistry
Heat capacity (C)	180.5 J/mol·K (dihydrate)[1]
Std molar entropy (S⦵298)	171.3 J/mol·K (dihydrate)[1]
Std enthalpy of formation (ΔfH⦵298)	−1888 kJ/mol (dihydrate)[1]
Hazards
GHS labelling:
Pictograms	[3]
Signal word	Warning
Hazard statements	H315, H319, H335[3]
Precautionary statements	P261, P305+P351+P338[3]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Structure

The most common form of FePO₄ adopts the structure of α-quartz. As such the material consists of tetrahedral Fe(III) and phosphate sites.^[6] As such the P and Fe have tetrahedral molecular geometry. At high pressures, a phase change occurs to a more dense structure with octahedral 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 ligands.^[7]

Uses

See also: Phosphate conversion coating

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.

Iron phosphate coatings are commonly used in preparation for painting or powder coating, in order to increase adhesion to the iron or steel substrate, and prevent corrosion, which can cause premature failure of subsequent coating processes.

It can also be used for bonding fabrics, wood, and other materials to iron or steel surfaces.^{[citation needed]}

Iron phosphate is used^[8] to make lithium iron phosphate, the cathode in lithium iron phosphate batteries.^[9][10]

Pesticide

Iron phosphate is one of the few molluscicides approved for use in the practice of organic farming.^[11] Pesticide pellets contain iron phosphate plus a chelating agent, such as EDTA.^[12]

Mineral

Strengite 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.

See also

• Iron(II) phosphate, commonly known as ferrous phosphate, the lower phosphate of iron
• Lithium iron phosphate battery, a battery that uses iron phosphate
• Phosphate conversion coating, an industrial process used to protect newly manufactured iron and steel from corrosion