Iron(III) phosphate

Iron(III) phosphate, also ferric phosphate, is the 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 materials have attracted much interest as potential cathode materials in batteries.

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. 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.

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.

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.

Iron phosphate is used to make lithium iron phosphate, the cathode in lithium iron phosphate batteries.

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

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 It was withdrawn from the list of allowed substances in the directive 2002/46/EC in 2007.