Iron(III) phosphate

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]

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]

Strengite is the mineral form of hydrated ferric phosphate.

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 It was withdrawn from the list of allowed substances in the directive 2002/46/EC in 2007.

• 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