Indium(III) phosphate is an inorganic compound of indium and phosphate with the chemical formula InPO4. It is a white solid that is practically insoluble in water.
Preparation Indium(III) phosphate can be prepared by precipitation from solutions containing In3+ and phosphate ions. For example, indium(III) chloride reacts with phosphoric acid to form InPO4:
InCl3 + H3PO4 → InPO4↓ + 3 HCl
More generally, soluble indium salts form precipitates of indium phosphate when treated with acidic or neutral soluble phosphates in neutral or weakly acidic solution.
Hydrated phases can be prepared hydrothermally. A monoclinic dihydrate, InPO4·2H2O, has been obtained from indium(III) hydroxide and phosphoric acid in the presence of hydrofluoric acid and quinuclidine.
Structure and properties At ambient pressure, anhydrous InPO4 crystallizes in the orthorhombic crystal system in space group Cmcm. Its structure is of the CrVO4 type: In3+ is six-coordinate in InO6 octahedra, while phosphorus is tetrahedrally coordinated in PO4 groups.
At pressures above about 12 GPa, InPO4 undergoes a structural transformation. High-pressure diffraction studies indicate coexistence of tetragonal zircon-type (I41/amd) and scheelite-type (I41/a) phases together with the ambient orthorhombic phase. In the high-pressure phases, indium becomes eight-coordinate while phosphorus remains four-coordinate.
Several hydrates are known. InPO4·H2O crystallizes in the triclinic crystal system, space group P, with lattice parameters a = 5.4342 Å, b = 5.5508 Å, c = 6.5446 Å, α = 97.593°, β = 94.558° and γ = 107.565°. Its three-dimensional framework contains In2O8(H2O)2 dimers linked by PO4 tetrahedra.
On heating between about 370 and 480 °C, the monohydrate loses its coordinated water to form anhydrous InPO4. This dehydration corresponds structurally to condensation of the hydrated indium dimers into chains of edge-sharing InO6 octahedra.
The dihydrate InPO4·2H2O is also polymorphic. A monoclinic form with space group P21/n is isostructural with metavariscite-type AlPO4·2H2O, and an orthorhombic form with space group Pbca is also known.
Applications Indium phosphate has been investigated for use in electronic and optoelectronic materials. Its pressure-induced structural changes and the associated changes in indium coordination are of interest for understanding the behaviour of phosphate materials under extreme conditions.
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