Phosphates: Solid-State Chemistry
Published Online: 15 DEC 2011
Copyright © 2011 John Wiley & Sons, Ltd. All rights reserved.
Encyclopedia of Inorganic and Bioinorganic Chemistry
How to Cite
Attfield, J. P. 2011. Phosphates: Solid-State Chemistry. Encyclopedia of Inorganic and Bioinorganic Chemistry.
- Published Online: 15 DEC 2011
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Phosphates contain oxyanions of phosphorus(V). The most common anion is the orthophosphate group PO43−, which is found in many solid phosphates including hydrated and acid phosphate salts. Oligomeric polyphosphate chains PnO3n+1(n+2)− up to n = 6 and cyclophosphate rings PnO3nn− up to n = 12 are also observed, and a unique cage ultraphosphate anion P8O236− is found in the Na3FeP8O23 structure. Polymeric phosphate anions include the infinite chain catenaphosphate anion (PO3−)∞ and the ultraphosphate networks found in CaP4O11 and the lanthanide MP5O14 structures. Many compound anions and neutral frameworks such as silicophosphates, aluminophosphates, and molybdenum phosphates also exist. Phosphate species are characterized through PO stretching and bending vibrations in infrared and Raman spectroscopies, and by 31P NMR. Phosphate structures are generally rigid, resistant to chemical attack, and (when anhydrous) insoluble and thermally stable. Some phosphate crystals have important physical properties such as ferrolectricity in KH2PO4 (potassium dihydrogenphosphate, KDP). Diffusion of extra-framework ions leads to potential uses of solid phosphates as ion exchangers and conductors, for example, NASICON based on NaZr2(PO4)3, and as cathode materials for lithium batteries. Vanadium phosphates are important selective oxidation catalysts. Large pore phosphates in which molecules can be adsorbed and undergo acid-catalyzed reactions have been intensively studied. These materials are crystalline microporous solids such as aluminophosphates, or lamellar structures, typically based on α-Zr(HPO4)2·H2O that can be pillared. This area of interest has led to the synthesis of many new organically templated metal phosphates in recent years, usually through hydrothermal reactions. Hydrated and acid orthophosphates can also be prepared in solution, and anhydrous phosphates are synthesized by direct high-temperature reactions and from phosphate-rich melts or fluxes. Slow cooling of melts is used to grow crystals, and quenching can result in many stable phosphate glasses. Phosphate anions do not absorb significantly in the UV-visible region and so solid phosphates can also find use as optical materials such as phosphors, nonlinear crystals, for example, KTiOPO4, and lasers. Solid phosphates constitute many minerals, notably Apatites which are also found in living organisms as rigid components such as bones and teeth. Synthetic calcium phosphate biomaterials are used for bone implants.
- ionically conducting phosphates;
- microporous phosphate materials;
- mineral phosphates;
- nonlinear optical phosphate materials;
- condensed phosphate polyanions;
- layered phosphate materials;
- biomaterial phosphates