Phosphates: Solid‐State Chemistry

Abstract

Phosphates contain oxyanions of phosphorus(V). The most common anion is the orthophosphate group PO 4 3− , which is found in many solid phosphates including hydrated and acid phosphate salts. Oligomeric polyphosphate chains P n O 3 n +1 ( n +2)− up to n = 6 and cyclophosphate rings P n O 3 n n − up to n = 12 are also observed, and a unique cage ultraphosphate anion P 8 O 23 6− is found in the Na 3 FeP 8 O 23 structure. Polymeric phosphate anions include the infinite chain catenaphosphate anion (PO 3 − ) ∞ and the ultraphosphate networks found in CaP 4 O 11 and the lanthanide MP 5 O 14 structures. Many compound anions and neutral frameworks such as silicophosphates, aluminophosphates, and molybdenum phosphates also exist. Phosphate species are characterized through PO stretching and bending vibrations in infrared and Raman spectroscopies, and by 31 P 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 KH 2 PO 4 (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 NaZr 2 (PO 4 ) 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(HPO 4 ) 2 ·H 2 O 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, KTiOPO 4 , 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.