Gadolinium phosphate is an inorganic compound of gadolinium and phosphate with the chemical formula GdPO4. Both hydrated and anhydrous forms are known. The compound is notable for its very low solubility and high neutron absorption associated with gadolinium.
Preparation Hydrated gadolinium phosphate can be prepared by precipitation from solutions containing Gd3+ and phosphate ions. For example, reaction of gadolinium(III) chloride with phosphoric acid at low temperature gives hydrated GdPO4.
The product obtained depends on temperature, pH and ageing conditions. Hemihydrate, monohydrate and more highly hydrated forms have been reported.
Calcination of hydrated GdPO4 at about 800–1000 °C produces anhydrous material with the monazite structure.
Structure and properties Anhydrous gadolinium phosphate can occur in both monazite-type and xenotime-type structures. The monazite form is monoclinic, space group P21/n, whereas the xenotime form is tetragonal and isostructural with zircon, space group I41/amd.
In monazite-type GdPO4, isolated PO4 tetrahedra are linked through irregular GdO9 coordination polyhedra.
GdPO4 lies close to the structural boundary between the monazite-type light rare-earth phosphates and the xenotime-type heavy rare-earth phosphates. Consequently, both structural forms have been reported under different synthesis and temperature conditions.
Hydrated gadolinium phosphate is extremely insoluble in water. Solubility-product measurements on GdPO4·H2O gave Ksp values in the approximate range 2 × 10−14 to 5 × 10−13.
Applications Because naturally occurring gadolinium has a very high thermal-neutron absorption cross section, GdPO4 has been investigated as a chemically durable neutron absorber. Its low aqueous solubility and thermal stability have led to study of the material for incorporation into systems used for storage or disposal of spent nuclear fuel.
Gadolinium phosphate is also used as a host material for luminescent rare-earth ions. Solid solutions based on LaPO4–GdPO4, doped with ions such as Sm3+, have been investigated as phosphors for white-light-emitting devices.
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