Lanthanum phosphate is an inorganic compound of lanthanum and phosphate with the chemical formula LaPO4. It occurs in an anhydrous monazite-type form and in hydrated rhabdophane-type forms.
Preparation Lanthanum phosphate can be prepared by precipitation from aqueous solutions containing La3+ and phosphate ions:
La3+ + PO43− → LaPO4↓
Hydrated lanthanum phosphate can, for example, be obtained by reaction of soluble lanthanum salts with sodium phosphate. Hydrothermal synthesis has been used to prepare rhabdophane-type LaPO4 nanofibres.
Depending on the synthesis conditions, precipitation can give either hydrated rhabdophane-type LaPO4·nH2O or anhydrous monazite-type LaPO4.
Structure and properties Anhydrous lanthanum phosphate adopts the monazite structure. It crystallizes in the monoclinic crystal system in space group P21/n (No. 14). In this structure La3+ is nine-coordinate, while phosphorus occurs in isolated PO4 tetrahedra.
Hydrated lanthanum phosphate forms the rhabdophane structure, with an approximate composition LaPO4·nH2O. Synthetic rhabdophane commonly contains roughly 0.5–1 molecule of water per formula unit.
On heating, hydrated LaPO4 first loses water while retaining the rhabdophane framework. At higher temperatures it irreversibly transforms into the monoclinic monazite phase. The rhabdophane-to-monazite transformation generally occurs between about 500 and 900 °C, depending on preparation conditions. Rhabdophane-type LaPO4 has been reported to remain structurally stable to approximately 650 °C before conversion to monazite.
Lanthanum phosphate is chemically durable and only very slightly soluble in water. Monazite-type LaPO4 also shows high thermal and radiation stability.
Applications Lanthanum phosphate has been investigated as a component of high-temperature ceramic composites. Monazite-type LaPO4 forms weak interfaces with materials such as alumina and has therefore been studied as a coating and interphase material in ceramic-matrix composites.
Hydrated rare-earth phosphates, including LaPO4·nH2O, have also been investigated as proton conductors for intermediate-temperature electrochemical applications.
Monazite-type rare-earth phosphates are also studied as durable host materials for immobilization of radioactive elements because of their chemical stability, radiation resistance and ability to accommodate a wide range of cations in the crystal structure.
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