Conditions of the Formation of Rare Earth Phosphates and the Colors of Their Powders

Abstract Rare earth oxides, such as La2O3, Pr6O11, Nd2O3, and Sm2O3, can react with phosphoric acid to give four kinds of rare earth phosphates (orthophosphates LnPO4·0.5H2O and LnPO4, catena-polyphosphates Ln(PO3)3 and ultraphosphates LnP5O14), while in the reaction of CeO2 with phosphoric acid there are formed five kinds of phosphates (orthophosphate CePO4, pyrophosphate CeP2O7, two types of catena-polyphosphate Ce(PO3)3 and Ce(PO3)4, and ultraphosphate CeP5O14). The formation of these products in the CeO2–H3PO4 system are notably different from these in other systems, showing that cerium(III) phosphates as well as cerium(IV) phosphates can exist stably. It can be seen from the X-ray diffraction patterns that each corresponding phosphate (ortho, catena-Qoly, and ultraphosphates) of lanthanum, cerium(III), praseodymium, neodymium, and samarium is isomorphous. This may be caused by the facts that the valencies of all these metals are +3 and that their ionic radii are almost equal to one another (La(III) 1.06 Å–Sm(III) 0.96 Å). The conditions of the formation of each phosphate were investigated, and the colors of the crystals, the densities, and the infrared absorption spectra of various rare earth phosphates were also examined.