Synthesis, crystal chemistry and physical study of a new series of rare earth phosphates with specific open structure

Hexagonal rare earth phosphates with the composition LnP0₄,xKOH (Ln = Eu - Lu, Y; 0 < x ≤ 1), were prepared by coprecipitation of rare earth nitrates and potassium phosphate in basic medium. These compounds were characterized by X-ray diffraction and chemical analysis. The best crystallinity was obtained for samples heat-treated at 660-750°C in air during 24 h, in which the K content was determined to be near 1% (wt). A Rietveld refinement of these hexagonal orthophosphates was performed in the same conditions as used for the hexagonal holmium orthophosphate reported earlier by us (rhabdophane-type, space group P6₂22). It indicated that a part of KOH is incorporated in the hexagonal form and that the other part remains in an amorphous form. Between 800 and 1200°C the two forms gradually transform into a zircon-type LnP0₄ phase for Ln = Tb-Lu, Y and a monazite-type LnP0₄ phase for Ln = Eu, Gd. The infrared spectra for all these compounds were obtained by the KBr disk method.