X-ray diffraction and Ir-absorption characteristics of lanthanide orthophosphates obtained by crystallisation from phosphoric acid solution

Powders of LnPO₄ · H₂O (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) prepared by crystallisation from boiling phosphoric acid (2 M H₃PO₄/1) solution were characterised by X-ray diffraction and FTIR-spectroscopy. Hexagonal LnPO₄ · H₂O (La Tb), tetragonal (Ho Lu and Y) and orthorombic DyPO₄ · H₂O crystalline modifications were identified. Ir-spectra of the hydrated hexagonal, anhydrous tetragonal LnPO₄ · H₂O (Dy, Ho, Er, Tm, Yb, Lu and Y) and anhydrous monoclinic (La Tb) are consistent with those reported in the literature. However, the hydrated tetragonal LnPO₄ · H₂O (Ho, Er, Tm, Yb, Lu and Y) display a surplus band (625 cm^–1) in the region of ₄, which was not reported in the literature. The band disappears after ignition at 950°C, while the tetragonal structure is still maintained, which may imply that it is attributed to hydrogen bonding of H₂O molecules to the phosphate oxygen in hydrated salts. Some of the phosphates, after ignition at 950°C, display additional P₂O₇^4– band at 1265–1267 cm^–1. That may be resulted from HPO₄^2– for PO₄^3– substitution in the phosphates crystallised in acidic (2 M H₃PO₄/1) solution.