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

Abstract

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 ν4, 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.