Europium(III) Orthophosphates: Synthesis, Characterization, and Optical Properties

Abstract

Hydrated europium(III) orthophosphates EuPO4 × nH2O (rhabdophane) of a nanocrystalline particle size ranging from 5 to 40 nm were precipitated from aqueous solution at neutral pH. The hexagonal crystal structure of the synthesized phase remained stable up to a calcination temperature of 600 °C according to X-ray diffraction (XRD) analysis. The complete loss of water at temperatures exceeding 600 °C caused the transformation into monoclinic nonhydrated EuPO4 isomorphous to monazite. The typical Eu3+ luminescence emissions excited at 396 nm for hexagonal EuPO4 × nH2O as well as for the monoclinic nonhydrated EuPO4 were attributed to magnetic-dipole and vibronic as well as forced electric-dipole 5D0 → 7FJ (J = 1, 2, 3, 4) transitions. If the trivalent europium ion lies on an inversion center, the hypersensitivity is absent. The intensity ratio of the magnetic-dipole 5D0 → 7F1 transition to the electric-dipole 5D0 → 7F2 transition decreased with increasing calcination temperature up to 600 °C, indicating the presence of a hypersensitive, forced electric-dipole 5D0 → 7F2 transition due to the lack of inversion symmetry sites. The loss of water during heating up to 600 °C was considered to be responsible for the variations in the emission characteristics of the EuPO4 × nH2O.