Europium(III) orthophosphates: Synthesis, characterization, and optical properties

Hydrated europium(III) orthophosphates EuPO₄ × nH ₂O (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 Eu^3- 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 → ⁷F_J (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 ⁵D ₀ → 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 EuPO ₄ × nH₂O.