Comparative investigations of the crystal structure and photoluminescence property of eulytite-type Ba3Eu(PO4)3 and Sr3Eu(PO4)3

Abstract

In this study, the Ba₃Eu(PO₄)₃ and Sr₃Eu(PO₄)₃ compounds were synthesized and the crystal structures were determined for the first time by Rietveld refinement using powder X-ray diffraction (XRD) patterns. Ba₃Eu(PO₄)₃ crystallizes in cubic space group I3d, with cell parameters of a = 10.47996(9) Å, V = 1151.01(3) ų and Z = 4; Ba²⁺ and Eu³⁺ occupy the same site with partial occupancies of 3/4 and 1/4, respectively. Besides, in this structure, there exists two distorted kinds of the PO₄ polyhedra orientation. Sr₃Eu(PO₄)₃ is isostructural to Ba₃Eu(PO₄)₃ and has much smaller cell parameters of a = 10.1203(2) Å, V = 1036.52(5) ų. The bandgaps of Ba₃Eu(PO₄)₃ and Sr₃Eu(PO₄)₃ are determined to be 4.091 eV and 3.987 eV, respectively, based on the UV–Vis diffuse reflectance spectra. The photoluminescence measurements reveal that, upon 396 nm n-UV light excitation, Ba₃Eu(PO₄)₃ and Sr₃Eu(PO₄)₃ exhibit orange-red emission with two main peaks at 596 nm and prevailing 613 nm, corresponding to the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of Eu³⁺, respectively. The dynamic disordering in the crystal structures contributes to the broadening of the luminescence spectra. The electronic structure of the phosphates was calculated by the first-principles method. The analysis elucidats that the band structures are mainly governed by the orbits of phosphorus, oxygen and europium, and the sharp peaks of the europium f-orbit occur at the top of the valence bands.