Interplay between local environments and photoluminescence of Eu2+ in Ba2Zr2Si3O12: blue shift emission, optimal bond valence and luminescence mechanisms

Abstract

Ba_2(1−x)Zr₂Si₃O₁₂ (BZSO):xEu²⁺ (x = 0.005–0.06) phosphors have been prepared by a high temperature solid state reaction. Using X-ray powder diffraction, the structural properties including the phase purity were analyzed through Rietveld analysis. The BZSO:Eu²⁺ phosphors exhibit a broad excitation band ranging from 200 to 450 nm, and an intense asymmetric green emission band centered at 501 nm under an excitation of 365 nm. The optimum doping concentration of Eu²⁺ was found for x = 0.03. The detailed energy transfer mechanism among Eu²⁺ in BZSO is found to be a dipole–dipole mechanism, and the critical distance (R_C) for the Eu²⁺ ions calculated by the concentration quenching and spectral overlap methods are 20.45 and 25.83 Å, respectively. Furthermore, the unexpected blue shift (from green to cyan) in the emission and the increase in the thermal quenching barrier upon cation substitutions (Ca²⁺/Sr²⁺ for Ba²⁺) in the BZSO:0.03Eu²⁺ system have been investigated, which is due to the variation of the crystal field strength that the 5d orbital of the Eu²⁺ ion experiences. The underlying mechanism is ascribed to the differences between the average structure and the local coordination environments on the activator ions (Eu²⁺), as confirmed by the refinement results. Considering the merits of the near-UV light excitation, broad band emission, and good thermal stability, these materials have a potential application as white light emitting diode phosphors.