Dopant Concentration induced optical changes in Ba1−xEuxMoO4: A green and facile approach towards tunable photoluminescent material

Abstract

Ba1−xEuxMoO4 phosphors were synthesized by complex polymerization method. The prepared phosphor material was extensively characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetry (TG) and differential thermal analysis (DTA). The introduction of europium upto 2mol% doesn’t change the crystal structure. The influence of Eu concentration on the luminescent properties of phosphors was analyzed. It was seen that, under UV irradiation, the pure BaMoO4 sample shows multicolor emission. The blue and green emission was attributed to 1T2 → 1A1 transition within MoO42− moeity (shallow defects), whereas the red- orange emission to the defect MoO3 group (deep defects). The intensity of magnetic dipole transition (MDT) was found to be lower than that of electric dipole transition (EDT) which indicates the low -⁠ symmetry acentric environment of Eu3+ ions. Based on Stark splitting, the actual site symmetry of Eu3+ in BaMoO4 was clarified as C6v; reduced from actual S4 for Ba2+ ion in BaMoO4. The decay curve shows two different lifetime value 11.2 µs (τ1) (host contribution) and 693 µs (τ2) (europium sitting at Ba2+ site) with magnitudes 14% and 86% respectively. The value of host dopant energy transfer efficiency calculated under forbidden f-f transition excitation was found to be lower at 12.6% as compared to 76% in case of excitation by allowed charge transfer band. To find out the concentration quenching mechanism, the critical energy-transfer distance for the Eu3+ ions was evaluated to be 33.81Å, which was found to be greater than 10Å and signify electric multipolar interactions as the only mode of energy transfer The intensity of peaks corresponds to EDT was found to be high in comparison to the peak due to MDT in all the Eu- doped samples: indicating the asymmetric local site symmetry around Eu3+. This is also reflected in the JO (Judo Ofelt) parameter trend; at all concentrations, Ω2 value is greater than Ω4 indicating the high covalency of the Eu3+ ligand bonding and asymmetric environment around europium ion in BaMoO4, which is highly probable because it occupies the asymmetric Ba2+ position.