Effect of Ca~(2+) Doping on Luminescence Properties of BaMgSiO_4:Eu~(2+) Phosphor

The Eu2+-doped silicate based phosphors are very suitable for LEDs.These phosphors show broad emission colors which may vary from near UV to red light through 4f-5d transition of Eu2+ activator and strongly depend on the host lattice,thus the emission wavelength is tunable by modifying the composition and structure of the hosts.This provides a possible way to fabricate color-tunable phosphors for LEDs.In this article,a new composition of color-tunable phosphor material Ba1-xCaxMgSiO4:Eu2 + doped with Ca2 +(x≤0.15) was synthesized by conventional solid-state reaction in a reductive atmosphere(95% N2 + 5% H2),their luminescent properties and the influence of Ca2 + content were investigated by X-ray diffraction(XRD) and fluorescence spectrometer.The results showed that the emission of the phosphors turns from 498 to 450 nm,when the content of Ca2 + increases in 0 ～ 0.05.The maximum emission and excitation intensity are reached when the content of Ca2 + is equals to 0.075,and the shape of the excitation spectra also changes as increasing the content of Ca2 +.The XRD curves show that the single phase limit of Ca2 + in the phosphors is below 0.075.The results can be explained by Ca2 + substitution for Ba2 + in different lattice sites and the crystal field strength.As reported by Mingying Peng et al in the structure of BaMgSiO4,there are three different barium sites Ba(1),Ba(2) and Ba(3) with equal amounts in the lattice.The 498 nm emission band corresponds to the Eu2 + emission on the Ba(1) and Ba(2) sites.And the two absorption bands at 360 and 400 nm in the excitation spectrum are assigned to the Eu2 + centers on Ba(1) and Ba(2) sites,respectively.The Ba2 + at the Ba(2) can be more easily substituted by Ca2 + because of a longer average Ba-O distances [Ba(1):0.289 nm;Ba(2):0.294 nm;Ba(3):0.274 nm].The strength of crystal field around the Eu2 + ion on Ba(1) sites changes when Ca2 + substitutes the Ba2 + at Ba(2),which leads to the change of excitation and emission spectra of the Eu2 + ion on Ba(1) sites.Then,when the content of Ca2 + is over 0.075,Ca2 + at Ba(2) reaches saturation,and the CaMgSiO4 phase occurs.This leads to impurity quenching.Ca2 + substitution for Ba2 + at Ba(1) also causes the change of the excitation spectrum.