Yihua Hu

Controllable optical properties by ratio of Sr/Ca in Sr1.97?xCaxMgSi2O7?:? {\rm Eu}_{0.01}^{2+} , {\rm Dy}_{0.02}^{3+} phosphors

The long afterglow phosphors Sr1.97−xCaxMgSi2O7u2009:u2009 , (x = 0, 0.4, 0.8, 1.2, 1.6, 1.97) were synthesized by the high temperature solid-state reaction method. The phases were analyzed by x-ray powder diffraction. All the samples have a similar structure. A slight shift of the peaks, which implied a change of crystal plane spacing, was found. With a decrease in the Sr/Ca ratio, the crystal plane spacing became smaller. The red shift was found in the emission spectra, from 468 to 529u2009nm. The 5d level of the Eu2+ ion splits into two sets. With the decrease in the crystal plane spacing, the energy of one set becomes higher while the other one becomes lower. This leads to the red shift of the emission spectra. The afterglow time becomes shorter with the decreasing Sr/Ca ratio. The thermoluminescence curves, which imply the depth of the traps, conform to the various afterglow times of the samples. This is due to the greater ionization potential of the Dy ions with the decreasing crystal plane spacing.

First-principles study of intrinsic vacancy defects in Sr2MgSi2O7 phosphorescent host material

Electronic structures of intrinsic vacancy defects in Sr2MgSi2O7 phosphorescent host material are investigated using first-principles calculations. Si vacancies are too high in energy to play any role in the persistent luminescence of Sr2MgSi2O7 phosphor. Mg vacancies form easier than Sr vacancies as a result of strain relief. Among all the vacancies, O1 vacancies stand out as a likely candidate because they are the most favorable in energy and introduce an empty triply degenerate state just below the CBM and a fully-occupied singlet state at ~1 eV above the VBM, constituting in this case effective hole trap level and electron trap levels, respectively. Mg vacancies are unlikely to explain the persistent luminescence because of its too shallow electron trap level but they may compensate the hole trap associated with O1 vacancies. We yield consistent evidence for the defect physics of these vacancy defects on the basis of the equilibrium properties of Sr2MgSi2O7, total-energy calculations, and electronic structures. The persistent luminescence mechanism of Sr2MgSi2O7:Eu2+, Dy3+ phosphor is also discussed based on our results for O1 vacancies trap center. Our results provide a guide to more refined experiments to control intrinsic traps, whereby probing synthetic strategies toward new improved phosphors.