SrAl2O4:Eu2+(,Dy3+) Nanosized Particles: Synthesis and Interpretation of Temperature-Dependent Optical Properties

Abstract

SrAl2O4 nanosized particles (NPs) undoped as well as doped with Eu2+ and Dy3+ were prepared by combustion synthesis for the discussion of their intensively debated spectroscopic properties. Emission spectra of SrAl2O4:Eu2+(,Dy3+) NPs are composed by a green band at 19 230 cm−1 (520 nm) at room temperature, assigned to anomalous luminescence originated by Eu2+ in this host lattice. At low temperatures, a blue emission band at 22 520 cm−1 (444 nm) is observed. Contrary to most of the interpretations provided in the literature, we assign this blue emission band very reliably to a normal 4f6(7FJ)5d(t2g)→4f7(8S7/2) transition of Eu2+ substituting the Sr2+ sites. This can be justified by the presence of a fine structure in the excitation spectra due to the different 7FJ levels (J=0⋯6) of the 4f6 core. Moreover, Fano antiresonances with the 6IJ (J=9/2,7/2) levels could be observed. In addition, the Stokes shifts (ΔES=1 980 cm−1 and 5 270 cm−1 for the blue and green emission, resp.), the Huang-Rhys parameters of S=2.5 and 6, and the average phonon energies of ħω=480 cm-1 and 470 cm−1 coupled with the electronic states could be reliably determined.