SrAl 2 O 4 :Eu 2+ (,Dy 3+ ) Nanosized Particles: Synthesis and Interpretation of Temperature-Dependent Optical Properties

SrAl 2 O 4 nanosized particles (NPs) undoped as well as doped with Eu 2+ and Dy 3+ were prepared by combustion synthesis for the discussion of their intensively debated spectroscopic properties. Emission spectra of SrAl 2 O 4 :Eu 2+ (,Dy 3+ ) NPs are composed by a green band at 19 230 cm −1 (520 nm) at room temperature, assigned to anomalous luminescence originated by Eu 2+ 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 4f 6 ( 7 F J )5d(t 2g )→4f 7 ( 8 S 7/2 ) transition of Eu 2+ substituting the Sr 2+ sites. This can be justified by the presence of a fine structure in the excitation spectra due to the different 7 F J levels ( J = 0 ⋯ 6 ) of the 4f 6 core. Moreover, Fano antiresonances with the 6 I J ( J = 9 / 2 , 7 / 2 ) levels could be observed. In addition, the Stokes shifts ( Δ E S = 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 ħ ω = 480 cm - 1 and 470 cm −1 coupled with the electronic states could be reliably determined.