High frequency EPR of europium(II)-doped strontium aluminate phosphors

The europium(II) sites in Sr4Al14O25:Eu phosphor with blue emission were examined by means of high frequency (90 and 180 GHz) EPR spectroscopy. The ions occupy two low symmetry sites in the Sr4Al14O25 lattice with \D\ = 0.100 and 0.0907 cm(-1) respectively; the ratio \E/D\ is estimated to be 1/3 regardless of the substitution site. This implies that the crystal field surrounding europium(II) ions has non-axial symmetry. EPR spectral simulation suggests that the europium(II) ions preferentially occupy the site with the larger \D\ when they substitute for strontium ions. Comparison of the EPR spectra of Sr4Al14O25:Eu and SrAl2O4:Eu,Dy phosphors shows that the Sr4Al14O25:Eu is present as an impurity in the non-stoichiometric SrAl2O4:Eu,Dy when prepared with excess amounts of Al2O3. Such materials have been found to have long phosphorescent characteristics.