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

The europium(II) sites in Sr4Al14O25:Eu phosphor with blue emission were ex amined by means of high frequency (90 and 180 GHz) EPR spectroscopy. The io ns 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 reg ardless of the substitution site. This implies that the crystal field surro unding europium(II) ions has non-axial symmetry. EPR spectral simulation su ggests that the europium(II) ions preferentially occupy the site with the l arger \D\ when they substitute for strontium ions. Comparison of the EPR sp ectra of Sr4Al14O25:Eu and SrAl2O4:Eu,Dy phosphors shows that the Sr4Al14O2 5: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 h ave long phosphorescent characteristics.