Luminescence of Exchange Coupled Pairs of Transition Metal Ions

The influence of exchange interaction between paramagnetic ions on the luminescence properties is investigated for Mn 2 + ­ Mn 2 + , Cr 3 + ­ Cr 3 + , Cr 3 + ­ Gd 3 + , and Mn 2 + ­ Gd 3 + pairs. Two effects are reported: shortening of the lifetime of the spin-forbidden emission and a shift of emission lines or bands to a longer wavelength. The shortening of the lifetime of the spin-forbidden emission is important in Mn 2 + doped phosphors. For application of Mn 2 + -doped phosphors in displays the long lifetime of the Mn 2 + emission is a problem. Shortening of the lifetime is possible by exchange coupling with Mn 2 + neighbors, or possibly other paramagnetic neighbors. The results reported here show that the lifetime shortening depends on the exchange-coupling parameter J . The strongest coupling is observed in Cr 3 + ­ Cr 3 + pairs and the lifetime shortening is strong ( e.g. , in LaAlO 3 : Cr 3 + J = 63 cm − 1 and the lifetime is about 30 times shorter for the pair emission). The coupling in Mn 2 + ­ Mn 2 + and Cr 3 + ­ Gd 3 + pairs is weak. Typically, values for J are around 1 cm −1 and the lifetime is reduced by a factor of 2-10 ( e.g. , J is −1.0 cm −1 for exchange coupling between Cr 3 + and Gd 3 + ; the lifetime of the spin-forbidden Cr 3 + emission is 14 ms in GdAlO 3 compared to 56 ms in LaAlO 3 ) . The lifetime shortening for the Mn 2 + emission is not related to the observed red shift for the pair emission. For the Mn 2 + ­ Gd 3 + pair the exchange interaction is very weak and no significant lifetime shortening could be measured for Gd 3 + ­ Mn 2 + pairs (24 ms for the Mn 2 + emission in YF 3 vs. 22 ms in GdF 3 ) .