Luminescence of exchange coupled pairs of transition metal ions

The influence of exchange interaction between paramagnetic ions on the luminescence properties is investigated for Mn2+-Mn2+, Cr3+-Cr3+, Cr3+-Gd3+, and Mn2+-Gd3+ 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 Mn2+ doped phosphors. For application of Mn2+-doped phosphors in displays the long lifetime of the Mn2+ emission is a problem. Shortening of the lifetime is possible by exchange coupling with Mn2+ 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 Cr3+-Cr3+ pairs and the lifetime shortening is strong (e.g., in LaAlO3:Cr3+ J = 63 cm(-1) and the lifetime is about 30 times shorter for the pair emission). The coupling in Mn2+-Mn2+ and Cr3+-Gd3+ 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 Cr3+ and Gd3+: the lifetime of the spin-forbidden Cr3+ emission is 14 ms in GdAlO3 compared to 56 ms in LaAlO3). The lifetime shortening for the Mn2+ emission is not related to the observed red shift for the pair emission. For the Mn2+-Gd3+ pair the exchange interaction is very weak and no significant lifetime shortening could be measured for Gd3+-Mn2+ pairs (24 ms for the Mn2+ emission in YF3 vs. 22 ms in GdF3). (C) 2001 The Electrochemical Society.