Understanding enormous redshifts in highly concentrated Mn²⁺ phosphors

Broad band near infrared (NIR) emission has recently been reported for a wide variety of concentrated Mn2+ phosphors. Typically, Mn2+ emits in the green to red spectral region, depending on local coordination. The enormous redshift to the NIR was explained by exchange coupling between Mn2+ neighbours at high Mn2+ dopant concentrations. However, the reported redshifts are an order of magnitude larger than expected for exchange coupling and also the absence of a shift in excitation spectra suggests that exchange coupling cannot explain the observations. Here, extensive concentration, temperature and time dependent luminescence studies are reported for Mg1-xMnxAl2O4 (x = 0.01-0.5). The results show that the broad band NIR emission originates from NIR emitting trap centers, possibly Mn3+. High Mn2+ dopant concentrations enable efficient energy migration over the Mn2+ sublattice to these traps, consistent with the same excitation spectra for the green Mn2+ and NIR trap emission. Upon cooling to cryogenic temperatures energy migration is hampered and the green Mn2+ emission increases, especially in the most concentrated systems. Finally, the relative intensity of the NIR emission was varied by changing synthesis conditions providing further support that the NIR emission in concentrated Mn2+ phosphors originates from NIR emitting centers and not exchange coupled Mn2+ pairs.