Optical characterization of fourfold (Td)- and sixfold (Oh)-transition-metal species in MgAl2O4:Co2+ by time-resolved spectroscopy

This work investigates the origin of novel visible photoluminescence (PL) bands observed in the spinel MgAl2O4:Co2+. Besides the well-known fourfold-coordinated Co2+(Td) PL at 670 nm [N.V. Kuleshov, V.P. Mikhailov, V.G. Scherbitsky, P.V. Prokoshin and K.V. Yumashev, J. Lumin. 55 (1993) 265.], a rich structured PL band at 686 nm was also observed that we associate with uncontrolled impurities of sixfold coordinated Cr3+(Oh) by time-resolved spectroscopy and lifetime measurements and their variation with temperature. We also show that the lifetime of the Co2+(Td) emission at 670 nm varies from τ=6.7 μs to 780 ns on passing from T=10 to 290 K. This unexpected behaviour for Td systems is related to the excited-state crossover (4T1↔2E), making the emission band to transform from a narrow-like emission from 2E at low temperature to a broad structureless band from 4T1 at room temperature.