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+(T-d) PL at 670 nm [N V Kuleshov, V.P. Mikhailov, VG. Scherbitsky, RV. 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+(O-h) by time-resolved spectroscopy and lifetime measurements and their variation with temperature. We also show that the lifetime of the Co2+(T-d) emission at 670 nm varies from tau = 6.7 mu s to 780 ns on passing frorn T = 10 to 290 K. This unexpected behaviour for T-d systems is related to the excited-state crossover (T-4(1)(sic)E-2), making the emission band to transform frorn a narrow-like emission from E-2 at low temperature to a broad structureless band from T-4(1) at room temperature. (c) 2009 Elsevier B.V. All rights reserved.