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Abstract: This work investigates the Mn2+-photoluminescence (PL) of [(CH3)4N]2MnX4 (X: Cl, Br) under high pressure (0–15GPa). We show that pressure induces structural modifications leading to changes of coordination geometry around Mn2+ from fourfold-coordinated MnX4 2−(Td) to sixfold MnX6 4−(Oh) along with we observe enormous redshifts of the green emission at 520nm as well as a new red emission about 700nm above 6GPa. The Mn2+-PL behaviour with pressure in these highly efficient materials is investigated by time-resolved spectroscopy (emission and excitation) under high pressure. Furthermore, the two-colour PL emissions induced by pressure experience noticeable redshifts (6nm/GPa) producing a marked piezo-PL effect. The large pressure hysteresis observed in downstroke makes these materials attractive as a new type for multiband emission, which can be tuned through pressure and eventually stabilised at ambient conditions. [Copyright &y& Elsevier]

Abstract: 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 670nm [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 686nm 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 670nm varies from τ=6.7μs to 780ns on passing from T=10 to 290K. 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. [Copyright &y& Elsevier]