Pressure-induced transformation from isolated MnX4(Td) to exchange-coupled MnX6(Oh) in A2MnX4 (X: Cl, Br) crystals. Structural correlations by time-resolved spectroscopy

This work investigates the Mn 2+ -photoluminescence (PL) of [(CH 3 ) 4 N] 2 MnX 4 (X: Cl, Br) under high pressure (0-15 GPa). We show that pressure induces structural modifications leading to changes of coordination geometry around Mn 2+ from fourfold-coordinated MnX 2- 4 (T d ) to sixfold MnX 4- 6 (O h ) along with we observe enormous redshifts of the green emission at 520 nm as well as a new red emission about 700 nm above 6 GPa. The Mn 2+ -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.