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 Mn2+-photoluminescence (PL) of [(CH3)4N]2MnX4 (X: Cl, Br) under high pressure (0–15 GPa). We show that pressure induces structural modifications leading to changes of coordination geometry around Mn2+ from fourfold-coordinated MnX42−(Td) to sixfold MnX64−(Oh) 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 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 (6 nm/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.