Electronic structure and luminescence of [(CH3)4N]2MnX4 (X=Cl,Br) crystals at high pressures by time-resolved spectroscopy: Pressure effects on the Mn-Mn exchange coupling

This work investigates the Mn2+ photoluminescence (PL) and electronic structure of [(CH3)(4)N](2)MnX4 (X:Cl,Br) and their variation with pressure through time-resolved spectroscopy in the 0-15 GPa range. The crystal-field excitation and emission spectra are explained on the basis of the MnX42-(T-d) tetrahedra. Their peaks experience large pressure redshifts, which are associated with the big crystal compressibility and the interaction between organic/inorganic ([CH3](4)N-/MnX42-) tetrahedra. The variation in the Racah parameters and crystal-field splitting with pressure indicates that the excitation and emission redshifts of Mn2+ are mainly governed by the increase in the Mn-X bond covalency (70%) rather than the increase in the crystal-field splitting (30%). Above 6 GPa, pressure induces structural modifications, which are related to aggregation of the MnX42- tetrahedra with change in Mn2+ coordination from fourfold MnX42-(T-d) to sixfold MnX64-(O-h). This process involves a drastic change in the PL behavior yielding a simultaneous two-color PL emission: green emission at 520 nm and a red emission around 650 nm. Both emissions experience noticeable redshifts with pressure producing a marked piezo-PL effect. The large pressure range of phase coexistence makes these materials attractive for multiband PL, the wavelengths of which can be tuned through pressure and eventually stabilized at ambient conditions.