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 ${\text{Mn}}^{2+}$ photoluminescence (PL) and electronic structure of ${[{({\text{CH}}_{3})}_{4}\text{N}]}_{2}{\text{Mn}X}_{4}$ $(X:\text{Cl},\text{Br})$ 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 ${\text{Mn}X}_{4}^{2\ensuremath{-}}({\text{T}}_{\text{d}})$ tetrahedra. Their peaks experience large pressure redshifts, which are associated with the big crystal compressibility and the interaction between organic/inorganic $({[{\text{CH}}_{3}]}_{4}{\text{N}}^{\ensuremath{-}}/{\text{Mn}X}_{4}^{2\ensuremath{-}})$ tetrahedra. The variation in the Racah parameters and crystal-field splitting with pressure indicates that the excitation and emission redshifts of ${\text{Mn}}^{2+}$ are mainly governed by the increase in the $\text{Mn-}X$ 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 ${\text{Mn}X}_{4}^{2\ensuremath{-}}$ tetrahedra with change in ${\text{Mn}}^{2+}$ coordination from fourfold ${\text{Mn}X}_{4}^{2\ensuremath{-}}({\text{T}}_{\text{d}})$ to sixfold ${\text{Mn}X}_{6}^{4\ensuremath{-}}({\text{O}}_{\text{h}})$. 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.