Progressive pressure-induced transformation from isolated MnX₄ (T_d) to exchange-coupled MnX₆ (O_h) systems in A₂MnX₄ (X:Cl,Br) crystals

This work reports a correlation study between the crystal structure and photoluminescence (PL) in A₂MnX₄ (A: [CH₃] ₄N; X: Cl, Br) by means of simultaneously optical spectroscopy and x-ray diffraction (XRD) under pressure. These materials are attractive systems for PL studies under pressure since they are very compressible and the interaction between organic/inorganic ((CH₃)₄N ^-/MnX₄^2-) tetrahedra strongly affects the PL efficiency, which is fully related to the isolated T_d MnX ₄^2-. We show that pressure induces strong redshifts of emission peak of MnX₄^2- as well as decomposition processes at higher pressures yielding new aggregated phases of Mn²⁺ and amorphization which deeply affect Mn²⁺ PL. The equation of state of each compound is reported from XRD data.