Tuning multifunctional stimuli-responsive behaviour through halogen exchange in hybrid ionic [(CH3CH2)3N(CH2X)]2[MnCl4] (X=Cl, Br)

Abstract

Hybrid metal halide materials have attracted significant attention due to their diverse functional properties. In this study, we prepared, characterized, and evaluated the multifunctional properties of two hybrid tetrachloromanganate (II) compounds with the general formula [(CH₃CH₂)₃N(CH₂X)]₂[MnCl₄], where X is Cl or Br. From the structural point of view, these compounds consist of two isolated ionic species with an antifluorite crystal arrangement: 0D tetrahedral [MnCl₂]^2– anions and large, quasi-spherical and polar [(CH₃CH₂)₃N(CH₂X)]⁺ cations. These cations are responsible for the first-order phase transition observed at T_t ~318 K for the Cl-compound and T_t ~343 K for the Br-compound. We investigated various functional properties—specifically dielectric, photoluminescent, and pressure responses—and our results demonstrate that these compounds are highly versatile, exhibiting broad responses to external stimuli. Furthermore, we found that substituting the halogen in the ammonium cations from Cl to Br induces important modifications in their functional properties. In particular, this halogen exchange increases the phase transition temperature, reduces the thermal hysteresis, improves the photoluminescence quantum yield, and enhances the reversible entropy changes under compression-decompression cycles.