Effect of the transition metal on the structure and order-disorder phase transition in layered hybrid metal halides (CH3CH2NH3)2[MCl4] (M = Mn and Co).

Layered hybrid organic-inorganic metal halides (CH₃CH₂NH₃)₂[MnCl₄] and (CH₃CH₂NH₃)₂[CoCl₄] were synthesized by the slow evaporation method to understand the relationship between the crystal structure and order-disorder phase transition. Calorimetric data and crystal structure determination across the phase transition temperature establish the order-disorder phase transition. (CH₃CH₂NH₃)₂[MnCl₄] undergoes the reversible structural phase transition from tetragonal I4/mmm to orthorhombic Pbca at 212/222 K (cooling/heating), whereas (CH₃CH₂NH₃)₂CoCl₄ demonstrates the phase transition at 220/239 K from orthorhombic Pnma to orthorhombic P2₁2₁2₁. Both compounds are characterized by disordered ethyl ammonium cations in the structure above the phase transition temperature, whereas they become ordered cations at temperatures below the phase transition. Dielectric results further support the observed structural phase transitions. Additionally, magnetic measurements show canted antiferromagnetic characteristics for (CH₃CH₂NH₃)₂[MnCl₄] and paramagnetic behaviour is observed for (CH₃CH₂NH₃)₂[CoCl₄]. The structural differences, the role of intermolecular interactions and the effect of transition metals on the phase transition were evaluated using Hirshfeld surface analysis and the topological properties of electron density distributions. An accurate description of the structure and intermolecular interactions is crucial for understanding the physical properties and designing multifunctional hybrid organic-inorganic metal halide perovskites.