Crystal structure, structural geometry, and molecular motion of organic–inorganic perovskite [N(CH3)4]2MnCl4 crystals at phases I, II, and III

Various aspects of the new development of organic-inorganic hybrid [N(CH₃)₄]₂MnCl₄ single crystals have been discussed. The phase transition temperatures were determined to be 268 K (T_C2) and 291 K (T_C1), and the thermodynamic stability was maintained at temperatures up to 669 K. The crystal structures at 250 K (phase III), 275 K (phase II), and 300 K (phase I) are monoclinic, orthorhombic, and orthorhombic, respectively. Notably, while the ¹H and ¹³C chemical shifts gradually changed near T_C1 and T_C2, the ¹⁴N resonance frequency exhibited a split in the number of signals near T_C1. Furthermore, the shorter spin-lattice relaxation time T_1ρ of ¹H than that of ¹³C suggests facile energy transfer for¹H. Additionally, analysing the temperature dependencies of T_1ρ for ¹³C revealed that the activation energy E_a in phase I is approximately five times greater than those in phases III and II. The high E_a observed in phase I primarily stems from the collective motion of the N(CH₃)₄ group, which contrasts with the considerable freedom observed for the CH₃ group in phases III and II. These distinctive physical properties suggest potential applications for [N(CH₃)₄]₂MnCl₄ as an organic‒inorganic hybrid material.