Organic-Inorganic Hybrid Perovskite [N(C₂H₅)₄]₂CoCl₄ Crystals: Growth, Phase Transition, Thermodynamic Property,...

The research has focused on candidates for application in various fields on the organic-inorganic hybrid perovskite-type [N-(C₂H₅)₄]₂CoCl₄. Here, [N-(C₂H₅)₄]₂CoCl₄ single crystals were grown, and their phase transition temperature (T _C) was about 228 K. Single-crystal X-ray diffraction performed at 300 K revealed that the crystals possess a tetragonal structure belonging to the P4₂/nmc space group, and their thermal behavior was subsequently analyzed. To investigate the influence of N-(C₂H₅)₄ cations near T _C, temperature dependences of the resonance frequency and chemical shifts in the ¹H, ¹³C, and ¹⁴N NMR spectra were analyzed. The abrupt change in the ¹H NMR resonance frequencies near T _C, in contrast to the variations observed in the ¹³C and ¹⁴N chemical shifts, is suggested to be closely related to the change in the Cl ligand of the CoCl₄ group nearest to the terminal methyl group. Furthermore, the activation energies for the molecular motions determined from the ¹H spin-lattice relaxation time in phases I and II were 4.16 and 1.08 kJ/mol, respectively. It can be inferred that CH₂ and CH₃ groups move more freely in phase II, in contrast to phase I, where the [N-(C₂H₅)₄]⁺ cation displays mostly unified overall motion. We compared the results for [N-(C₂H₅)₄]₂CoCl₄ obtained in this study and [N-(C₂H₅)₄]₂ BCl₄ (B=Cu and Zn) obtained by our previous group. The fundamental mechanism of the phase transition in the [N-(C₂H₅)₄]₂CoCl₄ crystal of the A ₂ BX ₄ type indicates that this material may be effectively utilized in diverse application areas.