Comprehensive study of organic-inorganic hybrid [N(C₂H₅)₄]₂CdBr₄: crystal structure, phase transitions, and...

The compound [N(C₂H₅)₄]₂CdBr₄, an organic-inorganic hybrid compound, has garnered attention for its potential applications across various fields. In this study, single crystals of [N(C₂H₅)₄]₂CdBr₄ were grown, and two distinct phase transition temperatures were identified: approximately 232 K (T _C1) and 476 K (T _C2). Single-crystal X-ray diffraction analysis at 300 K revealed a tetragonal structure belonging to the space group P4̄2₁ m. The thermal properties of the material were also briefly examined. Interestingly, while the ¹H NMR chemical shifts exhibited minimal variation around T _C1, the ¹³C NMR spectra showed a noticeable change in the number of peaks, suggesting a structural phase transition. These observations indicate that the crystal retains a tetragonal structure above T _C1 but transitions to a phase with lower symmetry below this temperature. Furthermore, ¹¹³Cd NMR chemical shifts showed a significantly more pronounced change near T _C1 compared to the shifts observed for ¹H and ¹³C. This behavior reflects alterations in the local electronic environment around the Cd²⁺ ions, implying reconfiguration of the surrounding atomic framework. Additionally, the spin-lattice relaxation times for ¹H and ¹³C, which are related to molecular motion, were nearly identical for both CH₂ and CH₃ groups in [N(C₂H₅)₄] cations. The phase transition observed at T _C1 is thus attributed to substantial structural rearrangements, particularly involving changes in atomic coordinates and the rotation of the CdBr₄ ^2- tetrahedra.