Correlation between structural properties and molecular dynamics in [N(C2H5)4]2CuBr4: a multinuclear NMR and X-ray diffraction study

The structural, thermal, and molecular dynamic properties of the organic–inorganic hybrid compound [N(C 2 H 5 ) 4 ] 2 CuBr 4 were investigated using differential scanning calorimetry, thermogravimetric analysis, single-crystal and powder X-ray diffraction (SCXRD, PXRD), and multinuclear solid-state nuclear magnetic resonance (NMR) spectroscopy. Three phase transitions at 205, 260, and 448 K, accompanied by thermal hysteresis, indicating partially irreversible structural transformations. SCXRD analyses at 240 and 300 K confirmed that the compound maintains a tetragonal structure with space group P \(\:\bar{4}\) 2 1 c, while exhibiting anisotropic lattice changes and temperature-dependent orientational disorder of the cations. Temperature-dependent 1 H and 13 C NMR spectra revealed the linewidth narrowing and changes in local environments associated with enhanced molecular motions of the cations. The 14 N static NMR results particularly indicate that the phase transition near 260 K is closely related to changes in local symmetry of the organic cations, two different [N(C 2 H 5 ) 4 ] groups. Spin–lattice relaxation times T 1ρ further confirmed site-dependent molecular dynamics within the [N(C 2 H 5 ) 4 ] groups. These results demonstrate that the phase transition behavior in [N(C 2 H 5 ) 4 ] 2 CuBr 4 is primarily governed by reorientational motions and ordering/disordering processes of the organic cations within the stable inorganic CuBr 4 framework.