Phase Transition and Cationic Motion in a Metal–Organic Perovskite, Dimethylammonium Zinc Formate [(CH₃)₂NH₂][Zn(HCOO)₃]

The antiferroelectric phase transition of a metal–organic perovskite with a dimethylammonium cation, [(CH₃)₂NH₂]­[Zn­(HCOO)₃], at T_c = 156 K was investigated using ¹H nuclear magnetic resonance spectroscopy. The temperature dependence of the spin–lattice relaxation time, T₁, was measured to elucidate the methyl group reorientation and cation reorientation. The proton–proton distance of NH₂ protons was estimated to be 1.71 ± 0.03 Å from line-shape measurements of the deuterated compound, [(CD₃)₂NH₂]­[Zn­(DCOO)₃], and the cationic motion was shown to be the 120° reorientation of the dimethylammonium ion around the axis through the two carbon atoms of the cation. The activation energy for cationic motion was determined to be 23 kJ mol^–1. The two methyl groups of the cation in the low-temperature antiferroelectric phase become nonequivalent and have activation energies of 8 and 10 kJ mol^–1 for reorientation about the methyl group C₃ axis. The phase transition was revealed to be first-order from line-shape measurements as a function of temperature. T₁ measurements indicated another first-order phase transition around 79 K.