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, [(CH 3 ) 2 NH 2 ][Zn(HCOO) 3 ], at T c = 156 K was investigated using 1 H nuclear magnetic resonance spectroscopy. The temperature dependence of the spin–lattice relaxation time, T 1, was measured to elucidate the methyl group reorientation and cation reorientation. The proton–proton distance of NH 2 protons was estimated to be 1.71 ± 0.03 Å from line-shape measurements of the deuterated compound, [(CD 3 ) 2 NH 2 ][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 C 3 axis. The phase transition was revealed to be first-order from line-shape measurements as a function of temperature. T 1 measurements indicated another first-order phase transition around 79 K.