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)(2)NH2][Zn(HCOO)(3)], at T-c = 156 K was investigated, using H-1 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 NH2 protons was estimated to be 1.71 +/- 0.03 angstrom from line-shape measurements of the deuterated compound, [(CD3)(2)NH2]-[Zn(DCOO)(3)], and the cationic motion was shown to be the 120 degrees 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 he 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 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.