Phase transition and cationic motion in the perovskite formate framework [(CH₃)₂NH₂][Mg(HCOO)₃]

The dielectric phase transition of a metal-organic perovskite with a dimethylammonium cation, [(CH₃)₂NH₂][Mg(HCOO)₃], at T_c = 270 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 results was very similar to that of the zinc analog, [(CH₃)₂NH₂][Zn(HCOO)₃], previously reported. The cationic motion was expected 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 22.7 kJ mol^-1. The two methyl groups of the cation in the low-temperature phase become nonequivalent and have activation energies of 9.1 and 7.0 kJ mol^-1 for reorientation about the methyl group C₃-axis. The T₁ measurements indicated that the T_c = 270 K phase transition is of first-order and another first-order phase transition was revealed at around 80 K. The transition entropy was estimated to be ΔS = 10 ± 1 J K^-1 mol^-1 for the T_c = 270 K phase transition in agreement with the simple three fold order-disorder model of dimethylammonium ion.