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

Abstract

The dielectric phase transition of a metal–organic perovskite with a dimethylammonium cation, [(CH3)2NH2][Mg(HCOO)3], at Tc=270K 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 22.7kJmol−1. The two methyl groups of the cation in the low-temperature phase become nonequivalent and have activation energies of 9.1 and 7.0kJmol−1 for reorientation about the methyl group C3-axis. The T1 measurements indicated that the Tc=270K phase transition is of first-order and another first-order phase transition was revealed at around 80K. The transition entropy was estimated to be ΔS=10±1JK−1mol−1 for the Tc=270K phase transition in agreement with the simple three fold order–disorder model of dimethylammonium ion.