We present a detailed multinuclear (13C, 15N, 25Mg) NMR study on a metal–organic framework material with a dimethylammonium (DMA+) cation, [(CH3)2NH2]Mg(HCOO)3, that shows a dielectric phase transition at 270 K. The mechanism underlying this phase transition is not fully understood as there are contrasting reports attributing the phase transition to the order–disorder dynamics of the dimethylammonium cation in the cavity or to the contraction of the metal formate framework. In this work, we use high-resolution solid-state NMR to understand the mechanism of this phase transition by analyzing the motional dynamics of nitrogen and carbon. Spin–lattice relaxation time (T 1) and BPP theory yield the correlation time for nitrogen hopping of 10–9 s and activation energy of 28.62 kJ/mol. The T 1 behavior and the chemical shifts of 13C and 15N show that the phase transition has both order–disorder and displacive characteristics. 25Mg NMR spectra show a change in line width with the change in DMA+ cation dynamics, but there is no change in its chemical shift implying that its local environment remains the same.
Mesoscale particles ranging from 2.3 μm down to 180 nm of dimethylammonium magnesium formate, DMMgF, are generated, and characteristics of the known solid–solid phase transition, in which dimethylammonium ions residing in cavities of the metal formate framework undergo an order–disorder transition, are investigated. Detected by powder X-ray diffraction, the mesoscale particles undergo the same solid–solid phase change characterized for bulk samples, but calorimetry measurements reveal that the phase transition temperature decreases in the reduced-size particles from 263 K for the bulk materials to 251 K for the 180 nm particles, while the thermal hysteresis associated with the transition increases as the particles become smaller. Despite the solid–solid phase change, the mesoscale particles are too small to detect the pyroelectric current observed for the bulk. Taken together, the changes with particle size point to the elastic framework distortion as the determinant of the phase transition temperature in DMMgF. Synthetic challenges associated with the isolation of reduced size particles, specifically the role of the kinetic product magnesium formate dihydrate, are described.
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