Measuring Motional Dynamics of (CH$_3$)$_2$ NH$_2^+$ in the Perovskite-Like Metal--Organic Framework [(CH$_3$)$_2$ NH$_2$][Zn(HCOO)$_3$]: The Value of Low-Frequency Electron Paramagnetic Resonance

英文摘要：

Dimethylammonium zinc formate (DMAZnF) is the precursor for a large family of multiferroics, materials which display co-existing magnetic and dielectric ordering. However, the mechanism underlying these orderings remains unclear. While it is generally believed that the dielectric transition is related to the freezing of the order-disorder dynamics of the dimethylammonium (DMA+) cation, no quantitative data on this motion are available. We surmise that this is due to the fact that the timescale of this cationic motion is on the borderline of the timescales of experimental techniques used in earlier reports. Using multifrequency EPR, we find that the timescale of this motion is ~ 5 x 10 -9 s. Thus, S-band (4 GHz) EPR spectroscopy is presented as the technique of choice for studying these motional dynamics. This work highlights the value of the lower-frequency end of EPR spectroscopy. The data are interpreted using DFT calculations and provide direct evidence for the motional freezing model of the ferroelectric transition in these metal-organic frameworks with the ABX3 perovskite-like architecture.