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

TL;DR

This study uses low-frequency EPR spectroscopy to quantify the motional dynamics of DMA+ cations in a perovskite-like MOF, revealing their role in ferroelectric transition mechanisms.

Contribution

It demonstrates the effectiveness of S-band EPR in measuring cation motion timescales, providing direct evidence for motional freezing in ferroelectric MOFs.

Findings

DMA+ cation motion timescale is ~5 x 10^-9 s

S-band EPR is optimal for studying these dynamics

Data supports motional freezing model of ferroelectric transition

Abstract

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.