Understanding Ferroelectricity in the Pb-Free Perovskite-Like Metal–Organic Framework [(CH3)2NH2]Zn(HCOO)3: Dielectric, 2D NMR, and Theoretical Studies

Dimethylammonium zinc formate ([(CH3)2NH2]Zn(HCOO)3 or DMZnF) is a model system for the study of hybrid perovskite-like dielectrics. It undergoes a phase transition from the paraelectric to ferroelectric phase at ∼166 K, as observed via NMR spectra. The mechanism of this phase transition has been shown to have contributions from ordering of the hydrogen bonds between [(CH3)2NH2]+ (DMA+) and the formate groups as well as buckling of the metal-formate framework, but the transition dynamics and atomistic mechanism are not fully clear. This work presents dielectric constant measurements as evidence of cluster formation of the low-temperature phase and the relaxor-like behavior of this metal–organic framework above the phase transition temperature. 13C CP-MAS is used to track the evolution of the chemical shift, T1, and T2 of the dimethylammonium cation and formate groups from room temperature to 120 K. 2D 13C–13C correlation measurements provide evidence of the formation of pretransitional clusters above the phase transition temperature. Density functional theory (DFT) calculations support the assignment of chemical shifts and the proposed model. The analysis of 13C CP-MAS spectra and DFT calculations is used to discuss the mechanism of the dielectric phase transition and the origin of relaxor-like behavior in DMZnF.