Understanding Ferroelectricity in Perovksite-like Metal-Organic Frameworks through EPR, NMR, and AC Dielectric Permittivity Measurements

Nandita Abhyankar

Florida State University

Perovskite-like metal-organic frameworks (MOFs) of the ABX3 type have received extensive research interest because of their ease of synthesis, compositional tunability, and multiferroicity. They are also of interest as potential lead-free alternatives to the technologically important class of PZT piezoelectrics. Dimethylammonium metal formates, with the general formula [(CH3)2NH2]M(HCOO)3; where M = Mn, Fe, Co, Ni, Zn, comprise the model system for these multiferroic MOFs. This talk will discuss the mechanism of the ferroelectric transition in [(CH3)2NH2]Mn(HCOO)3 (DMMnF) and [(CH3)2NH2]Zn(HCOO)3 (DMZnF). The size-dependence of dielectric properties of DMMnF shows the emergence of relaxor-like properties in ensembles of particles in the size range of ~1-10 um. A single-crystal X-band EPR study is used to provide an explanation for the apparent magnetoelectric effect in DMMnF. A comparison of the dielectric properties of DMMnF and DMZnF shows that DMZnF exhibits relaxor-like behavior while DMMnF does not. A high-resolution 13C CP-MAS study of DMZnF, combined with AC dielectric permittivity measurements and DFT calculations, is used: (i) to demonstrate the combined order-disorder and displacive nature of the phase transition in DMZnF, and (ii) to elucidate the relaxor-like behavior of DMZnF.