On the origin of ferroelectric structural phases in perovskite-like metal–organic formate

Abstract

Metal–organic frameworks (MOFs), formed of metal centers coupled by organic molecules, exhibit inherent porosity and crystallinity. Although these systems have been examined for many potential applications, their multiferroic properties remain poorly understood. One of the approaches to get an insight into the ferroic features of these materials is the study of frameworks templated by protonated organic molecules possessing rotational and conformational freedom. The embedded organic cation with an internal electric dipole moment may be ordered on cooling, leading to the appearance of ferroelectric-like properties. Herein, we report a study of two structural phase transitions in one such material, the methylhydrazinium zinc formate [CH₃NH₂NH₂][Zn(HCOO)₃] framework (MHyZn), using various experimental techniques. High-temperature (HT) improper ferroelectric phase transition and low-temperature (LT) biferroic phase transition in MHyZn single crystals have been investigated in a broad temperature range using X-ray diffraction, longitudinal ultrasonic velocity and attenuation measurements, pyrocurrent measurements, and broadband dielectric spectroscopy. In addition, noncentrosymmetric to centrosymmetric structural transformation was confirmed for the HT phase transition with the use of temperature-resolved second harmonic generation on powder samples, while polarizing microscopy observation of a large deformation of ferroelastic-like domains attested to the noncontinuous character of the crystallographic system transformation (trigonal to triclinic) in the LT biferroic phase transition. Finally, we proved that the spontaneous electric polarization occurs due to freezing of motion around the 2- and 3-axes of the methylhydrazinium cation.