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.

The dielectric spectra of lamellar compound CuInP₂S₆ is presented. Above 1 MHz the dielectric dispersion caused by diffuse soft mode appears. The ferroelectric dispersion in the vicinity of T_c begins at about 10 MHz and ranges up to the GHz region. Debye -type dispersion together with the critical slowing down shows the order -disorder type of phase transition

The temperature dependence of ultrasonic velocity and ultrasonic attenuation were measured in polyurea elastomer composites with inorganic nanoparticles. The decrease in the ultrasonic velocity along with an attenuation maximum was observed above the glass transition temperature, T_g. The shape and position of this peak are directly proportional to the amount of embedded nanoparticles. In addition, the sharp anomalies in the ultrasonic velocity and ultrasonic attenuation were observed above the glass transition temperature in semicrystalline polyurea elastomers. These anomalies are related to the corresponding first order phase transition of these crystalline domain-containing networks. The ultrasonic attenuation peak related to glass transition is considerably smaller in these polyurea networks showing a small influence of the soft domains below the melting temperature, T_m. The first order phase transition in the semicrystalline polyurea elastomers shows a large temperature hysteresis of more than 10 K, which was confirmed by dielectric investigations. The addition of small amounts of inorganic nanoparticles resulted in a shift of the first order phase transition temperature in semicrystalline nanocomposites.