Martynas Kinka

Broadband dielectric spectroscopy of water confined in MCM-41 molecular sieve materials : low-temperature freezing phenomena

Dielectric properties of water adsorbed in pure siliceous and aluminium containing mesoporous MCM-41 materials have been investigated in the frequency range from 20 Hz to 1 MHz. The dielectric spectra revealed three dispersion regions, liquid-like free water in the centre of the mesopores, an intermediate water layer with reduced mobility, and an interfacial water layer at the inner surface of the mesopores. The analysis of the relaxation time distribution by means of a double-well potential indicates a strong dependence of the barrier height of reorienting water molecules dipoles in the interfacial layer on the Si/Al ratio in the framework.

Dielectric Properties of Sodium Nitrite Confined in Porous Glass

In this paper the results of dielectric investigation of nanoconfined NaNO2 is presented. The phase transition is shifted to lower temperatures. Only one phase transition can be seen in the dielectric spectra and the intermediate incommensurate phase is suppressed. The increase in the frequency dependent dielectric permitivity on heating is mainly the result of a conductivity effects. The relaxational soft mode contribution is as well present, but is small as compared to the conductivity contribution. Such huge conductivity is caused by premelting phase, which is visible even at temperatures below the phase transition

Coexistence of ferroelectric and relaxor states in Ba 2 Pr x Nd 1-x FeNb 4 O 15

We have investigated the dielectric response of Ba₂Pr_xNd_1-xFeNb₄O₁₅ ceramics (x = 0, 0.2, 0.5, 0.6, 0.8, 1) in the frequency range from 20 Hz to 1 GHz. The obtained results confirmed the continuous transformation from the ferroelectric behavior of Ba₂NdFeNb₄O₁₅ to the pure relaxor response of Ba₂PrFeNb₄O₁₅ with increasing x. For intermediate x values, coexisting ferroelectric transition and relaxor dielectric signatures were observed, corresponding to two different phenomena in the framework of these materials. Increasing the amount of Pr decreases the ferroelectric phase transition temperatures in these ceramics; a large cooling-heating hysteresis exceeding 50K was also observed.

Broadband Dielectric Spectroscopy of Water Confined in MCM-41 Molecular Sieve Material

Dielectric properties of water adsorbed in pure siliceous mesoporous MCM-41 material with pore diameter of 3.7 nm have been investigated in the frequency range from 20 Hz to 1 MHz for temperature interval from 100 K to 400 K. The dielectric spectra revealed three dispersion regions of characteristic shape. Analysis of relaxation processes showed that dynamics of water molecules is strongly affected by interaction with the pore surface. Two types of water—relatively free water in the centers of mesopores and “bound” water on the pore surface—can be distinguished.

Dielectric and Ultrasonic Investigation of Phase Transitions in PbFe1/2Nb1/2O3 Ceramics

We have investigated dielectric and acoustic properties of PbFe1/2Nb1/2O3 ceramics, obtained by conventional ceramic technology, in 120–450 K temperature interval. Dielectric response showed diffused dielectric permittivity maximum caused by ferroelectric phase transition around 380 K, but no dielectric anomalies associated with magnetic phase transition at low temperatures were observed. Formation of the polar phase was confirmed by ultrasonically detected piezoelectric sensitivity. Ultrasonic investigations using pulse-echo method showed two anomalous regions in temperature dependencies of ultrasonic velocity and attenuation, associated with magnetic and ferroelectric phase transitions.

Effect of Confinement on the Dynamics of Methanol

In this paper the results of dielectric spectroscopy of MCM-41 mesoporous material with incorporated MEOH are presented. Three dispersion regions of the confined MEOH have been found. The dielectric dispersion in the low temperature region is caused by MEOH molecules forming an interfacial layer with the inner surface of the mesoporous support. MEOH in the middles of the pores shows typical saddle like behaviour of the relaxation time. With incorporation of Al atoms in the framework of MCM-41 molecular sieves the saddle process becomes much weaker and vanishes at Si/Al = 64. Also, this process is followed by increase of conductivity.

Effect of Confinement on the Freezing-Melting Dynamics of Water

Melting-freezing dynamics of water confined in three MCM-41 materials with pore diameters of 2.0 nm, 2.5 nm and 3.7 nm was analyzed using dielectric spectroscopy method. Obtained results show, that reorientation of water molecules is strongly affected by confinement and interaction with the pore surface, which leads to the formation of highly disordered hydrogen bonded network. Defects of this network play the key role in the overall relaxation of confined water.

Investigation of Dielectric Relaxation Processes in Ba2NdFeNb4-xTaxO15 Ceramics

Dielectric response of Ba2NdFeNb4-xTaxO15 (x = 0.3, 0.6 and 2) solid solutions with Tetragonal Tungsten Bronze structure has been investigated by means of broadband dielectric spectroscopy in the frequency range from 20 Hz to 37 GHz. Pure Ba2NdFeNb4O15 compound displays ferroelectric behavior. Substitution of Nb5+ by Ta5+ considerably alters dielectric response of this system. Ferroelectric state was observed only for x = 0.3 compound with slightly lowered ferroelectric phase transition temperature. Further increase of x causes formation of relaxor state in these solid solutions and considerably lowers corresponding dielectric permittivity dispersion temperature region.

Dielectric spectroscopy of Nano BaTiO/sub 3/ confined in MCM-41 mesoporous molecular sieve materials

Dielectric properties of barium titanate (BaTiO3) particles, synthesized directly in the pores of MCM-41 materials, have been investigated in the frequency range from 20 Hz to 1 MHz for temperature intervals from 100 K to 500 K. The dielectric spectra of BaTiO3 confined in these molecular sieves were compared with the results obtained from the investigation of pure MCM-41 materials. Obtained results confirmed successful incorporation of BaTiO3 into porous matrix, but no phase transition from paraelectric to ferroelectric phase was observed due to the particle size being smaller than the critical size. Also, the overall dielectric response of investigated materials is strongly influenced by adsorbed water molecules

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

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.