Dielectric switching in correlation with the structural phase transitions in tetrapropylammonium perchlorate.

The crystals of the tetrapropylammonium perchlorate ([(CH3CH2CH2)4N]ClO4, TePrAClO4) compound undergo two reversible phase transitions: at ca. T1 = 284 K and at ca. T2 = 445 K. The observed phase transitions and distinct dielectric and relaxation effects are due to the dynamic motions of the organic cations and anionic framework. The crystals become ordered at low temperatures, then disordered at room temperature (propyl chains of the organic part as well as perchlorate ions are disordered over the mirror plane at c = 1/4 and 3/4) and highly disordered at high temperatures. The comparable changes in the wavenumber and FWHM shifts (IR and Raman spectroscopy) in the case of tetrapropylammonium and perchlorate ions in the phase transition at T1 and slightly more significant changes for organic cations (juxtaposed with perchlorate ions) in the phase transition at T2 lead to a conclusion that the phase transition at T1 is equally driven by motions of the two ions, while the phase transition at T2 is more influenced by the motions of organic cations. The phase transition at T2 with its large entropy change resembles the behavior found in liquid crystals. The dielectric function values can be switched and tuned in the low- and high-dielectric states, which may indicate the potential application of this material in sensors or actuators.

New Insight into Phase Transitions of Porous Glass-Based Ferroelectric Nanocomposites.

The results of XRD, FTIR and differential scanning calorimetry (DSC) studies of empty porous silica matrices filled with binary mixtures of K1-xAgxNO3 (x = 0.05, 0.10) are reported in comparison with those obtained for bulk salts in the temperature range of structural phase transitions. Scanning electron microscopic data of the studied empty macroporous and microporous glasses confirmed differences in the pore morphology associated with the presence of silica gel. Accordingly, XRD and FTIR samples contain crystalline phase of KNO3 and AgNO3. The results of calorimetric investigation of porous glasses filled with binary mixtures of K1-xAgxNO3 (x = 0.05, 0.10) are presented. The results show that in the K1-xAgxNO3 nanocomposites, anomalies associated with phase transitions were detected. An influence of the mean value of pores sizes on the ferroelectric phase transition temperatures of K1-xAgxNO3 nanocrystals embedded into the porous matrices was determined. The impact of pore space structure on the phase transitions of ferroics nanocomposites was discussed.

Advances and Property Investigations of an Organic-Inorganic Ferroelectric: (diisopropylammonium)2[CdBr4].

The preparation of materials featuring more than one ferroelectric phase represents a promising strategy for controlling electrical properties arising from spontaneous polarization, since it offers an added advantage of temperature-dependent toggling between two different ferroelectric states. Here, we report on the discovery of a unique ferroelectric-ferroelectric transition in diisopropylammonium tetrabromocadmate (DPAC, (C6H16N)2[CdBr4]) with a Tc value of 244 K, which is continuous in nature. Both phases crystallize in the same polar orthorhombic space group, Iab2. The temperature-resolved second-harmonic-generation (SHG) measurements using 800 nm femtosecond laser pulses attest to the polar structure of DPAC on either side of the phase transition (PT). The dc conductivity parameters were estimated in both solid phases. The anionic substructure is in the form of [CdBr4]2- discrete complexes (0D), while in the voids of the structure, the diisopropylammonium cations are embedded. The ferroelectric properties of phases I and II have been confirmed by the reversible pyroelectric effect as well as by P-E loop investigations. On the basis of the dielectric responses, the molecular mechanism of the PT at 244 K has been postulated to be of mixed type with an indication of its displacive nature.

Ferroelectricity in Ethylammonium Bismuth-Based Organic-Inorganic Hybrid: (C2H5NH3)2[BiBr5].

The (C2H5NH3)2[BiBr5] (EBB) crystals adopt the one-dimensional (1D) polymeric anionic form [BiBr5]∞2-, which is preferred by halobismuthates(III) exhibiting polar properties and realized in R2MX5 stoichiometry. Differential scanning calorimetry and dilatometric measurements reveal reversible structural phase transitions: at 160 K (phase I → phase II) and 120 K (phase II → phase III). The resolved crystal structures of EBB show the centrosymmetric space group in phase I (Aeam), polar (Pca21) in phase II, and polar (Aea2) in phase III. The presence of dielectric hysteresis loops in phases II and III evidence ferroelectric properties. The dielectric response [ε*(ω,T)] of EBB close to 160 K is characteristic of ferroelectrics with a critical slowing down process. The molecular mechanism of a paraelectric-ferroelectric phase transition at 160 K is explained as "order-disorder" (assigned to the dynamics of the ethylammonium cations) and dominating "displacive" (related to strong distortion of the 1D anionic network). The optical band gap obtained from UV-vis measurements is about 2.6 eV. The conduction band minimum is formed by the hybridized Bi 6p and Br 4p states. An analysis of the CSD results for haloantimonates(III) and halobismuthates(III) ferroelectrics characterized by [MX4]-, [M2X9]3-, [MX5]2-, and [M2X11]5- anions is given.

Phase transition in NH4HSO4-porous glasses nanocomposites.

The results of investigations of porous glasses (PG) and porous glasses-ammonium hydrogen sulfate ferroelectric nanocomposites (AHS-PG) are presented. On the basis of dielectric and calorimetric measurements it was shown that in the AHS-PG nanocomposites with average pore size of 44, 68, 95, and 320 nm the anomalies of dielectric permittivity and specific heat similar to those in bulk crystals AHS are observed. An influence of the mean value of pores sizes on the ferroelectric phase transition temperatures of AHS nanocrystals embedded into the porous matrices was determined. It was shown that in AHS-PG dispersion of the dielectric permittivity is observed in both para- and ferro-electric phases and above room temperature AHS-PG nanocomposites exhibit the ionic conductivity.