The structure and switchable dielectric properties of a dabco complex with chromium chloride.

We report a metal-organic material of the following formula [DabcoH22+]·CrCl3(H2O)3·2(Cl-) (DabcoH22+ = C6H14N22+, diprotonated 1,4-diazabicyclo[2.2.2]octanium). This compound exhibits a dielectric anomaly, which is attributed to the rotatory fluctuation of the Dabco molecule. The complementary results of single-crystal X-ray diffraction, DSC, dielectric, NMR and Raman spectroscopy provide information about the general mechanisms of the phase transition, which results from the ordering of the DabcoH22+ molecules. The reversibility of dielectric switching with no observable attenuation of the dielectric signal during multiple cycling is observed. The dielectric switching characteristic of the crystal makes it an interesting material for potential application in smart devices.

A paraelectric-ferroelectric phase transition of an organically templated zinc oxalate coordination polymer.

Water-presence dependent switchable ferroelectricity was discovered in the hybrid organic-inorganic zinc oxalate 1D coordination polymer (DABCOH2)[Zn(C2O4)2]·3H2O (DZnOH, where DABCOH2: diprotonated 1.4-diazoniabicyclo[2.2.2]octane). The compound undergoes a reversible para-ferroelectric phase transition at 207 K from room temperature centrosymmetric phase I (space group P21/n) to low-temperature non-centrosymmetric phase II (space group P21). The microscopic mechanism of the phase transition is directly associated with the reconstruction of the hydrogen-bond network. On heating, the crystals exhibit a reversible single-crystal to single-crystal transformation concerned with the removal of all water molecules giving anhydrous DABCO zinc oxalate (DABCOH2)[Zn(C2O4)2] (DZnO). The dehydrated compound does not show ferroelectric properties.

Widely used hardly known. An insight into electric and dynamic properties of formamidinium iodide.

The simple organic crystal formamidinium iodide (FAI) appeared to be a novel semiconducting material in a wide temperature range. The electric properties of FAI and the role of formamidinium cation (FA+) in the molecular mechanism of the solid-to-solid phase transitions (at 345 K (III → II) and 388 K (II → I)) were analysed. The creation of the ferroelastic domain structure in phases III and II was proved on the basis of observation under a polarizing microscope. Moreover, the molecular arrangement of dipolar organic FA+ was studied by 1H NMR (spin-lattice relaxation time) and vibrational spectroscopy supported by density functional theory. The theoretical results show a good agreement with the experimental data. The infrared spectrum in a harmonic approximation was calculated and a comparative vibrational analysis was performed. All used techniques showed that the prototypic phase I exhibits the feature of plastic phase.

The effect of Fe on the structure and electrical conductivity of sodium borosilicate glasses.

We report the synthesis and characterization of iron-bearing sodium borosilicate glasses with an Fe2O3 content lower than 10 mol%. Using Mössbauer spectroscopy we demonstrate that Fe ions most probably have a tetrahedral oxygen environment for an iron oxide content higher than 5 mol%. Additionally, the Mössbauer results along with the X-ray diffraction studies indicate the formation of magnetite nanoclusters. The electrical conductivity of iron-containing sodium borosilicate glasses is studied over a wide temperature range. The impact of iron content on the dielectric permittivity and electrical properties is discussed. Finally, ionic conduction is identified in the high temperature region and its physical origin is explained. A similar activation energy of the dc conductivity and the activation energy of the electrical relaxation suggest that both the relaxation and conduction processes can be ascribed to the same type of entities.

Size-driven ferroelectric-paraelectric phase transition in TGS nanocomposites.

Dielectric properties of porous glass nanocomposites with TGS crystals embedded into six porous matrices with average pore size from 5 to 312 nm were investigated in the temperature range from 280 to 380 K at selected frequencies. The results are discussed based on the effect of the particle size on the phase transition temperature of TGS nanocomposites. Temperature-size phase diagram of TGS composites was derived. Non-monotonic character of the temperature-driven phase transition (Tp) with the decreasing particle size was determined. The nature of the Tp variation can be ascribed to the size-effect theoretically predicted by Zhong et al. (Phys Rev B 50:698-703, 1994).