L. Szcześniak

Electron microscopy studies of domain structure in ferroelectric crystals

Abstract Results of the studies of ferroelectric domain structure and domain wall orientation in GASH and LAS single crystals obtained by the electron microscope decoration method are presented. In addition the influence of lattice defects on the domain pattern and the domain wall thickness in TGS is studied together with the domain wall clamping effect. A possible explanation of the AgCl decoration effect in the case of GASH crystals is given.

Pretransitional effects at the superionic phase transition of Rb3H(SeO4)2 protonic conductor

Abstract Rb 3 H(SeO 4 ) 2 single crystal undergoes a transition from low-temperature ferroelastic phase to superionic, paraelastic one at T S ≈456 K. The low-temperature monoclinic structure contains a zero-dimensional hydrogen bond network. Above T S , the reorienting HSeO 4 groups form a dynamic hydrogen bond network in the (001) plane. The presented results of proton conductivity, ferroelastic domain pattern and differential scanning calorimetry (DSC) studies in the vicinity of T S show that the transition region is stretched into ∼10 K and the transition process proceeds in steps: the changes in the ferroelastic domain pattern close to T S are accompanied by DSC anomalies. The temperature dependence of the conductivity σ [100] on heating and cooling revealed ∼2 K hysteresis of the superionic phase transition. The experimental results support the suggestion of recent papers on theoretical studies of the behaviour of protonic conductivity in M 3 H(XO 4 ) 2 crystal group in the superionic phase transition region.

Study of ferroelectric domain structure of lithium ammonium sulphate by scanning electron microscopy and electron microscope decoration technique

Abstract Ferroelectric domain structure of lithium ammonium sulphate single crystals is studied using scanning electron microscopy and electron microscope decoration technique. Domains of thickness 0. 1 ÷ 80 μ are observed on various crystal surfaces; average domain thickness is found to be ≈30 μm. Domain walls are found to be parallel to b-orthorhombic axis.

DSC study of superionic phase transition in (NH4)4H2(SeO4)3 single crystals

Abstract DSC and complex impedance studies of the protonic conductor (NH4)4H2(SeO4)3, which undergoes a superionic phase transition of first order at Ts = 378 K show that the activation energy of ionic conductivity d(lg σ)/dt and the ordering enthalpy ΔCp of the crystal are proportional: d(lg σ)/dT = XΔCp/RTs + const, as found for MAg4I5 crystals undergoing a second-order superionic phase transition. Thus the short-range order environment of the species involved in fast-ion transport plays the main role in the superionic phase transition. This is also supported by the value of the entropy change at Ts, ΔS = 43 J/mole·K. A new metastable phase was found to be induced on heating the (NH4)4H2(SeO4)3 crystal above Ts.

Ferroelectric domain wall studied by scanning electron microscopy and electron microscope decoration technique

Abstract Scanning electron microscope (SEM) and electron microscope decoration technique are very useful in revealing domain structure in ferroelectrics. The resolution power of these methods depends, however, on the studied crystal and experimental conditions. We present conditions and restrictions of the two methods applied to domain wall studies.

Anhydrous proton conductors for use as solid electrolytes

Anhydrous proton conductors in which proton transport proceeds via structure diffusion are interesting alternatives for membrane materials in hydrogen/air fuel cells. Understanding the mechanism of proton diffusion in candidate materials is necessary to improve their performance. We present the results of our studies of two classes of proton conductors in which conductivity does not rely on the presence of water molecules in their structure: crystalline hydrogen sulphates and selenates and new salts of nitrogen-containing heterocyclic molecules combined with a series of dicarboxylic acids. In the former group, superprotonic conductivity results from a high molecular dynamics of S/SeO4 groups, which creates an excess of structurally equivalent positions for protons. Ferroelastic properties of the crystals determine their physical behaviour at high temperatures. In particular, ferroelasticity makes a smooth and reversible character of superionic phase transition possible. In the second group of materials, the heterocycles exhibit extensive specific interactions that result in a fluctuating network of H-bonds. Thus, the proton conduction in these materials depends on the number of protons, which can be involved in the diffusion and on the fluctuation rate of the hydrogen bond network.

Ferroelastic-Superionic Phase Transition in (NH 4 ) 3 H(SO 4 ) 2 Single Crystals: FT NIR Raman, DSC and Ferroelastic Domain Studies

(NH 4 ) 3 H(SO 4 ) 2 , a member of M 3 H(XO 4 ) 2 hydrogen-bonded crystal family. The transition from the superionic phase (R3m) to the low-temperature phase (A2/a) at T S =413 K is accompanied by an anisotropic spontaneous deformation, which leads to the appearance of ferroelastic properties. The paper presents results of temperature variation of heat capacity, ferroelastic domain pattern and NIR Raman spectra in the close vicinity of T S .

Pyroelectric response of TGS-PEO composites

Abstract 0-3 type composites of triglycine sulphate, with high molecular weight polyethylene oxide were prepared by hot-pressing. FT Raman spectroscopy was used to control the interaction of glycine ions with the polymer. Pyroelectric and electric response was studied for composite samples in the form of diaphragm fixed on its perimeter and poled in low dc field (E p = 6MV/m). Pyroelectric response in the current mode of TGS/PEO composites decreases on increasing the light modulation frequency. Pyroelectric coefficient evaluated for TGS/PEO composites at 10 Hz amounts to ∼ 7.10−5 C/m2K.

High temperature phase transition of β-LiNH4SO4 single crystal

Abstract Very detailed optical studies, dielectric and calorimetric measurements were carried out in the close vicinity of high temperature phase transition (Tc ∽ 462 K) of β-LiNH4SO4. Temperature dependences of dielectric permittivity and specific heat revealed some anomalies near Tc. The anomalies were explained in terms of regular large scale structure which appears in a very narrow range of temperature near Tc. An attempt to explain the origin of this structure is presented.

DTA and DSC studies of GASH

Abstract The two stage dehydration process of guanidinium aluminium sulphate hexahydrate (GASH) was studied with the DTA, TG and DSC methods. The dehydration energies of these processes were determined from DTA and DSC curves. The low values of these energies can explain a high density of defects for crystals grown at high temperatures (60° - 95°).