H. Warhanek

Symmetry analysis of domain structure in KSCN crystals

Abstract Basic symmetry properties of domain states, domain pairs, domain twins and walls in KSCN crystals are examined by group theoretical methods. Relations between domain states and domain pairs are expressed in matrix form and by permutation representation. Black and white space groups describing the symmetry of domain pairs are found and layer group symmetries of domain twins and domain walls are determined as their sectional subgroups. Microscopic structures of domain walls are given for coherent ferroelastic walls and for translation (antiphase) walls. Exposition and obtained results provide an illustrative example of symmetry analysis of domain structures.

Optical studies of the ferroelastic phase transitions in Rb3H(SeO4)2, (NH4)3H(SO4)2 and (NH4)3H(SeO4)2

Abstract Optical observations of growth twins and ferroelastic domains and measurements of the rotation of the optical indicatrix were carried out for Rb3H(SeO4)2 and (NH4)3H(SO4)2 using an optical microscope. Taking into account the symmetry reduction from the rhombohedral (Rm) to the monoclinic phase (B2/a) the occurrence of domains and growth twins can be well described. The orientations of oblique ferroelastic walls are well determined by the spontaneous strains s e 11 and s e 23 at room temperature.

Anomalous dielectric behaviour of crystals in the ferroelectric phase

The temperature- and frequency-dependent dielectric susceptibility parallel to the ferroelectric axis of crystals was studied in the ferroelectric phase. The dispersion corresponding to a critical slowing down of the thermal dipole relaxation was observed in the vicinity of in the frequency range 1 - 10 MHz. Besides, a dispersion which is related to the dipole reversal within the domain walls was found at lower frequencies. The latter is much more pronounced in the deuterated sample and is thought to be connected with the dynamics of hydrogen bonds.

On the domain structure of kscn

Abstract The domain structure of plate-like KSCN crystals has been studied between room temperature and 150[ddot]C. There are 4 different domains due to the symmetry reduction 14/mcm (D 18 4h ) to Pbcm (D 11 2 h) at Tc = 140[ddot]C. Two different types of pattern were observed: Lense shaped patterns which correspond to the 90[ddot] domains and curved trench like pits with preferential crystallographic orientations. It is suggested that this pattern corresponds to the antiphase-boundaries.

A dielectric study of the domain freezing in KD2AsO4

The temperature- and frequency-dependent dielectric susceptibility of KD2AsO4 was studied in the ferroelectric phase. A dispersion which is related to the dipole reversal was found in the high-frequency range (107 Hz). The dispersion is described by a Gaussian distribution of relaxation times which show Vogel-Fulcher temperature dependences. The high value of the dielectric constant below the phase transition is related to the mobility of the domain walls. This mobility steeply decreases near the freezing temperature Tf and therefore a sudden decrease in the susceptibility takes place. A non-linear superposition of the piezoelectric resonance and the relaxational dispersion was observed.

NMR study of proton self-diffusion in (NH4)4LiH3(SeO4)4 and (NH4)4LiH3(SO4)4

Abstract The appearance of a “liquid-like” proton T 2 component and the relatively high value of the proton self-diffusion coefficient D = (2−6) × 10 -7 cm 2 s -1 in the high temperature phase of (NH 4 ) 4 LiH 3 (SeO 4 ) 4 and (NH 4 ) 4 LiH 3 (SO 4 ) 4 demonstrate the onset of super-ionic like fast protonic conduction in these crystals. The linear Larmor frequency dependence of the proton spin-lattice relaxation time at low fields further demonstrates that we deal with low-dimensional acid proton translational diffusion already below room temperature.

Group-theoretical description of the 413 K phase transition in KSCN

Primary- and secondary-order parameters are found in the 413 K phase transition of KSCN by symmetry rules. The soft mode polarisation is predicted. A closed form of all the invariants of the free energy is given and used to analyse the existing experimental data.

2D 39K NMR study of the phase transition in KSCN

Abstract Two-dimensional (2D) “separation of interactions” 39K 1 2 → −1 2 NMR measurements of KSCN show that the line broadening around and above Tc is purely homogeneous. The disordering process above Tc is thus of dynamic and not of static origin. The lifetimes of the orthorhombic microdomains which appear static on the neutron scattering time scale are between 10−3 and 10−7 s.

Optical properties of the ferroelastic phase transition in (NH4)4LiH3(SO4)4

The ferroelastic phase transition in (NH4)4LiH3(SO4)4 at 232 K has been studied by birefringence measurements in the temperature range 190-300 K. The temperature dependence of the birefringence can be well described by the Landau theory for a proper ferroelastic phase transition of second order. The domain structure is compatible with the symmetry reduction from the tetragonal to the monoclinic phase.

Optical investigations in [N(CH3)4]2MnCl4

Abstract Some optical investigations including measurements of birefringence and angle between the extinction positions in different domains, have been performed on [N(CH3)4]2MnCl4, In addition the domain structure has been studied using an optical microscope. The observed anomalies in the birefringence measurements clearly confirm the existence of an incommensurate phase in a temperature interval of 0.6°C.