H. Kabelka

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.

Thermodynamic properties at the phase transition of Pb(Zr, Sn, Ti)O3 solid solutions

Abstract Field induced deformation and electrocaloric effect are investigated in the vicinity of phase transition at 163°C. The sharp increase of electrostriction below Tc is explained by the field induced ferroelectric to antiferroelectric phase transition. The antiferroelectric phase appears and remains stable below Tc in the absence of field. Elastic compliance and thermal expansion as functions of temperature are studied.

Giant domain wall response of highly twinned ferroelastic materials

Many ferroelastic crystals display at sufficiently low measurement frequencies a huge elastic softening below Tc which is caused by domain wall motion. Materials range from perovskites to iron based superconductors and shape memory materials. We present a model—based on Landau-Ginzburg theory including long range elastic interaction between needle shaped ferroelastic domains—to describe the observed superelastic softening. The theory predicts that the domain wall contribution to the elastic susceptibility is different for improper and proper ferroelastic materials. A test of the theory against experimental data on SrTiO3, KMnF3, LaAlO3, LaAlO3,La1-xNdxP5O14, and NH4HC2O4·12H2O yields excellent agreement.

Phase transitions of Pb0.99Nb0.02(Zr0.75Sn0.20Ti0.05)O3ceramics

Abstract The dielectric, elastic and electromechanical properties, electrocaloric effect and thermal expansion of poled and depoled Pb0.99Nb0.02(Zr0.75Sn0.20Ti0.05)O3 samples are presented to evaluate the nature of polar phases existing in the solid solution above room temperature. The Kittels free energy expansion is used to explain some essential features of physical properties.

Elastic stiffness constants and elastic relaxation around the first low temperature phase transition in LiKSO4

Abstract The elastic siffness constants Cn, C33, C44 and C66 of LiKSO4 have been measured in the temperature range 190 K to 300 K, by the ultrasonic pulse echo overlap method. The time dependence of the elastic constant C11 at 200 K showed that there are at least two relaxation times involved in the phase transition process. The first one is (15 ± 4) min. While the second one is (12 ± 2) h.

“No-permissible” W”-domain walls in K2Cd2(SO4)3 ferroelastic crystals

Abstract The domain structure of the ferroelastic crystal K2Cd2(SO4)3 was observed and investigated in detail, in different temperature regions below Tc using a polarizing microscope. It is shown that the appearence of a domain structure depends on crystal thickness and the domain structure can have a time dependent dynamic. The orientation of the domain walls in potassium-cadmium langbeinite crystals has been determined.

Dielectric dispersion in the ferroelectric phase of Rb2CoCl4 crystals

Abstract Data are presented on dielectric properties of Rb2CoCl4 crystals. A wide tail of enhanced permittivity extends into the ferroelctric phase. It shows a relaxation in the MHz region with a wide distribution of relaxation times. On lowering temperature, the decrease in permittivity is accompanied by dielectric softening as revealed by decreasing mean relaxation frequency.

Superelastic Softening of Ferroelastic Multidomain Crystals

Many proper and improper ferroelastic materials display (at sufficiently low measurement frequencies) a huge elastic softening below Tc. This giant elastic softening, which can be suppressed with uniaxial stress, is caused by domain wall motion. Here we shortly review our results on frequency and temperature dependent elastic measurements of perovskites which exhibit improper ferroelastic phase transitions. We also present a new model – based on Landau-Ginzburg theory including long range interaction of needle shaped ferroelastic domains - which describes superelastic softening observed in many of the perovskite systems very well. We also show how the theory can be extended to proper ferroelastic materials and apply it to describe the elastic behaviour of the proper ferroelastic material La1-xNdxP5O14 (LNPP).

The dielectric behaviour of KSCN

Abstract All three components of the dielectric constant of KSCN have been measured in the frequency range 103–107 Hz and 1011–1012 Hz in a wide temperature interval around and below the order-disorder transition at 415 K. The increase of the low frequency permittivity e i (T) (i = a, b, c) with temperature T is correlated to the appearance of the disorder near the transition with the law (1/χ i (T) - 1/χ i (B) α Q 2(T) where 1/χ i (B) is the background permittivity extrapolated from the high-temperature phase and Q is the order parameter. The origin of the anomaly are the submillimetre and millimetre absorption processes induced by the disorder. The low-frequency dielectric losses above room temperature are dominated by ionic conduction.

KSCN and its phase transition at 415K

Abstract The order-disorder phase transition of KSCN at 415K has been investigated by birefringence-, ultrasonic-, dielectric-and NMR-measurements. A pseudospin model which takes into account the interaction between the four pseudospins S1, S2, S3 and S4 corresponding to the four SCN-ions in the unit cell has been calculated and used to describe the experimental data.