Kenji Ema

Critical Phenomena and Anomalous Thermal Hysteresis Accompanying the Normal-Incommensurate-Commensurate Phase Transitions in Rb2ZnCl4

Measurements were made of the dielectric constant along the ferroelectric axis e a , the spontaneous polarization P s and the heat capacity c p of Rb 2 ZnCl 4 crystals in the temperature range from -150^° C to 150°C. The normal-incommensurate transition point determined as the peak position of c p lies at 30.1°C, which is about 6°C lower than the temperature for the maximum of e a . It was found that e a and P s show a characteristic thermal hysteresis over a wide temperature range including the incommensurate-commensurate trarasition point (-80^° C ). The thermal hysteresis occurs even when heating and cooling runs are cycled within the incommensurate phase. The observed thermal hysteresis is understood if we assume that defects such as impurities act as obtacles to diffusion of discommensurations and prevent the crystal from reaching thermal equilibrium.

Nonadiabatic scanning calorimeter

A high-resolution computerized calorimeter capable of fully automatic operation in either ac or relaxation modes is described. Emphasis is given to a new version of the relaxation technique in which the heater power is ramped linearly in time. This improvement results in superior performance and convenience in studying both first- and second-order phase transitions and allows quantitative evaluation of latent heats as well as pretransitional heat capacity variations. Examples are given for the use of this calorimeter in the study of liquid crystal phase transitions.

Effect of impurities on the incommensurate-commensurate phase transitions in Rb2ZnCl4, K2ZnCl4 and Rb2ZnBr4

Abstract The thermal hysteresis accompanying the incommensurate-commensurate phase transitions in Rb2ZnCl4, K2ZnCl4 and Rb2ZnBr4 is assumed to be caused by pinning of the incommensurate wave by defects in the crystal. The results obtained for (Rb1-xKx)2ZnCl4 mixed crystal system are also well explained in terms of such pinning effect of defects.

Transient Dielectric Behavior during the Commensurate-to-Incommensurate Phase Transition in Rb2ZnCl4

The kinetic process of the commensurate-to-incommensurate phase transition in Rb 2 ZnCl 4 has been studied by tracing the time variations of the polarization and dielectric constant during the transition induced by removing the applied high dc electric field from the sample. The polarization shows an abrupt dropat the instant of field removal and then decays monotonically. The dielectric constant first increases, reaches a maximum and then decreases toward the thermal equilibrium value. The relaxation process slows down as T c is approached from above. A qualitative explanation of the observed transient characteristics is presented on the basis of a nucleation model. It is suggested that at an early stage of the transition the nucleated discommensurations are spaced more widely than in the final thermal equilibrium state.

Liquid crystalline amorphous blue phase and its large electrooptical Kerr effect

An amorphous blue phase III with low and wide thermal range (∼20 °C) including room temperature is induced by doping a bent-core nematic with a strong chiral material. We confirm that the electrooptical response is due to the Kerr effect, with the Kerr constant being up to two orders of magnitude larger than conventional Kerr materials such as nitrobenzene.

Effect of purification on dielectric properties near the commensurate-incommensurate transition point of Rb2ZnCl4

Abstract Purification of Rb2ZnCl4 by recrystallization and zone melting is described in detail and the results of chemical analysis are given. Single crystals were grown from an aqueous solution which had been purified by repeated recrystallization. The thermal hysteresis of the dielectric constant in the purified crystal was reduced to 0.2 K from 2.8 K in the unpurified crystal. The switching time of the field-induced commensurate-incommensurate transition was shortened significantly in the vicinity of Tc . These indicate the weakening of pinning in the purified crystal. The low relaxation frequency of domain wall motion in the commensurate phase starts to rise abruptly when the transition to the incommensurate phase begins. This is understood to reflect a process in which the isolated domain walls are incorporated into the discommensuration lattice of the incommensurate phase. The mathematical analysis of the ordinary recrystallization process and the reverse process is given in Appendix.

Thermal Hysteresis Accompanying the Incommensurate-Commensurate Phase Transitions in Rb2ZnBr4 and Rb2ZnCl4

The dielectric constants of Rb 2 ZnBr 4 and Rb 2 ZnCl 4 exhibit remarkable thermal hysteresis over wide temperature ranges including their incommensurate-commensurate transition points. The most characteristic feature of this thermal hysteresis is that it occurs even when a cooling run is turned to a heating run in the incommensurate phases, i.e., the thermal hysteresis occurs even when the crystal has no experience of phase transition. This phenomenon was studied in detail in this work. If the turning temperature T turn is in the range where the incommensurate wave number is independent of temperature (the range of constant δ), the thermal hysteresis does not occur, while if T turn is in the range of varying incommensurate wave number (the range of varying δ), the thermal hysteresis appears. This fact seems to support our idea that the thermal hysteresis of the dielectric constant is caused by pinning of the incommensurate wave.

Multiple nematic phases observed in chiral mesogenic dimers

A sequence of seven nematic phases has been observed in chiral mesogenic dimers, having odd number of carbon atoms in the spacer, thus a bent shape. The highest temperature phase is a chiral nematic (cholesteric) phase on heating or blue phases on cooling. The lowest temperature nematic phase expels the chiral twist and exhibits spontaneous bent–splay modulation.

Study of Thermal Expansion of NaNO2 with Special Emphasis on the Vicinity of the Transition Points

By use of a capacitance method, the thermal expansions of NaNO 2 ( T C =163.9°C, T N =165.2°C) along the a -, b - and c -axes were measured from -180°C to 240°C with the accuracy of 1×10 -6 cm in Δ l . Anomalies associated with the phase transitions were observed over the wide temperature range 100 to 200°C. The thermal expansion showed a distinct jump at T C , and the thermal expansion coefficient exhibited a divergent character near T N . The critical exponents were found to be 0.34∼0.36 for T > T N and 0.09∼0.20 for T < T N . These agree very well with those of the specific heat. It was revealed that the Neel temperature determined by the analysis of the thermal expansion data lies 0.1∼0.2 deg below the peak position of the dielectric constant. The pressure dependences of the transition points deduced from the Clausius-Clapeyron equation and the Pippard equation were found to agree with the result of the hydrostatic pressure experiment.

Critical Region in the Specific Heat of Ferroelectric TGS

Temperature dependence of the specific heat of TGS was investigated by an a.c. calorimetry technique. It was revealed that the anomalous specific heat above T c depends logarithmically on T - T c in a range \(T-T_{\text{c}}{\lesssim}20\) K, which is far wider than that reported so far.