S. A. Taraskin

Ferroelectric properties of smectic liquid crystal

Thermodynamical, optical and dielectric properties of chiral smectic liquid crystal DOBAMBC were studied. It was found that the ferroelectric phase transition in DOBAMBC is of the second order, and critical exponents for 6 (order parameter), Ps (spontaneous polarization), Cp. (specific heat) were estimated. The phenomenological description of static and dynamic phenomena, connected with helix unwinding in the electric field is performed.

Ferroelectric behaviour of different classes of smectic liquid crystals

Abstract It is shown that two new classes of smectic liquid crystals - esters and salicelidenanilines - have ferroelectric properties. The new ferroelectric smectics have a hightened hydrolitical stability. Their electrical and heat properties are investigated, the correlation between a chemical composition and spontaneous polarisation values is discussed.

Critical phenomena in perfect and inperfect TGS crystals

Abstract The heat and elastic properties of TGS crystals grown above (type A) and below (type B) Tc were investigated. The critical anomalies of specific heat and sound velocity of type A crystals are closely follow Landau theory and should be considered as “perfect”. The type B crystals and small doses γ-irradiated type A crystals reveal the very similar critical anomalies which main features are the “tail” in paraelectric and small anomalous change of specific heat and sound velocity in ferroelectric phase. The thermal and elastic anomalies for both perfect and inperfect crystals obey Pippard-Janovec relations.

Thermal and Dielectric Properties of a New Ferroelectric LaBGeO5

Thermal and dielectric properties of LaBGeO 5 , a new ferroelectric with a stillwellite-type structure are investigated. The specific heat, C p , shows one clear anomaly (Δ S =0.114 R) at 802.5 K. The basic thermodynamic parameters are determined on the basis of the Landau theory. However, an additional hump in C p was found in a different sample just above T c . This dependence on samples is confirmed by the measurements of dielectric constants. The appearance of the new intermediate phase may be caused by some impurities or defects. The Rhodes-Wohlfarth plot suggests that the nature of the phase transitions is of the displacive-type.

Specific heat and heat conductivity of BaTiO3 polycrystalline films in the thickness range 20–1100 nm

Thermal properties - specific heat and heat conductivity coefficient - of polycrystalline BaTiO3 films on massive substrates were studied as a function of the temperature and the film thickness by ac-hot probe method. The anomalies of specific heat with decreasing of the film thickness from 1100 to 20 nm revealed the reducing of critical temperature (Tc) and excess entropy of the ferroelectric phase transition, which becomes diffused. The critical thickness of the film at which Tc = 0 estimated as 2.5 nm.Thermal properties - specific heat and heat conductivity coefficient—of polycrystalline BaTiO3 films on massive substrates were studied as a function of the temperature and the film thickness by the ac-hot probe method. The anomalies of specific heat with the film thickness decreasing from 1100 to 20 nm revealed the reduction of Tc and excess entropy of the ferroelectric phase transition which becomes diffused. The critical thickness of the film at which Tc = 0 has been estimated as 2.5 nm.

Defects and ferroelectric phase transitions

Abstract With TGSe crystal as an example the y-irradiation effects upon the ferroelectric phase transition including dose dependence of Landau expansion coefficients are considered and the interpretation of the results is given.

Anomalous effect of γ-irradiation on the phase transition in diglycine nitrate crystals

Abstract The essential difference of γ-irradiation induced changes of the thermodynamical properties of DGN crystals as compared with TGS crystals is found. A surplus energy of the phase transition decreases under the successive accumulation of the radiation dose but in contrast to TGS it was not observed the threshold dose which is enough to destroy the ferroelectric order. The stationary defect concentration is seems to settle for doses near 10 MR and more. For this and greater doses (up to maximal ones which were about 50 MR) the transition point shift ΔTc ∼ 2 K and relative surplus phase transition energy ΔQD /ΔQo ∼ 0,5 were obtained. The time dependence of the crystal properties after irradiation is traced.

Thermodynamical Properties of the Thin Polycrystalline BaTiO 3 Films on Substrates

The results of measurements of the specific heat and heat conductivity coefficient of the polycrystalline BaTiO 3 thin films on substrates are presented. The range of the film thickness 20-1100 nm. The reducing of the ferroelectric transition point and excess entropy, diffusion of the transition are revealed. The critical thickness of the film when the transition temperature goes to zero is estimated as 2.5 nm.

Specific heat overcritical behaviour in (CH3NH3)5Bi2Cl11 (MAPCB) crystals

The results of the study of thermal properties of MAPCB crystals in the temperature interval 90-330 K are presented. Specific heat anomalies at 160 K and 307 K are revealed and discussed. It is shown that both anomalies can be described quantitatively in the phenomenological model considering the low-temperature anomaly as an overcritical “trace” of the first order isomorphic phase transition.

Effect of growth defects, impurities and γ-radiation upon phase transitions in ferroelectrics

Abstract The influence of different types of defects upon dielectric, thermal and elastic properties of TGS-type ferroelectric crystals, and some effects in γ-irradiated and doped crystals are discussed.