B. A. Strukov

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 thermal conductivity of BaTiO3 polycrystalline thin films

The results of measurements by the ac heater-probe method of the specific heat and thermal conductivity of polycrystalline BaTiO3 films with thickness between 1 and 0.1 μm in a temperature range 120–440 K are presented. In the heating and cooling runs, the phase transition anomalies of the thermal properties in the region of each phase transition in the sequence of symmetry change m3m-4mm-mm2-3m have been observed. A temperature hysteresis is observed, which is the signature of the first order phase transitions. As film thickness decreases, anomalies shift to lower temperatures and became smeared.

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.

Optical Phonons and Ferroelectric Phase Transition in the LaBGeO5 Crystal

Infrared (IR) reflectivity and submillimeter transmission measurements of ferroelectric and paraelectric LaBGeO 5 single crystals were carried out in the temperature range of 300 to 870 K. From the classical oscillator fit to the spectra, the complex dielectric function in the range of 4 to 3000 cm -1 was calculated. In addition, room temperature polarized Raman spectra were recorded and evaluated which, together with the known high temperature Raman data, enabled us to identify and classify all of the 46 transverse optical modes predicted by the factor-group analysis and their longitudinal optical analogues. The order-disorder type of the ferroelectric transition at T c 800 K was confirmed by revealing a microwave critical relaxation near and above T c . However, this relaxation vanished below 700 K and an anomalous IR phonon mode was detected which softens partially from 87 cm -1 at 300 K down to 60 cm -1 near T c , where it becomes nearly overdamped and coupled to the relaxation. This represents a novel partial displacive feature which is revealed when the structure below the order-disorder transition becomes ordered.

Global hysteresis in ferroelectrics with incommensurate phases

Abstract A review of the basic experimental results connected with the formation of long-living metastable states in the incommensurate phases of ferroelectric crystals and related phenomena is given. The interpretation and appropriate physical mechanisms underlying the global hysteresis phenomena associated with these states are considered.

Clarification of Size Effects in Polycrystalline BaTiO3 Thin Films by Means of the Specific Heat Measurements: Grain Size or Film Thickness?

Specific heat of polycrystalline BaTiO3 thin films on the fused quartz substrate was measured by ac-hot probe method. Phase transition temperature and excess entropy were determined as a function of film thicknesses and grain sizes. The variation of the latter was obtained in the limits 30–150 nm by changing of the temperature of the substrate during sputtering while thickness of films 20–1100 nm was controlled by the time of sputtering. It was found that the relation between the thickness and grain size is important for the size effects in polycrystalline films.

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.