R. Tchukvinskyi

Characterization of ferroelectric phase transition in GPI crystal

The dielectric behaviour of a glycine phosphite [GPI: Gly.H 3 PO 3 ] crystal is investigated around its ferroelectric phase transition. The temperature dependence of the dielectric constant is measured for dc bias fields up to 200 kV/m. The dielectric constant is also measured in the paralectric phase as a function of electric field at constant temperatures. The results are discussed on the basis of the classical state equation and the equation coefficients are estimated. The obtained results lead to the conclusion that the ferroelectric phase transition in GPI has an intermediate character between critical and tricritical one.

Dielectric relaxation in high temperature phases of (NH3C2H5)2CuCl4 crystals

On the basis of dielectric investigations of (NH 3 C 2 H 5 ) 2 CuCl 4 crystals the existence of phase transitions at T 1 = 364 K and T 2 = 356 K (in a heating run) was confirmed. In the temperature region of 300-370 K Debye relaxation was observed. The change of the dielectric dispersion character around T 3 = 330 K is related to the phase transition. The determined values of the activation energy and the times of the dielectric relaxation for all phases are characteristic of the combined reorientation motions of the alkylammonium cation. The conclusions concerning the dynamics of the hydrogen bonds were confirmed by IR spectroscopy.

Dielectric and Electrooptic Properties of the DMAMeS (Me = Al, Ga) Crystals in the Region of Low Temperature Phases

This paper presents the results of investigations of the dielectric, birefringent and electrooptic properties of NH 2 (CH 3 ) 2 Me(SO 4 ) 2 .6H 2 O (Me = Al, Ga) crystals. The data obtained are compared with the predictions of a phenomenological theory. In particular, the temperature behaviour of the dielectric susceptibility components and their anomalies in the vicinity of phase transition points were investigated in details. The specific behaviour of the studied parameters in the low temperature region was related to the coexistence of ferroelectric and antiferroelectric ordering of the dimethylammonium groups carrying a dipole moment.

Electron–Phonon Interaction and Phase Transitions in [C2H5NH3]2CuCl4 Crystals

Optical and spectral investigations confirmed the availability of phase transitions in [C 2 H 5 NH 3 ] 2 CuCl 4 at T 1 = 364 K, T 2 h = 356 K, T 3 = 232 K. It has been found that the empirical Urbach rule is fulfilled in all phases. It is shown, that in the high-temperature phases skeletal vibrations of the C-N-C group interact with phonons, whereas in the low-temperature phases internal vibrations of the metal-halogen complex occur. A ferroelastic domain structure was observed using the polarising microscope in the temperature region 232 to 364 K.

Phase Transitions in [N(C2H5)4]2MeCl2Br2 (Me = Zn, Co, Cu) Solid Solutions

In this paper dielectric permittivity and birefringence of [N(C 2 H 5 ) 4 ] 2 MeCl 2 Br 2 (Me = Zn, Co, Cu) have been investigated. As the results of our measurements we present structural phase transitions in these ferroelectric crystals.

Thermal Expansion and Optical Properties of DMAGS Crystals around Their Phase Transitions

This paper presents the results of the investigations of thermal expansion, birefringent and electrooptic properties of NH 2 (CH 3 ) 2 Ga(SO 4 ) 2 . 6H 2 O (DMAGS) crystals. It has been shown that the ferroelectric phase in DMAGS is limited by the first-order phase transitions at T c1 h = 136 K and T c2 h = 119 K. Both phase transitions are connected with a considerable reconstruction of the crystalline structure. Absence of the linear electrooptic effect below T c2 reflects the disappearence of spontaneous polarization along the investigated X-direction.