Dielectric, dilatometric measurement and polarized microscopic observation were performed for single crystal of Cs_2CdBr_4. These investigations showed clearly the phase transitions at 252 K, 236 K and 153 K. The jump-like change of the crystal volume corresponding to the first-order phase transition was found at temperature of 236 K. Ferroelastic domain structure was observed along the a-axis below 236 K and it is preserved below the transition at 153 K. Observation along the c-axis revealed new domain structure below 153 K.
Temperature dependences of the linear dimension L(T) and spontaneous polarization P_s(T) for triglycine sulphate and glycine posphite single crystals have been measured in the same run of temperature. A decrease in the spontaneous electrostriction coefficient Q_L of both crystals studied is displayed when approaching the corresponding phase transition point T_c. A temperature dependence of the coefficients Q_L(T) shows that the rates of temperature changes of displacement of the atomic complexes of corresponding crystal structure forming the square of spontaneous polarization P_s^2 (primary order parameter) and the displacements of all atomic complexes forming the spontaneous linear thermal expansionδ L_s/L (secondary order parameter) are different. The applying external electric field E=3×10 ^5 V/m to triglycine sulphate crystal does not influence essentially the temperature dependence of Q_L(T) in the temperature range of phase transition below T_c.
Dielectric, dilatometric, and ultrasonic studies around the ferroelectric phase transition at 319 K were done. Measurement of relative electric permittivity showed the existence of a strong bias field which causes in studied samples some diffused character of phase transition. However, all presented measurement confirmed the continuous character of phase transition. The obtained data are similar to those received earlier in the case of hydrogenated glycine phosphite crystal.
The crystal of (CH_{3})_{2}CHNH_{3}CdCl_{3} was grown and its physical properties were investigated. On the ground of DSC, dylatometric and dielectric investigations the phase transition at 390 K on heating process was revealed. On cooling two phase transitions at 382 K and 352 K were found. Polarized microscope observations show appearance of ferroelastic domain structure and cracks at 390 K on heating. On cooling the observed domain structure vanishes at 352 K.
On the basis of the optical and dielectric investigations of [(CH_3)_2NH_2]_5Cd_2CuCl_{11} solid solutions the existence of phase transitions at T_1=175 K and T_2=117.5 K was confirmed. Both phase transitions were found to be shifted toward lower temperatures with respect to the corresponding transitions in the "host" [(CH_3)_2NH_2]_5Cd_3Cl_{11} crystals. It was found that the proton conductivity in [(CH_3)_2NH_2 ]_5Cd_2CuCl_{11} crystals is realized through the Grotthus mechanism. The investigations of the birefringent properties confirmed existence of the structural changes at T_0=320 K connected with the complex co-operative effect involving weakening of the hydrogen bonds and modification of the Jahn-Teller distortion with temperature.
On the basis of dilatometric and dielectric investigations of NH(CH_3)_3CuCl_3·2H_2O crystals the earlier unknown phase transition with considerable temperature hysteresis was found at T_1^c=198 K and T_1^h=223 K, respectively, in cooling and heating runs. Existence of the characteristic dielectric dispersion was revealed within the high-temperature phase. It corresponds to the single dielectric relaxator below 250 K as well as to co-existence of two relaxators above this temperature. The determined values of the activation energy and relaxation time are characteristic of the co-operative reorientation of the trimethylammonium cation (high-frequency relaxator) whereas the combined reorientation motions of the CuCl_2·2H_2O chains would be responsible for the low-frequency relaxation process. It was found that the above mentioned quasi-Debye type processes are involved into the mechanisms of the protonic conductivity.
This paper presents the results of investigations of the temperature dependence of heat capacity and dielectric dispersion in the vicinity of ferroelectric-ferroelastic phase transition of dimethylammonium metal sulphate hexahydrate crystals DMAAl_{1-x}Cr_{x}S. In particular, it is shown that the isomorphous substitution of metal ion noticeably changes the temperature of phase transition and parameters of the fundamental ferroelectric dispersion observed around T_{c1}. These changes are explained in terms of clusters sizes and dynamics in the framework of order-disorder type phase transition mechanism.
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