Pyridinium perchlorate and pyridinium fluoroborate were grown and their successive phase transitions were studied by means of polarized microscopic observations and measurements of linear birefringence temperature changes. The microscopic observations revealed domain structure appearance at 245 K and 238.7 K for pyridinium perchlorate and pyridinium fluoroborate, respectively. This domain structure exists also in lower phases. Birefringence measurements confirmed two first-order phase transitions at 245.3 K and 231.5 K for pyridinium perchlorate and two continuous phase transitions at 238.7 K and 205.4 K for pyridinium fluoroborate.
The domain nucleation and growth during switching process in telluric acid ammonium phosphate crystals has been investigated by nematic liquid crystal decoration technique. The domain configuration arising during polarization reversal has been discussed, considering that the type of nucleation is the controlling factor in the propagation of the domain walls.
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.
The paper presents the new data of dielectric dispersion in DMAGaS crystal. The very pronounced dispersion above T_{c1} =136 K is observed in the frequency range 100 Hz-30 MHz with a single relaxation process. The relaxation time τ of an electric dipole reaches 2.17×10^{-7} s at the T_{c1}. The activation free energy ΔF of the dipoles is estimated to be equal to 0.087 eV. The order-disorder type of phase transition is confirmed.
Linear birefringence temperature changes are measured in the betaine phoshite crystal. The linear birefringence increments induced by antiferrodistortive, structural and ferroelectric phase transitions are analysed. Temperature derivatives of the linear birefringences are shown for comparison. Critical exponents are determined for both transitions. The antiferrodistortive phase transition is close to a tricritical one.
Tetraguanidinium dichloride-sulphate crystal, [C(NH_2)_3]_4Cl_2SO_4, abbreviated as G_4Cl_2SO_4 was investigated. The vibrational infrared spectra of powdered G_4Cl_2SO_4 crystal in Nujol mull were studied in the wide range of temperature, from 298 K to 377 K. This temperature range contains all the phases in the crystal (named III, II, I on heating, respectively). The temperature changes of wavenumbers, centre of gravity, and intensity of the bands were analyzed to clarify the molecular mechanism of the phase transitions. It was shown that in cooling from 377 K to 313 K the phase II is the same as the room temperature phase. Information about hydrogen bonds was obtained. The time dependence of internal vibrations at 356 K was observed and it was connected with slow transition III → II. For more detailed band assignment Raman spectrum at room temperature, at ferroelectric phase was carried out. Theoretical calculations were made based on density functional theory, with the B3LYP method using 6-311 + G(d,p) basic set. Calculated normal vibrational modes of the molecule, their frequencies and intensities were compared with these recorded in experiment. Theoretical description of the molecule including hydrogen bonds were optimized and the bond parameters were obtained. The Mulliken charges population analysis was performed.
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.
The "steady-state" non-linear optic polarization arising in lithium sulphate monohydrate crystals under the influence of the short pulse laser irradiation has been investigated in the temperature range of 95-305 K. The reversal of the sign of the components of the generalised non-linear susceptibility tensorχ has been found at about 105 K. The change of the photoresponse kinetics and a drastic temperature dependence of the componentsχ_{ijk} in the temperature range of 95-105 K, as well as the change of the spectral dependence of the photoresponse were observed.
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 paper reports the electron paramagnetic resonance investigation of Cu^{2+} doped [(CH_3)_2NH_2]_5Cd_3Cl_{11} (DMACC) monocrystal. DMACC:Cu^{2+} was studied in the temperature range (6÷293) K. We did not observe any anomalies of spin-Hamiltonian parameters of Cu^{2+} ion corresponding to the temperature anomalies observed in dielectric studies. The electron paramagnetic resonance anomaly recorded at about 40 K results from the subtle changes in bonds angles and Cu-Cl distances and suggests the smooth second order transition.
This paper presents the results of investigations of the dielectric properties of NH_2(CH_3)_2Al(SO_4)_2×6H_2O crystals performed in a wide frequency range. In particular, the temperature and frequency dependences of the dielectric permittivity were measured in the region of ferroelectric phase at different rates of temperature change. The thermal dipole relaxation with a critical slowing-down at T_{c1} was observed at comparatively low frequencies of measuring field. The relaxation connected with the domain freezing takes place at still lower frequencies (below 100 kHz) and was found to be strongly dependent on the rate of the temperature change. It was shown that deuteration results in a noticeable change of the parameters characterising this process including the temperature of domain wall freezing.
Investigations of temperature dependence of a nuclear quadrupole spin-lattice relaxation time of ^{79}Br nuclei in ferroelectric (CH_{3})_{4}NCdBr_{3} are presented. It is shown that in paraelectric phase in the vicinity of phase transition temperature relaxation process is determined by critical fluctuations of electric field gradient. In ferroelectric phase relaxation is related to three different mechanisms, namely: lattice vibrations, CH_{3¯} group reorientations and TMA cation motions. According to the results of measurements it is concluded that the phase transition is connected with ordering of TMA cations.
X-band electron paramagnetic resonance investigations of single crystals of Cr^{3+}-doped dimethylammonium gallium sulphate hexahydrate are presented from room temperature down to 110 K. The crystal undergoes a order-disorder phase transition to ferroelectric phase at 134 K and additionally a first-order transition into a low temperature non-ferroelectric phase at T_{c2}=116 K. The spin-Hamiltonian parameters were determined for paraelectric and ferroelectric phases. The spin-Hamiltonian parameters in the paraelectric phase are: g=1.982±0.002, b_2^0=(890±10)×10^{-4} cm^{-1}, b=(386±15)×10^{-4} cm^{-1}. Remarkable EPR line width changes confirm the order-disorder character of the ferroelectric phase transition. Additionally observed triplet lines demonstrate freezing-out of dimethylammonium reorientations which can be considered as a prime reason for this transition similarly as observed in dimethylammonium aluminium sulfate hexahydrate.
Results of optical thickness and refractive indices measurements for (NH_{4})_{2}SbF_{5} crystal in the temperature range 100-310 K are presented. The measurements were done with interference method using He-Ne laser (λ = 632.8 nm). At T_{c1} = 292 K and T_{C2} = 168 K two continuous phase transitions with critical indices β_{1} = 0.38 and β_{2} ≤ 0.57 were observed. Additional anomaly of temperature changes of interference order with maximum at T_{c3} ≈ 132 K were found. This anomaly resembles a diffused transition. Temperature-averaged values of derivatives of refractive indices dn_{g} /dT = -3.3 × 10^{-5} K and dn_{m}/dT = -2.9 × 10^{-5} K^{-1} evidenced a decrease in optical anisotropy (birefringence) along the b-axis on heating run.
Experimental investigation of the thermal linear expansion l(T) and temperature dependencies of the interference optical path difference D(T) were carried out for (CH_{3})_{2} NH_{2}Al(SO_{4})_{2}·6H_{2}O crystal, in the range of 120-310 K for three crystal physical directions. Decrease in electrostriction coefficients g of crystal was observed on approaching the phase transition point T_{c} ≈ 150 K in ferroelectric phase. Relative temperature changes of the lengthening δl/l and refractive index δn/(n-1) are of the same order of magnitude. Peculiarities of temperature dependence of dilatometric and optical parameters of (CH_{3})_{2}NH_{2}Al(SO_{4})_{2}·6H_{2}O testify for qualitative reconstruction of chemical bonds in the crystal when temperature changes in the same phase.
The complementary measurements of the dielectric dispersion in ferroelectric phase of glycine phosphite crystal at the frequency range of 200 Hz - 1 MHz have been carried out. The analysis of the dielectric spectra of the low frequency relaxation process showed that it can be fitted by two Debye-type relaxators. The activation energy of the both processes has been calculated. The observed behaviour of the relaxators suggests that they are connected with a domain structure; one with the growth of the nuclei whereas the second with the domain wall movement.
The critical temperature dependence of the nuclear quadrupole- resonance ^{79}Br frequencies at the phase transition (T_{c} = 163 K) was found in (CH_{3})_{4}NCdBr_{3}. It was concluded that bromine nuclei take part in the phase transition not directly but via the interaction with the crystal polarization.
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.
Chemical preparation, differential scanning calorimetry and thermal stability differential thermal gravimetry studies, positron annihilation lifetime investigations, optical observations as well as electric properties of the NH₃(CH₂)₃NH₃CuCl₄ crystal are presented. On the basis of the differential scanning calorimetry response the structural phase transition of the first order was observed at 436 K. The enthalpy and entropy of the phase transition are equal to 1120 J/mol and 2.57 J/(mol K), respectively. Differential thermal analysis and thermogravimetric analysis studies confirmed the phase transition at 436 K and one can conclude the chemical and thermal stability of the compound up to about 480 K. Optical observations showed a continuous change of colour from yellow to dark brown above the phase transition to 436 K. Dielectric measurements showed a significant increase of conductivity upon approaching the phase transition regions, with a significant increase above the phase transition temperature. An activation energy dependent on the temperature range, and different for each particular phase, is obtained from measurements of complex impedance.
Dielectric dispersion in ferroelectric hydrogen bonded glicyne phosphite crystal was investigated in the frequency range 100 Hz - 27 GHz. Dielectric relaxation of Debye type observed in the paraelectric phase shows a critical slowing down of the polarization fluctuations. The relaxation frequency decreases with temperature according to f_{s} = 0.305(T-T_{0}) GHz in the paraelectric phase. The activation energy for flipping dipole motion ΔU = 2.07kT_{c} confirms order-disorder character of the phase transition. In the ferroelectric phase pronounced low frequency (100 Hz - 1 MHz) dispersion related to domain contribution to permittivity was found.
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