The non-isotropic character of electric and dielectric properties of ammonium zinc chloride crystal

The dielectric constant, ε, and the d.c. conductivity, σ, were measured along the a-, b- and c-axes of (NH₄)₂ZnCl₄ (AZC) crystal in the 300-450 K temperature range. Crystals of AZC grown from aqueous solutions containing excess of ZnCl₂ were used. The value of the dielectric permittivity of AZC is extremely small compared to other ferroelectric crystals. Pronounced broad or step-like peaks at the phase transition temperatures were detected along the a- and b-axes, while ε along the c-axis is temperature independent up to the end of the measuring range. Reciprocal of the dielectric permittivity in the range of the commensurate to incommensurate phase transition obeys a relation similar to the Curie-Weiss law that is valid for second order ferroelectric/paraelectric phase transitions. The constants of the proposed relationship applied to the cooling run are given. The J-E characteristics along the three crystallographic axes were measured in the normal, incommensurate, commensurate and antiferroelectric phases. Hence, the type of conduction mechanism has been estimated. Parameters of Poole-Frenkel and Richardson-Schottky types of conduction mechanism have been determined. The effect of applied electric field on the conductivity measurement was also tested. Conductivity anomalies with different character were observed at the phase transition temperatures. The lnσ−1000/T dependence revealed thermal activation energy of conduction along the a-, b- and c-axes with different values in different phases of AZC.     

Properties of the secondary order parameter in Rb2ZnCl4

The dielectric constants and the spontaneous polarization of Rb₂ZnCl₄-crystals have been investigated in order to characterize the nature of the transitions at 303, 195 and 74.5 K. The dielectric anomalies around 303 K hint at a critical exponent β = 0.36 ± 0.03 in the incommensurate phase. Close to 195 K the commensurate state can be induced by electric fields in agreement with an appropriate Clausius-Clapeyron relation. The polarization measurements further show that Rb₂ZnCl₄ has a (second) ferroelectric transformation at 74.5 K.     

Coexistence of magnetic and ferroelectric properties in Y0.1Co1.9MnO4

The magnetic,conductivity,and dielectric properties have been investigated in single-phase polycrystalline Y0.1 Co1.9 MnO4.The temperature-dependent magnetisation reveals the ferromagnetic transition in sample at a low temperature (～186 K).Magnetisation as a function of field H (M-H loop) indicated the weak ferromagnetism of the sample at room temperature.The constant ε and dielectric loss tgδ measurements represent a ferroelectric phase transition at a higher temperature (～650 K),while the conductivity shows an insulator-metallic transition.The ferroelectric hysterisis loops and capacitance-voltage measurements confirm the ferroelectric nature of the sample at room temperature.The observed ferromagnetism and ferroelectric nature in this material suggests a potential multiferroic application.     

Evidence for low temperature phase transition in Rb2ZnCl4 from dielectric constant and birefringence measurements

Temperature dependences of the dielectric constants ?_i and of the birefringences Δn_i for light propagation directions along all the three crystallographic axes (i = a, b, c) have been measured between 5 and 350 K. The optical measurements clearly reveal a new phase transition at T_L = 75 K, below which the crystal structure is transformed from the ferroelectric Pna2₁ phase into an unknown low temperature phase, possibly the monoclinic space group P112₁. Small peaks in the dielectric constants ?_a and ?_b have been observed at this transition temperature.     

Ferroelectric phase transitions in NH4HSeO4 crystals

Results are presented of measurements of spontaneous polarization, electric permittivity and electrical conductance of NH₄HSeO₄ crystals in three principal crystallographic directions from liquid nitrogen temperature to 320 K. NH₄HSeO₄ crystals exhibit ferroelectric properties in the a-axis direction between 106–250 K, whereas in the b-direction they are pyroelectrie over the entire temperature range. The calculated activation energy (1.42 eV) is indicative of semiconducting behavior.     

Electrical investigations in the normal and incommensurate phases of [N(CH3)4]2ZnCl4 single crystals

The dielectric permittivity (?) of TMA-ZC single crystals was measured along the mean crystallographic axes a, b and c, in a temperature range from 273 to 340 K. The ?-T relationship exhibited peak values at T_i=296 K and T_c1=279 K for the three axes. These peaks are attributed to the contribution of discommensurations. The d.c. and a.c. electrical conductivity showed anomalous variation at the same transition temperatures, with a remarkable change in the value of the activation energy around the transition temperatures. The j-E characteristic indicates different types of electrical conduction. The mechanism of the phase transition and the electrical process were discussed on the basis of Shottky and Frenkel conduction mechanisms.     

Time relaxation of dielectric constant in the commensurate phase of TlGaSe2

The results of measurements of the time dependences of the dielectric constant of TlGaSe₂ in the commensurate ferroelectric phase are presented. From the result of the observation of the decay of ε at different stabilized temperatures below the commensurate phase transition temperature after cooling from the incommensurate phase, the presence of two different characteristic relaxation time constants with the same temperature behaviour has been revealed. This peculiarity is considered as a result of a coexistence of two polar sublattices in the temperature range below 110 K. According to these results, the previously reported dielectric anomaly at about 103 K is considered as a final lock-in phase transition accompanied by the forming of the antiferroelectric state in TlGaSe₂.     

Ferroelectric phase transition in Ga2Te3 single crystals

Measurements of the electrical conductivity and Hall effect were carried out in a wide temperature range (200-500 K) for Ga₂Te₃ crystals. The crystals were grown in single crystalline form by making a modification of the travelling heater method technique. The measurements revealed unusual observations in the electric conductivity and Hall mobility indicating the presence of some type of phase transitions at about 430 K. So, ferroelectric behavior was examined for confirming the presence of second-order (ferroelectric) phase transition. An energy gap of 1.21 eV and depth of the impurity center of 0.11 eV were found.     

Electrical and thermal studies in the commensurate incommensurate phase region of (NH4)2ZnCl4 single crystal

DC electrical conductivity for a virgin and poled annealed (NH₄)₂ZnCl₄b-axis single crystal shows a defect controlled property. A Schottky mechanism is a probable mechanism of conduction in regions of strong structural transitions. The rise of conductivity in the incommensurate and paraelectric phases is linked to an increase in discommensurations density. The activation energies (ΔE) in the three phases region were calculated. DTA measurements shows that the crystal is stable up to 200 °C and the phase transition temperatures were observed at 42, 94.8 and 137 °C. The effective activation energy (E_e) was obtained using Kissinger and Mahadevan equations. It was found to be equal to 0.49 eV. This correlates with the value obtained through DC conductivity.     

Thermodynamic properties of successive phase transitions in Rb2ZnBr4

Heat capacity measurements have given the following transition temperatures and entropies: 0.027 J K^?1mol^?1 at 76.5 K, 0.47 J K^?1mol^?1 at 111.7 K, and 0.048 J K^?1mol^?1 at 193.6 K. These are interpreted as “lock-in”, soft-mode, and “lock-in” transitions, respectively, from comparison with analogous results for Rb₂ZnCl₄ and K₂SeO₄. The N-IC transition was located at 347 K by DTA.     

Ferroelectricity and phase transition from incommensurate phase into commensurate one in (NH4)2ZnCl4

Phase transition has been found in (NH₄)₂ZnCl₄ at T = 266 ± 0.5 K by NQR method. There is a ferroelectric phase below T_c with a space group P2₁cn and with the trebling of the elementary lattice parameter along the axis c. Above the phase transition temperature in the crystal (NH₄)₂ZnCl₄ an incommensurate phase is realized.     

Dielectric properties of deuterated ammonium Rochelle salt

The dielectric constant of deuterated ammonium Rochelle salt along the three crystallographic axes as well as the pyroelectric charge density developed on the crystal surface perpendicularly to the b-axis have been measured with a high temperature resolution. The para- to ferroelectric transition at T_c = ?159°C shows a thermal hysteresis of ΔT = 0.15°C. This and the discontinuities in the dielectric constant along all three axes show that the phase transition is of the first-order. No phase intermediate between the para- and the ferroelectric phases could be detected.     

Decoupling of the order-disorder and displacive contributions to the phase transition in NH4H(ClH2CCOO)2

The effects of hydrostatic pressure and substitution of Rb⁺for the ammonium cations on the ferroelectric phase transition temperature in NH₄H(ClH₂CCOO)₂ have been studied by electric permittivity measurements. The transition temperature (T_c) decreases with increasing pressure up to 800 MPa and the pressure coefficient dT_c/dp=−1.4×10⁻² [K/MPa] has been experimentally determined. The substitution of Rb⁺ for the ammonium cations has been shown to considerably lower the ferroelectric phase transition temperature T_c. In mixed crystals, additional electric permittivity anomaly has been clearly evidenced. The results are discussed assuming a model, which combines polarizability effects, related to the heavy ion units, with the pseudo-spin tunnelling.     

Slowing down of the relaxational dynamics at the ferroelectric phase transition in one-dimensional (TMTTF)2AsF6

We present measurements of the dielectric response of quasi one-dimensional system (TMTTF)₂AsF₆ in a wide temperature and frequency range. We provide a thorough characterization of the relaxational dynamics observed close to the ferroelectric-like transition at T_c=100 K. Our measurements, extending up to 100 MHz, reveal a continuous slowing down of the mean relaxation time when approaching T_c from high as well as from low temperatures. The simultaneous critical rise of the dielectric constant and relaxation time point to an explanation of the transition in terms of a classic ferroelectric scenario.     

Temperature dependence of the indirect band gap, steepness parameter and related optical constants of [Kx(NH4)1−x]2ZnCl4 mixed crystals

Optical transmittance measurements near the absorption edge of [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals, where x=0.00, 0.232, 0.522, 0.644, 0.859 and 1.00, are reported over 276–350 K range. Analysis reveals that the type of transition is the indirect allowed one. The absorption edge shifted towards lower energy with increasing temperature. It is shown that [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals with x0.644 reveal a phase transition at 319 K, this phase disappeared at high concentrations of K⁺ ions. The steepness parameter is given, its value is used to estimate the temperature dependence of the indirect energy gap. In the region of the absorption edge, the absorption coefficient obeys Urbach's rule. Urbach parameters are investigated as a function of temperature.     

Influence of crystal synthesis on the incommensurate phase and lock-in transition of Rb2ZnCl4 crystals

A comparative study of several crystals of Rb₂ZnCl₄, obtained by different crystal growing methods, has allowed us to determine the influence of growth defects on the incommensurate phase and on the lock-in transition of these samples. X-ray diffraction has allowed us to complete previous dielectric measurements realized on the same samples and to relate the crystalline quality to the evolution of the modulation as a function of the temperature. The principal influence of an increasing defect density seems to be a stronger pinning of the modulated phase and this induces a lower lock-in temperature and a wider hysteresis.     

Infrared dispersion of potassium zinc chloride

The infrared reflection spectra for the three main polarizations of K₂ZnCl₄ in the orthorhombic commensurate ferroelectric phase are reported at room temperature. Phonon frequencies are assigned to internal and external modes by comparison with K₂SeO₄ and Rb₂ZnCl₄ data. The dielectric constant along the ferroelectric axis is essentially due to the weight of the low-frequency heavily-damped mode.     

Proton conduction in (NH4)3 H(SO4)2 single crystals

D.C. electrical conductivity, DTA and coulometric studies on (NH₄)₃ H(SO₄)₂ single crystals are made. Conductivity is markedly anisotropic with maximum along c¹ direction. A sudden jump in the conductivity plot along c¹ direction at 413 K is supported by a large endothermic peak in DTA, confirming the presence of transition at this temperature. The values of activation energy calculated from conductivity measurements indicated that the charge carriers are protons. This was further confirmed by coulometric experiment where the gas evolved was hydrogen, as established by a gas chromatograph and the volume of H₂ released agreed with that expected from electrolysis. The mechanism of protonic conduction in this crystal is discussed.     

Effect of purification on dielectric properties near the commensurate‐incommensurate transition point of Rb2ZnCl4

Purification of Rb₂ZnCl₄ by recrystallization and zone melting is described in detail and the results of chemical analysis are given. Single crystals were grown from an aqueous solution which had been purified by repeated recrystallization. The thermal hysteresis of the dielectric constant in the purified crystal was reduced to 0.2 K from 2.8 K in the unpurified crystal. The switching time of the field-induced commensurate-incommensurate transition was shortened significantly in the vicinity of T_c . These indicate the weakening of pinning in the purified crystal. The low relaxation frequency of domain wall motion in the commensurate phase starts to rise abruptly when the transition to the incommensurate phase begins. This is understood to reflect a process in which the isolated domain walls are incorporated into the discommensuration lattice of the incommensurate phase. The mathematical analysis of the ordinary recrystallization process and the reverse process is given in Appendix.     

A low-temperature tilter system and its application to the measurement of the anisotropy of optical rotation in K2Zncl4 in the vicinity of the phase transition at 145 k

The ferroelectric/ferroelastic phase transition of K₂ZnCl₄ at 145 K has been investigated employing the tilter method (Kaminsky and Glazer (1996) Ferroelectrics 183, 133) which was adapted to low-temperature experiments. We were able to observe the anisotropy of optical rotation in K₂ZnCl₄ which shows a distinct discontinuity at the transition temperature. A precursor-optical rotation in the ferroelectric phase is discussed in connection with a cluster ordering scheme suggested in an earlier X-ray study. The rotation of the optical indicatrix which accompanies the transition from the orthorhombic to the monoclinic system shows a Landau-type temperature dependence. Model calculations based on the dipole-dipole interaction between the atoms of the structures at different temperatures are in qualitative agreement with the experimental results.