Dipolar Glass Research Articles

Dielectric properties in the vicinity of the ferroelectric phase transition in a mixed crystal of deuterated betaine phosphate0.03 betaine phosphite0.97

The dielectric behaviour of ferroelectric hydrogen bonded deuterated betaine phosphate0.03 betaine phosphite0.97 (DBP0.03DBPI0.97) is investigated in the region of the ferroelectric phase transition. Dielectric dispersion is investigated in the frequency range up to 12 GHz. The dielectric dynamics shows a critical slowing down. The frequency of the relaxational soft mode in the paraelectric phase varies according to the quasi-one-dimensional Ising model and decreases up to 0.31 GHz at the phase transition temperature (TC = 272 K). The obtained activation energy for the deuteron flipping motion ΔU = 2.3kTC = 0.054 eV shows the order-disorder character of the ferroelectric phase transition. The spontaneous polarisation, obtained from pyroelectric measurements, is also well explained using the quasi-one-dimensional Ising model. At low temperatures, the freezing phenomena in DBP0.03DBPI0.97 revealed the complex dielectric permittivity behaviour characteristic for the transition into the dipolar glass state. The activation energy of this low temperature process was found to be Eb = 1297 K (0.041 eV) and the glass temperature was estimated to 73 K. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Adaptation and performance of the fast multipole method for dipolar systems

We have developed a new specialized version of the fast multipole method (FMM) for dipolar systems. For this purpose we have derived general expressions of the multipole expansion coefficients (in spherical coordinates) for a system of point dipoles with the potential ϕ dip∼1/ r 2. Our version is especially useful for simulations of fine magnetic particle systems (magnetic nanocomposites, ferrofluids), molecular dipolar fluids or electric dipolar glasses.

Dielectric Properties of Relaxor-Like P(VDF-TrFE-CFE) Terpolymer

The dynamic processes in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymer with a content of 68/32/9 mol% have been studied by measurements of the temperature and frequency-dependent linear and third-order nonlinear dielectric constants. Analysis of the dielectric response has revealed that dielectric dynamics in this organic system is very similar to behavior observed in various dipolar glasses and relaxors.

Features of Dielectric Properties and Photoluminescence of Glass SbSI

The temperature dependence of dielectric properties and photoluminescence (PL) of glass semiconductor SbSI is studied. According to measurements of the dielectric constant and polarization the first-order structural phase transition into the polar state (dipolar glass) is realized at T = 198 K. A complex spectrum of the found PL in the glass SbSI is related to two types of structural units. A wide radiation band, PL maximum location and large Stocks shift prove the existence of a strong electron-phonon interaction.

High-Frequency Dielectric Spectroscopy and Soft Lattice Dynamics of Disordered Ferroelectrics

Broad-band dielectric spectroscopy including the infrared response of several examples of disordered ferroelectrics is reviewed. The discussed examples belong to dipolar glasses (RADP and DRADP), relaxor ferroelectrics (PMN, PLZT), relaxor-like ferroelectrics (PZN-PT) and doped incipient ferroelectrics (KLT, KTN, SBiT). Common features to all the examples are summarised and differences among the mentioned classes of materials are pointed out.

Recent Understanding the Dielectric Relaxation Processes and Dipolar Disordering in High-Symmetry Relaxor System Cd2Nb2O7

Unusual polydispersive dielectric response of undiluted Cd2Nb2O7 pyrochlore between 100 Hz to 6 MHz and 77 to 300 K is reported. Below TC ≈ 198 K, the relaxation typical of (i) a normal ferroelectric (FE), (ii) a relaxor FE with glassy behaviour, and (iii) a dipolar glass develop in the system. Based on a weak bonding between the pyrochlore-structure-forming (NbO6)n− and (CdO8)n− networks, a mechanism is proposed that explains an occurrence of the orientational dipolar disorder and disordered states of different nature with decreasing temperature.

Description of Dynamic Dielectric Response in Poly(vinylidene Fluoride-Trifluoroethylene) Copolymers

We propose a theory of dynamic dielectric response in organic ferroelectrics PVDF, P(VDF/TrFE) copolymer before and after irradiation by fast electrons. For this purpose, we use random field formalism. In the adopted model we consider above polymers as disordered ferroelectrics in mixed ferroglass phase (FG) with a coexistence of polar short- and long-range order regions. Short-range order regions correspond to amorphous layers, while long-range order regions appears in the crystalline phase of all-trans conformation. In our formalism, we account for change of VDF content as well as for the influence of irradiation, by variation of content of electric dipoles and other random field sources. The downward shift of para-ferroelectric phase transition temperature and gradual transformation of mixed FG phase into dipole glass state with the increase of the irradiation dose was shown to be related to the self-consistent change of parameters of random field distribution function. We were able to fit the low-temperature dynamic dielectric response related to amorphous phase by Vogel-Fulcher (V-F) law with parameters close enough to those observed in the experiment. The coincidence between calculated and measured behaviour (namely, almost complete disappearance of high-temperature maxima, corresponding to ferroelectric phase transition, their submergence into low-temperature maxima and V-F law for low-temperature maxima of absorption) of irradiated samples speaks in favour of irradiation induced relaxor behaviour of P(VDF/TrFE) copolymers. We also discuss the physical reasons for choice of parameters necessary to fit our theory to the experiment.

Random Field Distribution Function in the Relaxor Ferroelectric Thin Films

The calculation of random field distribution function in the relaxor ferroelectrics was performed in the statistical physics framework. The point charges and electric dipoles randomly distributed in Burns reference phase were considered as the main sources of random electric field. The influence of the surfaces was calculated in the model of image charges and dipoles distributed in all the space. The contribution of these real and image sources of random field was taken into account. The characteristic features of calculated random field distribution function namely its half-width and the most probable electric field value were shown to be dependent both on the field sources parameters and on the dielectric permittivity of the surrounding media and ferroelectric material. It was shown that in general case the degree of disorder in the film of a relaxor could be either larger or smaller than in the bulk. Because of this the change of phase diagram with transformation of dipole glass state into ferroglass and vice versa can be expected. The developed theory opens the way of calculation of the disordered ferroelectrics film properties by averaging of the properties, calculated in host phase (Burns reference phase for the relaxors) as functions of electric field.

Nonlinear Dielectric Susceptibility in Doped Incipient Ferroelectrics

The theory of dc-electric field nonlinear susceptibility in the doped incipient ferroelectrics is developed in a random field approach. The calculations of the susceptibility χ(E, T) had shown that the coefficients χ nl (k) (T, n) in power serie of susceptibility χ(E, T) = Σχ nl (k) E 2k (k = 0 and k = 1, 2, 3…correspond respectively to linear and nonlinear susceptibility of the first, second etc. order) diverge at the same temperature T = T m that depends on the impurity dipoles concentrations. The critical indexes of the divergency for the concentration of the dipoles which correspond to mixed ferroglass phase with coexistence of short- and long-range polar order were shown to be the same as those calculated in mean field approximation. The comparison of the theory with experimental data obtained for KTa1−x Nb x O3 (0.0075 ≤ x ≤ 0.05) is performed. The calculations describe the main peculiarities of temperature, electric field and concentrational dependence of observed nonlinear dielectric susceptibility. Since the temperature T m corresponds to the transition from dipole glass to ferroglass phase the transition is proven to be equilibrium one in the disordered ferroelectrics.

Anomalies of Dielectric Response in Mixed Ferro-Glass Phase of Potassium Tantalate Doped by Lithium

The measurements of dynamic dielectric response and dc-electric field nonlinear susceptibility in K1−x Li x TaO3 for x = 0.01; 0.016 and 0.03 at 4 ≤ T ≤ 150 K has been performed. It was founded that the field decrease the susceptibility ϵ′ for the two smaller x. For x = 0.03 both the decrease (E > 1 kV/cm) and increase (E < 0.5 kV/cm) of ϵ′ value was revealed. The observed dependence of the position of ϵ′ maxima on temperature and frequency was described by Arrhenius and Vogel–Fulcher law respectively for x = 0.01, 0.016 and x = 0.03. The theoretical description was performed in random field theory framework. The calculations fitted pretty good the experimental data. It was shown that the Arrhenius law is related to the behavior of dipole glass state while Vogel–Fulcher law is the characteristic feature of mixed ferro-glass phase with coexistence of short- and long-range polar order. Therefore the measurements of dielectric response in any disordered ferroelectrics, including relaxors, can be used for identification of ferro-glass phase.

Dipolar-glass-like behaviour of 0.7PMN–0.3PZ ceramic

Relaxor ferroelectric behaviour has been observed for lead magnesium niobate–lead zirconate (0.7PMN–0.3PZ) ceramic. The freezing temperature obtained from fitting of dielectric data to the Vogel–Fulcher equation is very close to the dielectric-maximum temperature and to the temperature of collapse of remanant polarization. The temperature dependence of the spontaneous polarization follows a modified Neel's equation. Specific-heat measurement shows canonical glass-like behaviour, and the value of the glass-transition temperature is very close to the freezing temperature obtained from the Vogel–Fulcher equation. All the observations together indicate that 0.7PMN–0.3PZ ceramic behaves like a dipolar glass.

Theory of radiation induced relaxor behavior of poly(vinylidene fluoride-trifluoroethylene) copolymers

We propose a theory of the dynamic dielectric response in the organic ferroelectrics polyvinylidene fluoride (PVDF), poly(vinylidene fluoride/trifluoroethylene) [P(VDF/TrFE)] copolymer before and after irradiation by fast electrons. For this purpose, we use our random field formalism. In the adopted model we consider the earlier polymers as the disordered ferroelectrics in a mixed ferroglass phase (FG) with a coexistence of polar short- and long-range order regions. Short-range order regions correspond to amorphous layers, while long-range order regions appears in the crystalline phase of all-trans conformation. In our formalism, we account for the change of VDF content as well as for the influence of the irradiation, by variation of the content of electric dipoles and other random field sources. The downward shift of the paraferroelectric phase transition temperature and gradual transformation of the mixed FG phase into a dipole glass state with the increase of the irradiation dose was shown to be related to the self-consistent change of parameters of random field distribution function. We were able to fit the low-temperature dynamic dielectric response related to the amorphous phase by Vogel–Fulcher (V–F) law with parameters close enough to those observed in the experiment. The coincidence between calculated and measured behavior (namely, almost complete disappearance of high-temperature maxima, corresponding to the ferroelectric phase transition, their submergence into low-temperature maxima and V–F law for the low-temperature maxima of absorption) of the irradiated samples speaks in favor of irradiation induced relaxor behavior of P(VDF/TrFE) copolymers. We also discuss the physical reasons for the choice of the parameters of material necessary to fit our theory to the experiment.

Glass behavior of a relaxor Cd2Nb2O7 in a weak DC field

The temperature behavior of the dielectric response ehis(T) at a frequency of 1 kHz (110 K< T<300 K), δe(T)=[ehis(T)-eZFC(T)], and of the order parameter q(T), characterizing e(T) in a relaxor, is investigated for the relaxor Cd2Nb2O7 on samples with different thermal and electric histories in a dc field much weaker than the polarization saturation field. In a weak field (E dc=0.95 kV/cm), the behavior of δe(T) ∝ χnl changes in the region T f =184 K and the ehis(T) curves diverge monotonically at lower temperatures, indicating the development of the glassy state in the system. Analysis of the behavior of q(T) in the framework of the model of spin and dipole glasses shows that random interactions of polar microscopic regions in the presence of random fields play a dominating role in the phase formed below T f in the ZFC and FH/ZFC regimes. The relative contribution of random fields increases in the FC regime in a weak field and is manifested in the ZFH/FC regime both below and above T f .

Unusual impurity effects on the dielectric properties of CaCu 3− xMn xTi 4O 12

CaCu 3Ti 4O 12 has a large dielectric constant (ε∼10 4 at 300 K) , which is almost constant above 100 K , followed by a 100-fold reduction below 100 K . The physical mechanism of this anomalous behavior has been extensively investigated. We found that only 2% substitution of Mn for Cu dramatically quenches the huge ε of 10 4 down to 100 over the measured temperature range from 4.2 to 300 K . In usual substitution effects, 2% impurity induces a tiny change in ε of the order of 2%. Thus the present substitution effect suggests that dipole moments cause fluctuating ferroelectric domains in CaCu 3Ti 4O 12, as is similar to the frustrated dipole glass in LuFe 2O 4.

Phonon Spectra and Phase Transitions in CuInP 2 (Se x S 1 − x ) 6 Ferroelectrics

The investigations of dielectric permittivity, optic birefringence and Raman scattering spectra for CuInP 2 (Se x S 1 m x ) 6 mixed crystals give evidence about rapid lowering of the temperature and smearing of first order transition between monoclinic paraelectric and ferroelectric phases at sulfur by selenium replacement. In the concentration interval x = 0.3 m 0.75 the state similar to dipole glass obviously exists. The temperature dependence of the low energy part of Raman spectra for CuInP 2 S 6 crystals and concentration transformation of these spectra in CuInP 2 (Se x S 1 m x ) 6 solutions leads to suggestion that the morphotropic phase transitions between monoclinic and trigonal phases can be associated with the instability of a "rigid layer" shifting mode.

Distribution of correlation radii in disordered ferroelectrics

The calculation of the correlation radius distribution function is performed for the cases of undamped and overdamped soft mode dispertion laws. Taking into account the correlation radius dependence on the random field and this field distribution function, we carried out the calculation of the correlation radius distribution function dependence on temperature, damping coefficient and random field distribution function parameters. It was shown that in paraelectric phase (T&amp;gt;Td, Td is the Burns temperature) the most probable value of the correlation radius is equal to its maximal value, independently of the degree of system disorder, while at the dipole glass state it is close to the minimal value, with a broad “tail” of the distribution function in the region of larger radii.

Correlation between infrared phonon modes and dielectric relaxation in Bi2O3–ZnO–Nb2O5 cubic pyrochlore

Analysis of the dielectric relaxation phenomena in Bi2O3–ZnO–Nb2O5 cubic pyrochlore utilizing the Arrhenius equation is revisited to include recent high-frequency measurements. The fitting parameters obtained are further analyzed by comparison with theoretical and experimental activation energies of better understood relaxation systems such as dipolar glasses and relaxor ferroelectrics. The attempt jump frequency obtained from the Arrhenius fit (ν0∼4.385 THz) is found in correspondence with the resonant frequencies of the polar phonon modes previously identified by infrared spectroscopy. This correlation, and in particular the role of the O–A–O and O′–A–O′ bending phonon modes in driving the relaxation, is discussed.

Dynamic dielectric susceptibility of the betaine phosphate (0.15) betaine phosphite (0.85) dipolar glass

Dielectric susceptibility measurements in a wide frequency spectrum were used to obtain the distribution of relaxation times. This was performed by solving the integral equation for the susceptibility with the Tikhonov regularization technique. This method allows to extract the distribution of the relaxation times and resolves multiple dynamical processes. As an experimental example the investigation on mixed crystals of protonated and deuterated ${\mathrm{BP}}_{1\ensuremath{-}x}{\mathrm{BPI}}_{x}$ with $x=0.85$ are presented. The dipole-freezing phenomena results in a broad asymmetrical distribution of the relaxation times. The parameters of the double-well potentials of the hydrogen bonds, the local polarization distribution function and the glass order parameter have been extracted from the dielectric measurements. The microscopic parameters obtained are in good agreement with magnetic resonance data.

Random fields in dipolar glasses and relaxors

In dipolar glasses and relaxors both site and substitutional charge disorder generally occur simultaneously albeit with different weight. While random interactions control dipolar glassy behavior of K 1− x Li x TaO 3 ( x<0.02), quenched random-fields (RFs) due to charge disorder dominate the uniaxial relaxor Sr 1− x Ba x Nb 2O 6 (SBN, x≈0.4). RFs smear the glass transition of Rb 1− x (DH 4) x H 2PO 4 (DRADP, x≈0.5) as described by a random-bond random-field (RBRF) Ising model, while cubic relaxor glasses like PbMg 1/3Nb 2/3O 3 (PMN) rather obey spherical RBRF modeling. In the case of relaxors spatial fluctuations of the RFs correlate the dipolar fluctuations and give rise to polar nanoregions in the paraelectric regime. The order parameter dimension decides upon whether the ferroelectric phase transition is destroyed (PMN) or modified towards random-field Ising model behavior (SBN).

Dipolar and orientational glass formation in two-dimensional films of CO mixtures

Heat-capacity and dielectric measurements have been performed to study orientational and dipolar order and disorder phenomena in submonolayer films of CO and binary mixtures of CO physisorbed on graphite and boron nitride (BN). With both methods transitions to quadrupolar and dipolar ordered low-temperature phases have been found for pure CO. Calorimetric measurements revealed that the dipolar transition of CO on graphite belongs to the universality class of the two-dimensional (2D) Ising model. It will be shown that in dilute mixtures of CO with CH 4 molecules a dramatic suppression of this phase transition occurs. This effect turned out to be much stronger than that previously observed in (CO) 1− x (N 2) x and (CO) 1− x (Ar) x mixtures, which could be successfully described by the random-field theory. From dielectric experiments first information on the dynamics of CO and (CO) 1− x (Ar) x films was gained and activation energies and attempt frequencies for reorientation processes of CO determined. Both employed methods consistently indicate that at impurity concentrations beyond 6–7% dipolar and orientational glass states are formed.