Size dependence of dielectric properties and structural metastability in ferroelectrics

The size effect of the dielectric properties and the barrier height was investigated in the ferroelectric solid solution Ba_xSr_1-xTiO₃ system. The decrease of the grain size causes the suppression of the ferroelectricity, and the increase of the relaxation frequency. Barrier heights increase with increasing grain size. The result is analogous to magnetic phase transitions in nanocrystals and other solid-solid phase transitions in nanocrystals. It suggests a general rule that may be of use in the discovery of new metastable phases. An explanation of this phenomenon was given by an electric potential model that agrees well with the experimental results. For Ba_xSr_1-xTiO₃ system, the decrease of xcauses the decrease of the barrier height. Received 3 August 1998 and Received in final form 22 November 1998     

Griffiths phase manifestation in disordered dielectrics

We predict the existence of a Griffiths phase in dielectrics with a concentrational crossover between dipole glass (electric analog of spin glass) and ferroelectricity. Particular representatives of the above substances are KTaO₃:Li, Nb, Na, or relaxor ferroelectrics like Pb_1–xLa_xZr_0.65Ti_0.35O₃. Since this phase exists above the ferroelectric phase-transition temperature (but below that temperature for ordered substances), we call it a “para-glass phase”. We assert that the difference between paraelectric and para-glass phases in the above substances is the existence of clusters (inherent to the “ordinary” Griffiths phase of Ising magnets) of correlated dipoles. We show that randomness plays a decisive role in the Griffiths (para-glass) phase formation: this phase does not exist in a mean field approximation. To investigate the Griffiths phase properties, we calculate the density of Yang-Lee (YL) zeros in the partition function and find that it has “tails” inherent to the Griffiths phase in the above temperature interval. We perform calculations on the basis of our self-consistent equation for the long-range order parameter in an external electric field. This equation has been derived in the framework of the random field theory. The latter automatically incorporates both short-range (due to indirect interaction via transverse optical phonons of the host lattice) and long-range (ordinary dipole-dipole) interactions between impurity dipoles, so that the problem of long-range interaction considerations does not appear in it. Received 17 May 2000     

Aging of the field-induced asymmetry in a disordered ferroelectric

The isothermal aging of the asymmetry induced in the disordered dielectric crystal (x=0.027) submitted to the biasing electric field E, is investigated. To this end, the response of the complex dielectric constant to infinitesimal field changes , applied to the sample after a variable aging delay, has been measured for different magnitudes of E and after different aging delays. Two different experimental procedures have been used: in both cases the response strongly depends on the time spent under field. For short aging delays, the response has a strong contribution proportional to δE and a weak quadratic contribution proportional to . As time elapses, the linear and the quadratic contributions age in opposite ways: the former decreases whereas the latter increases. This paradoxical behaviour is analyzed in the framework of a model which attributes aging and the related effects (rejuvenation, memory) to the evolution of polarization domain walls: the decrease of the linear contribution is related to the decrease of the total wall area, while the increase of the quadratic term is attributed to wall reconformations.     

Ac behaviour and phase transition in PLZT 6/80/20 ferroelectric ceramics

The Ac behaviour of PLZT 6/80/20 ferroelectric ceramics was analyzed around and above the phase transition. Two relaxation processes are identified, showing that the so-called ‘universal relaxation law’ holds for the ceramics. A critical point in the values of the Ac conductivity, around the temperature corresponding to the maximum of the dielectric losses, is observed below the transition temperature due to the relaxor behaviour. The frequency dependence of the Ac conductivity at various temperatures and the hysteresis loops show classical relaxor behaviour with a diffuse phase transition.     

Phase transitions sequence in pyrochlore Cd2Nb2O7 studied by IR reflectivity

The infrared reflectivity of Cd₂Nb₂O₇ single crystal was studied in the temperature interval of 10-540 K, together with complementary dielectric measurements. A ferroelectric soft mode was revealed above the ferroelectric phase transition at T _c = 196 K coupled with a central-mode type dispersion in the near-millimetre range. This proves the mixed displacive and order-disorder nature of the transition. Below T _c many new modes were detected due to lowering of the symmetry, especially below the previously suggested incommensurate transition at 85 K. Discussion of the possible phase transitions based on symmetry considerations is presented with the conclusion that the ferroelectric transition is proper with the F_1u symmetry of the order parameter, whereas the intermediate ferroelastic transition is improper and triggered by the coupling with the ferroelectric order parameter. Received 17 July 2000     

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.     

The pressure influence on the incommensurate-commensurate ferroelectric phase transition in [4-NH2C5H4NH][SbCl4]

The dielectric properties of the [4-NH₂C₅H₄NH] SbCl₄ (abbreviated as 4-APCA) crystal were investigated under hydrostatic pressure up to 300 Mpa. The pressure-temperature phase diagram was given. The paraelectric-ferroelectric phase transition (II→III) temperature (T_c) increases linearly with increasing pressure with a slope dT_c/dp=21×10⁻² K/MPa. The pressure dependence of Curie-Weiss constants has been evaluated also. In the paraelectric phase (II) the Curie constant (C₊) was pressure dependent whereas the C₋ constant over the ferroelectric phase (III) was almost constant. The results are interpreted in terms of improper and displacive type phase transition model with a soft phonon at a zone boundary.     

Dielectric relaxation of poly(ethylenglycol)- b-poly(propylenglycol)-b-poly(ethylenglycol) copolymers above the glass transition temperature

The complex dielectric permittivity has been measured for three poly(ethylenglycol)-b-poly(propylenglycol)-b-poly(ethylenglycol) copolymers with different content of poly(ethylenglycol) (15%, 33% and 80%), and increasing degree of crystallinity (0%, 10% and 20%, respectively). Only the non-crystalline sample shows the normal mode relaxation together with the segmental (α-relaxation) and the Johari-Goldstein (β-relaxation) modes. The crystalline samples show also polarization contributions due to the existence of interfaces between the crystallites and the amorphous phase. The relaxation times of the (α and normal modes can be described by a VFT equation with the same value of T₀. There is a slowing-down of the segmental mode due to the presence of crystallites. The temperature dependence of the α and β relaxations in the copolymers is very similar to that found in pure PPG, while there are significant differences in the case of the normal mode of the non-crystalline sample. The size of the cooperatively rearranging regions CRR, and the width of the glass transition region increase slightly with the degree of crystallinity. The temperature dependence of the size of CRRs is compatible with the prediction of fluctuation theory. No systematic effect of the degree of crystallinity on the β-relaxation has been found. Near T _g the β-relaxation time is close to the primitive time of the coupling model. Received: 31 May 2000     

Non ergodic aging in potassium niobo-tantalate crystals

Slow dynamics has been studied in various potassium niobo-tantalate crystals (KTN) by recording the complex dielectric constant after several thermal histories: isothermal evolution following controlled cooling or rapid quenching, positive or negative temperature cycles. The results reveal most of the behaviours of aging already found in lithium-potassium tantalate (KLT): effective ergodicity breaking (the asymptotic value of the dielectric constant varies as the logarithm of the cooling rate R), quasi-independence of the isothermal evolution with respect to a sojourn at lower temperature. But some differences between KTN and KLT are noticeable: coefficients of with opposite signs, role of the quenching temperature on the subsequent evolution, no overshot after a temperature jump. In order to explain the results we propose to extend the model initially developped for KLT taking into account the different nature of the low temperature phase, paraelectric for KLT and ferroelectric for KTN. In this model the variations of the dielectric constant are attributed to the slow movements of polarization domain walls hindered by static random fields. By measuring the dielectric constant during cooling and immediate heating at the same rate, an illustrative comparison is provided, showing that the evolution of the domain size is reversible in KLT and not in KTN. Received 17 November 1999     

Investigation of size-driven phase transition in bismuth titanate nanocrystals by Raman spectroscopy

Raman spectroscopy was used to investigate the lattice dynamics and structural transformations in bismuth titanate (Bi₄Ti₃O₁₂) nanocrystals prepared by a chemical coprecipitation technique. The crystal structure of the samples of different grain sizes was determined by X-ray-diffraction analysis. The evolution of the Raman spectrum with grain size was characterized by an intensity decrease, a broadening of the line width, a frequency shift, and the disappearance of the Raman mode. The results revealed the appearance of a size-driven phase transition from orthorhombic to tetragonal phases at a critical size of 44 nm. This result is quite consistent with the X-ray-diffraction measurement and differential thermal analysis. The origin was attributed to the grain-size effect and explained by the surface-energy mechanism. Received: 26 June 2002 / Accepted: 18 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +86-25/3595535, E-mail: msz@nju.edu.cn     

Low frequency dielectric permittivity of quasi-one-dimensional conductor (TMTTF)2Br

Conductivity and permittivity of the organic transfer salt (TMTTF)₂Br have been measured at low frequencies (10²-10⁷ Hz) between room temperature down to 4 K. The real part of the permittivity, , is shown to grow below the temperature at which the conductivity is maximum due to charge localization of Mott-Hubbard type. reaches a maximum of 10⁵-10⁶ at 35 K-50 K depending on the samples. Decreasing temperature below , sharply decreases down to helium temperature through the antiferromagnetic phase transition at T _N = 15 K. We explain the magnitude, the temperature and frequency dependence of as resulting from short range charge density wave states in the temperature range where charge localization occurs. This interpretation is supported by recent X-ray scattering measurements. Received: 10 October 1997 / Revised: 28 February 1998 / Accepted: 3 March 1998     

Memory against temperature or electric field sweeps in potassium niobo-tantalate crystals

Aging, memories after temperature sweeps (double ramp and double jump) and memories after electric field sweeps (double ramp and double jump) were studied as a function of frequency. The experiments were performed at low temperatures in the ferroelectric phase of two potassium niobo-tantalate crystals K Ta_1-xNb_xO₃ with the niobium concentration x close to 0.02. Five complex quantities are defined, which respectively characterize these five phenomena. The main feature is that isothermal aging and memories after temperature sweeps have exactly the same frequency dependence while after electric field sweeps the frequency dependence is clearly different. Additionally, the role of the characteristics of the sweeps (amplitudes, rates of changes, durations) on these memories were measured. The observed behaviours are discussed in term of a model which attributes the time dependent effects to growth and reconformation of ferroelectric domains and takes into account that the domain wall motion is hindered by pinning sites. The difference in the frequency dependences against the nature of the swept parameters shows that the distribution of the reconformation time is sensitive to the biasing electric field. Received 28 May 2001 and Received in final form 10 October 2001     

Temperature dependence of infrared conductivity of manganites (x = 0, 0.05 and 0.2)

The temperature dependence of the reflectivity spectra of three manganites ceramics with compositions Pr_0.7Ca_0.3MnO₃, Pr_0.7Ca_0.25Sr_0.05MnO₃ and Pr_0.7Ca_0.1Sr_0.2MnO₃ has been investigated by infrared reflectivity spectroscopy in the wave number range 0.005-1.1 eV. The compound Pr_0.7Ca_0.25Sr_0.05MnO₃ which shows the largest conductivity jump at the ferromagnetic-paramagnetic phase transition has been studied in details. The optical conductivity of this compound is deduced from the best fit to reflectivity spectra of a “double-damping Drude” model, itself derived from the factorized form of the dielectric function. Excellent agreement with Kramers-Kronig transformation is reported. The model allows in particular to discriminate the contributions to the optical conductivity of trapped charges (polarons) and mobile charge carriers. Received 20 July 1999 and Received in final form 15 October 1999     

Submillimeter and infrared spectroscopy on La0.85Sr0.15MnO3

The optical conductivity of La_0.85Sr_0.15MnO₃ single crystals was studied by means of submillimeter and infrared spectroscopy for frequencies cm^-1 and temperatures 10 K < T <300 K. The submillimeter conductivity follows the temperature dependence of the dc-data. The phonon spectrum of La_0.85Sr_0.15MnO₃ changes considerably below K revealing a structural phase transition induced by charge or orbital order. At T =10 K a number of phonon modes can be identified in addition to the room-temperature spectrum. The optical conductivity () in the mid-infrared reveals the characteristics of small polaron absorption. Below the magnetic ordering temperature the polaron binding energy is highly reduced, but the onset of charge order interrupts the formation of free charge carriers with a Drude-like behavior. The frequency and temperature dependence of in this regime qualitatively resembles the small polaron predictions by Millis et al. (Phys. Rev. B 54, 5405 (1996)). Received 5 November 1999     

Anharmonic behaviour of BaFCl using Raman scattering

Raman scattering from oriented single crystals of BaFCl was recorded at various temperatures from 20 to 1073 K for the first time. The Raman spectra, corrected for phonon population, were fitted to the sum of four Lorentzian peaks. The peak frequencies and full width at half maximum (FWHM) of the peaks were obtained from the fit. The FWHM is accounted for by cubic and quartic anharmonic processes. The quartic anharmonicity of the mode increases with the mode frequency. The quartic anharmonicity of the fluorine mode is exceptionally high. The peak frequencies decrease linearly with the increasing temperature. Fluorine mode frequencies decrease more than the internal mode frequencies do. The LO-TO splitting of the fluorine modes and that of the internal modes increases with temperature indicating the increase of the ionic bonding character. The results are discussed. Received: 12 May 1997 / Accepted: 25 July 1997     

Scaling of broadband dielectric data of glass-forming liquids and plastic crystals

The Nagel scaling and the modified scaling procedure proposed recently by Dendzik et al. have been applied to broadband dielectric data on two glass-forming liquids (glycerol and propylene carbonate) and three plastic crystals (ortho-carborane, meta-carborane, and 1-cyano-adamantane). Our data extend the upper limit of the abscissa range to considerably higher values than in previously published analyses. At the highest frequencies investigated, deviations from a single master curve show up which are most pronounced in the Dendzik scaling plot. The loss curves of the plastic crystals do not scale in the Nagel plot, but they fall onto a separate master curve in the Dendzik plot. In addition, we address the question of a possible divergence of the static susceptibility near the Vogel-Fulcher temperature. For this purpose, the low-temperature evolution of the high-frequency wing of the dielectric loss peaks is investigated in detail. No convincing proof for such a divergence can be deduced from the present broadband data. Received 14 June 1999 and Received in final form 4 October 1999     

Probing the material properties and phase transitions of ferroelectric liquid crystals by determination of the Landau potential

The full Landau potential of several, widely varying ferroelectric liquid-crystalline materials has been experimentally determined. Tilt angle and polarisation data is analysed across the SmA to SmC transition for varying applied electric-field amplitudes, allowing the determination of all the coefficients of the generalised Landau model of ferroelectric liquid crystals. The materials investigated encompass different materials, including low-polarisation mixtures to high-polarisation single-component materials. The materials also possess a variation in the order of the SmA to SmC phase transition from strongly first order to strongly second order. The effects of both the polarisation and order of phase transition of the system are discussed with respect to the various terms of the generalised Landau model. Further, the mechanisms behind the difference between a first- and second-order phase transition are discussed with respect to the Landau potential and the second Landau coefficient b .     

Light scattering spectra of polarization fluctuations and models of the phase transition in KDP type ferroelectrics

Abstract

Recent light scattering studies in KH₂PO₄ type hydrogen-bonded crystals are reviewed. Dynamical properties of these crystals have been discussed in terms of the coupled tunneling proton-optical phonon model up to date. Light scattering spectra in several GHz and cm^?1 regions with various scattering geometries observed in KH₂PO₄, KD₂PO₄ and their mixed crystals, however, revealed that all these spectra are well analysed in terms of a coupled polarization relaxation mode-acoustic phonon system. Since most of the spontaneous polarization is due to shifts of K, P and O ions, relaxation motions of H₂PO₄ dipoles arc concluded to be the origin of these low-frequency spectra. Models of the phase transition mechanism are discussed on the basis of these results and other recent experimental studies including the explanation of large isotope effects in these crystals.