High-pressure and variable temperature single crystal synchrotron X-ray measurements combined with first-principles based molecular dynamics simulations study diffuse scattering in the relaxor ferroelectric system PSN (PbSc$_{1/2}$Nb$_{1/2}$O$_3$). Constant temperature experiments show pressure induced transition to the relaxor phase at different temperatures characterized by butterfly and rod shaped diffuse scattering around the $\{$h00$\}$ and $\{$hh0$\}$ Bragg spots, respectively. The simulations reproduce the observed diffuse scattering features as well as their pressure-temperature behavior, and show that they arise from polarization correlations between chemically-ordered regions, which in previous simulations were shown to behave as polar nanoregions. Simulations also exhibit radial diffuse scattering (elongated towards and away from {\bf Q}=(000)), that persists even in the paraelectric phase, consistent with previous neutron experiments on (PbMg$_{1/3}$Nb$_{2/3}$O$_3$) (PMN)
We report the observation of a "transparency window" in the dielectric resonant absorption spectrum of the relaxor ferroelectric K1-xLixTaO3 (KLT) in the vicinity of its weakly first order transition. This phenomenon is shown to be conceptually similar to the electro-magnetically induced transparency (EIT) phenomenon observed in atomic vapors, which can be modeled classically by a driven master -slave oscillator system. In KLT, it reveals the presence of macroscopic hetero-phase fluctuations and provides unique information on the nature and mechanism of the phase transition in relaxors.
We present here the results of frequency dependent dielectric, polarization and powder X-ray powder diffraction studies in the 300 to 100K temperature range for Ba_0.55Sr_0.45TiO_3. The dielectric results indicate a succession of three relaxor ferroelectric transitions accompanying the cubic to tetragonal to orthorhombic to rhombohedral phase transitions confirmed by XRD studies. Our results confirm the coexistence of the relaxor ferroelectric and ferroelectric behaviours.
A key to optimizing the growth of the new single-crystal relaxor ferroelectrics is resolving basic questions concerning their structural properties and energetics. We report on initial first-principles total energy and force calculations, examining the energetics of local order in PZN type relaxors.
Relaxor ferroelectrics are complex materials with distinct properties different from classic ferroelectrics. Probing frequency depended dielectric susceptibility is its trait. Other empirical laws build on classic ferroelectrics are still applicable to relaxor ferroelectrics. Here we report an abnormal improved dielectric in La doped relaxor ferroelectric 0.4PNN-0.6PHT, which deviated from the consistent variation relationship with piezoelectric coefficient. A mesoscale mechanism is proposed to reveal the origin of the improved dielectric response in 0.4PNN-0.6PHT, where the polar nanoregions …
New relaxor ferroelectric system has been synthesized. BLTN exhibits a smeared maximum of permittivity, characteristic of classic relaxor behaviour, with a peak shift from 185 K at 100 Hz to 300 K at 1 GHz. BNTN undergoes a first order ferroelectric phase transition at 389 K and BLNTN exhibits both a ferroelectric phase transition at 274 K and relaxor behaviour at higher temperatures.
The polarization and strain response of ferroelectric materials at fields below the macroscopic coercive field is of a paramount importance for the operation of many electronic devices. The response of real ferroelectric and related materials is in general complex and difficult to interpret. The reason for this is that many processes in a ferroelectric material contribute to its properties, often concurrently. Examples include motion of ferroelectric and ferroelastic domains, presence of domains within domains, dynamics of different types of polar nano-entities, interaction of polar nano-entit…
A first-principles-based model is developed to investigate the influence of lead vacancies on the properties of relaxor ferroelectric Pb(Sc1/2Nb1/2)O3 (PSN). Lead vacancies generate large, inhomogeneous, electric fields that reduce barriers between energy minima for different polarization directions. This naturally explains why relaxors with significant lead vacancy concentrations have broadened dielectric peaks at lower temperatures, and why lead vacancies smear properties in the neighborhood of the ferroelectric transition in PSN. We also reconsider the conventional wisdom that lead vacancie…
SrNaBi2Nb3O12 powder was prepared via the conventional solid-state reaction method. X-ray structural studies confirmed the phase to be a three-layered member of the Aurivillius family of oxides. SrNaBi2Nb3O12 (SNBN) ceramics exhibited the typical characteristics of relaxor ferroelectrics, associated with broad and dispersive dielectric maxima. The variation of temperature of dielectric maxima (Tm) with frequency obeyed the Vogel-Fulcher relationship. Relaxor behavior was believed to be arising from the cationic disorder at A-site. Pinched ferroelectric hysteresis loops were observed well above…
We suggest a random field based model for calculation of physical properties of mixed ferroelectric relaxors. Our model naturally incorporates the different orientations of electric dipoles (related to different solid solution components) as well as the contribution of nonlinear and correlation effects of random field. We calculate the transition temperature $T_c$ as well as concentrational and temperature dependence of order parameters.
High energy resolution neutron spin-echo spectroscopy has been used to measure intrinsic width of diffuse scattering discovered earlier in relaxor ferroelectric crystals. The anisotropic and transverse components of the scattering have been observed in different Brillouin zones. Both components are found to be elastic within experimental accuracy of 1 eV. Possible physical origin of the static-like behavior is discussed for each diffuse scattering contribution.
We have measured the bending-induced polarization of Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals with compositions at the relaxor-ferroelectric phase boundary. The crystals display very large flexoelectricity, with flexocoupling coefficients an order of magnitude bigger than the theoretical upper limit set by the theories of Kogan and Tagantsev. This enhancement persists in the paraphrase up to a temperature T* that coincides with the start of elastic softening in the crystals. Analysis of the temperature dependence and cross-correlation between flexoelectric, dielectric and elastic properties ind…
A mechanically coupled phase-field model is proposed for the first time to simulate the peculiar behavior of relaxor ferroelectrics. Based on the random field theory for relaxors, local random fields are introduced to characterize the effect of chemical disorder. This generic model is developed from a thermodynamic framework and the microforce theory and is implemented by a nonlinear finite element method. Simulation results show that the model can reproduce relaxor features, such as domain miniaturization, small remnant polarization and large piezoelectric response. In particular, the influen…
Raman scattering in relaxor ferroelectric PbMg1/3Ta2/3O3 (PMT) was investigated in the single crystalline form in the temperature range of 20 - 295 K. Anomalous temperature dependence of the integrated intensity and the Raman line contours were found at the diffuse phase transition. A correlation between the anomalies in the integrated intensities and the dispersion of the dielectric response was observed. The distortions of Raman lines with decreasing temperature are discussed.
We present the evidence of lattice rotation vortices having an average radius of ~7 nm at the monoclinic ferroelectric domain boundary of (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-xPT, x=0.08). Maps of crystal orientations, domain configurations, symmetry breaking are obtained using scanning convergent beam electron diffraction (SCBED). Such measurements suggest the merging of 2D and 1D topological defects, with implications for domain-switching mechanisms in relaxor ferroelectric crystals, and the possibility of a new form of nanoscale ferroelectric devices.
Ba(Zr,Ti)O$_3$ is a lead-free relaxor ferroelectric. Using the first-principles method, the ferroelectric dipole moments for pure BaTiO$_3$ and Ba(Zr,Ti)O$_3$ supercells have been studied. All possible ion configurations of BaZr$_{0.5}$Ti$_{0.5}$O$_3$ and BaZr$_{0.25}$Ti$_{0.75}$O$_3$ are constructed in a $2\times2\times2$ supercell. For the half-substituted case, divergence of ferroelectric properties has been found among these structures, which seriously depends on the arrangement of Ti and Zr ions. Thus our results provide a reasonable explanation to the relaxor behavior of Ba(Zr,Ti)O$_3$. …
The crystal structure of the PbMg$_{1/3}$Ta$_{2/3}$O$_3$ (PMT) relaxor ferroelectric was studied under hydrostatic pressure up to $\sim 7$ GPa by means of powder neutron diffraction. We find a drastic pressure-induced decrease of the lead displacement from the inversion centre which correlates with an increase by $\sim$ 50 % of the anisotropy of the oxygen temperature factor. The vibrations of the Mg/Ta are, in contrast, rather pressure insensitive. We attribute these changes being responsible for the previously reported pressure-induced suppression of the anomalous dielectric permittivity and…
Barkhausen noise, including both periodic and aperiodic components, is found in and near the relaxor regime of a familiar relaxor ferroelectric, PbMg$_{1/3}$Nb$_{2/3}$O$_3$, driven by a periodic electric field. The temperature dependences of both the amplitude and spectral form show that the size of the coherent dipole moment changes shrink as the relaxor regime is entered, contrary to expectations based on some simple models.
Relaxor ferroelectrics are applied in electronic devices such as actuators and sonars. Morphotrophic phase boundaries (MPBs) with monoclinic structures are known for their high piezoelectricity and electromechanical coupling factors in solid solutions of PbTiO$_{3}$ and relaxor ferroelectrics (Pb(Mg$_{1/3}$, Nb$_{2/3}$)O$_{3}$ or Pb(Zn$_{1/3}$, Nb$_{1/3}$)O$_{3}$). However, the monoclinic structures related to polar nanosize domains (polar nanoregions) exhibiting the relaxor properties of dielectric dispersion have not been reported. Using transmission electron microscopy and synchrotron x-ray…
Epitaxial thin films of relaxor PbMg1/3Nb2/3O3 and PbSc0.5Nb0.5O3, and ferroelectric PbZr0.65Ti0.35O3, Pb0.955La0.045Zr0.65Ti0.35O3, and Ba0.4Sr0.6TiO3 were prepared, and their dielectric properties were studied in a broad range of the measurement conditions. In the ferroelectric state, the presence and the change of configuration of the domains determined both the dynamic dielectric nonlinearity and the polarization hysteresis. In thin-film relaxors, the orientation of the randomly interacting dipoles in a random field was responsible for the dynamic dielectric nonlinearity, while the observe…
The model for calculation of relaxor ferroelectrics thin films properties is proposed. The basis of the model is the theory of random field. This field is originated from the chemical disorder allowing for influence of the film surfaces, which destroys the polar long-range order and transform it into mixed state of ferroelectric glass (FG) or dipole glass (DG). The spatial profiles and averaged over coordinate inside the film values of properties of relaxor ferroelectric film were calculated with the random field distribution function. As an example the dependence of the order parameter on fil…
Relaxor ferroelectrics are complex oxide materials which are rather unique to study the effects of compositional disorder on phase transitions. Here, we study the effects of quenched cubic random electric fields on the lattice instabilities that lead to a ferroelectric transition and show that, within a microscopic model and a statistical mechanical solution, even weak compositional disorder can prohibit the development of long-range order and that a random field state with anisotropic and power-law correlations of polarization emerges from the combined effect of their characteristic dipole fo…
Polycrystalline samples of CuCrO2 were synthesized by solid state reaction method. Temperature dependent dielectric measurements, synchrotron x-ray diffraction (SXRD), pyroelectric current and Raman measurements have been performed on these samples. Evidences of the presence of relaxor type ferroelectricity, which otherwise have gone unnoticed in CuCrO2 system (a member of delafossite family) near room temperature, have been presented. Presence of broad maximum in dielectric permittivity and its frequency dispersion indicates relaxor-type ferroelectricity in CuCrO2 near room temperature. Caref…
The relaxor ferroelectric PbMg_1/3Ta_2/3O_3 was studied by single-crystal neutron and synchrotron x-ray diffraction and its detailed atomic structure has been modeled in terms of static Pb-displacements that lead to the formation of polar nanoregions. Similar to the other members of the Pb-based relaxor family like PbMg_1/3Nb_2/3O_3 or PbZn_1/3Nb_2/3O_3 the diffuse scattering in the [H,0,0]/[0,K,0] scattering plane has a butterfly-shape around the (h,0,0) Bragg reflections and is transverse to the scattering vector for (h,h,0) peaks. In the [H,H,0]/[0,0,L] plane the diffuse scattering is elong…
We study a minimal model for a relaxor ferroelectric including dipolar interactions, and short-range harmonic and anharmonic forces for the critical modes as in the theory of pure ferroelectrics together with quenched disorder coupled linearly to the critical modes. We present the simplest approximate solution of the model necessary to obtain the principal features of the correlation functions. Specifically, we calculate and compare the structure factor measured by neutron scattering in different characteristic regimes of temperature in the relaxor PbMg$_{1/3}$Nb$_{2/3}$O$_3$.
Local inhomogeneities known as polar nanoregions (PNR) play a key role in governing the dielectric properties of relaxor ferroelectrics - a special class of material that exhibits an enormous electromechanical response and is easily polarized with an external field. Using neutron inelastic scattering methods, we show that the PNR can also significantly affect the structural properties of the relaxor ferroelectric Pb(Zn1/3Nb2/3)O3-4.5%PbTiO3 (PZN-4.5%PT). A strong interaction is found between the PNR and the propagation of sound waves, i.e. acoustic phonons, the visibility of which can be enhan…
The field-driven conversion between the zero-field-cooled frozen relaxor state and a ferroelectric state of several cubic relaxors is found to occur in at least two distinct steps, after a period of creep, as a function of time. The relaxation of this state back to a relaxor state under warming in zero field also occurs via two or more sharp steps, in contrast to a one-step relaxation of the ferroelectric state formed by field-cooling. An intermediate state can be trapped by interrupting the polarization. Giant pyroelectric noise appears in some of the non-equilibrium regimes. It is suggested …
We have recently observed a giant electrocaloric effect (12 K in 25 V) in 350 nm sol-gel Pb Zr_0.95 Ti_0.05 O_3 films near the ferroelectric Curie temperature of 242oC. Here we demonstrate a giant electrocaloric effect (5 K in 25 V) in 260 nm sol-gel films of the relaxor ferroelectric 0.9 PbMg_(1/3)Nb_(2/3)O_3 - 0.1 PbTiO_3 near the Curie temperature of 60oC. This reduction in operating temperature widens the potential for applications in novel cooling systems.
The microstructure and phase transition in relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 (PMN) and its solid solution with PbTiO3 (PT), PMN-xPT, remain to be one of the most puzzling issues of solid state science. In the present work we have investigated the evolution of the phase symmetry in PMN-xPT ceramics as a function of temperature (20 K < T < 500 K) and composition (0 <= x <= 0.15) by means of high-resolution synchrotron x-ray diffraction. Structural analysis based on the experimental data reveals that the substitution of Ti^4+ for the complex B-site (Mg1/3Nb2/3)^4+ ions results in the develop…
We demonstrate that recent experimental data (E. Castel et al J.Phys. Cond. Mat. {\bf 21} (2009), 452201) on tungsten bronze compound (TBC) Ba$_2$Pr$_x$Nd$_{1-x}$FeNb$_4$O$_{15}$ can be well explained in our model predicting a crossover from ferroelectric ($x=0$) to orientational (dipole) glass ($x=1$), rather then relaxor, behavior. We show, that since a "classical" perovskite relaxor like Pb(Mn$_{1/3}$ Nb$_{2/3}$)O$_3$ is never a ferroelectric, the presence of ferroelectric hysteresis loops in TBC shows that this substance actually transits from ferroelectric to orientational glass phase wit…
CuCrS2, a Heisenberg antiferromagnet with layered edge sharing triangular lattice, exhibits a spiral magnetic order. Dielectric (E) and polarization studies show magnetoelectric (ME) coupling at Neel temperature (T_N=38 K) where simultaneous dielectric and magnetic long range order occur. The compound shows a diffused ferroelectric (FE) transition and slow relaxation dynamics above T_N, indicative of relaxor FE behavior. Interestingly, memory effect and magnetic field induced rejuvenation are also observed in E, establishing cooperative glassy dynamics and ME coupling even above T_N. We discus…
Herein, we elucidate the impact of tubular confinement on the structure and relaxation behaviour of poly(vinylidene difluoride) (PVDF) and how these affect the para-/ferroelectric behavior of this polymer. We use PVDF nanotubes that were solidified in anodic aluminum oxide (AAO) templates. Dielectric spectroscopy measurements evidence a bimodal relaxation process for PVDF nanotubes: besides the bulk-like -relaxation, we detect a notably slower relaxation that is associated with the PVDF regions of restricted dynamics at the interface with the AAO pore. Strickingly, both the bulk-like and the i…
Ferroelectric domain structure evolution induced by an external electric field was investigated by means of nematic liquid crystal (NLC) method in two strontium-barium niobate single crystals of nominal composition: Sr_{0.70}Ba_{0.30}Nb_{2}O_{6} (SBN:70 - relaxor) and Sr_{0.26}Ba_{0.74}Nb_{2}O_{6} (SBN:26 - ferroelectric). Our results provide evidence that the broad phase transition and frequency dispersion that are exhibited in SBN:70 crystal have a strong link to the configuration of ferroelectric microdomains. The large leakage current revealed in SBN:26 may compensate internal charges acti…
Neutron and x-ray scattering studies on relaxor ferroelectric systems Pb(Zn$_{1/3}$Nb$_{2/3}$)O$_3$ (PZN), Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$ (PMN), and their solid solutions with PbTiO$_3$ (PT) have shown that inhomogeneities and disorder play important roles in the materials properties. Although a long-range polar order can be established at low temperature - sometimes with the help of an external electric field; short-range local structures called the ``polar nano-regions'' (PNR) still persist. Both the bulk structure and the PNR have been studied in details. The coexistence and competition of l…
Synchrotron x-ray scattering studies of anti-ferroelectric nanoregions (AFR) as fluctuations, which give rise to 1/2(hk0) superlattice reflections (alpha spots), were systematically performed in Pb(Mg1/3Nb2/3)O3 (PMN) ferroelectric relaxor. Separation of the alpha spots from underlying diffuse scattering background allowed studying them as separate entities for the first time. AFR fluctuations were shown to be different from the chemical nanodomains (CND) and ferroelectric polar nano-regions (PNR). Instead, they are formed by anti-parallel short-range correlated Pb^2+ displacements that lead t…
A review is given of the phonon anomalies, which have been termed ``waterfalls,'' that were recently discovered through a series of neutron inelastic scattering measurements on the lead-oxide relaxor systems PZN-xPT, PMN, and PZN. We discuss a simple coupled-mode model that has been used successfully to describe the basic features of the waterfall, and which relates this unusual feature to the presence of polar micro-regions.
A new class of superparamagnetism was found in relaxor ferroelectric 2/3BiFeO$_{3}$-1/3BaTiO$_{3}$. The size of the magnetic particle, estimated from the superparamagnetic magnetization curve, coincides with the size of the polar nanoregion (PNR), which governs the relaxor ferroelectric property. This suggests that the magnetic domain is identical to the PNR. The temperature variations in the sizes of the magnetic domains and PNRs estimated by our neutron diffraction measurements support this picture. Since the same domain provides both electric and magnetic properties, strong coupling between…
The comparative experimental analysis of polarization reversal kinetics in conventional homogeneous triglycine sulfate ((NH_{2}CH_{2}COOH)_{3} \cdot H_{2}SO_{4}; TGS) and relaxor strontium barium niobate (Sr_{0.61}Ba_{0.39}Nb_{2}O_{6}; SBN) crystals have been performed in a broad range of measurement conditions. The experimental data have been collected from microscopic observation of the domain structure, switching current and D-E hysteresis loop registration. The hysteresis loop and dielectric spectra have a strong link to the configuration of ferroelectric microdomains. The domain structure…
The low-energy part of vibration spectrum in PbMg$_{1/3}$Nb$_{2/3}$O$_3$ relaxor ferroelectric was studied by inelastic neutron scattering. We observed the coexistence of a resolution-limited central peak with strong quasielastic scattering. The line-width of the quasielastic component follows a $Γ_0+Dq^2$ dependence. We find that $Γ_0$ is temperature-dependent. The relaxation time follows the Arrhenius law well. The presence of a relaxation mode associated with quasi-elastic scattering in PMN indicates that order-disorder behaviour plays an important rôle in the dynamics of diffuse phase tran…
We report the spontaneous decay of a soft, optical phonon in a solid. Using neutron spectroscopy, we find that specific phonon lifetimes in the relaxor PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$ are anomalously short within well-defined ranges of energy and momentum. This behavior is independent of ferroelectric order and occurs when the optical phonon with a specific energy and momentum can kinematically decay into two acoustic phonons with lower phase velocity. We interpret the well-known relaxor "waterfall" effect as a form of quasiparticle decay analogous to that previously reported in quantum spin liq…
(1-x)Li0.12Na0.88NbO3-xBaTiO3 (0 to x to 0.40) ferroelectric ceramics were prepared using conventional ceramics route and their phase transitional behavior is investigated by using dielectric spectroscopy. The temperature-dependent dielectric permittivity epsilon'(T) shows a diffused ferroelectric-paraelectric transition for all compositions. An acceptable and competent characterizing parameter (D) of diffused phase transition (DPT), defined by Uchino et al. [J Am Ceram Soc 2010;93:4011], was measured and validated. Interestingly, a crossover from diffused ferroelectric phase transition (FE-DP…
Neutron inelastic scattering measurements of the polar TO phonon mode dispersion in the cubic relaxor Pb(Zn_1/3Nb_2/3)_0.92Ti_0.08O_3 at 500K reveal anomalous behavior in which the optic branch appears to drop precipitously into the acoustic branch at a finite value of the momentum transfer q=0.2 inverse Angstroms, measured from the zone center. We speculate this behavior is the result of nanometer-sized polar regions in the crystal.
Short-range polar orders in the relaxor ferroelectric material PbMg$_{1/3}$Nb$_{2/3}$O$_3$-$28\%$PbTiO$_3$ (PMN-28PT) have been studied using neutron diffuse scattering. An external electric field along [110] direction can affect the diffuse scattering in the low temperature rhombohedral/monoclinic phase. Diffuse scattering intensities associated with [110] short-range polarizations are partially suppressed, while those arising from [1$\bar{1}$0] polarizations are enhanced. On the other hand, short-range polar orders along other equivalent $\langle110\rangle$ directions, i.e. [101],[10$\bar{1}…
Relaxor ferroelectrics have been a focus of intense attention due to their anomalous dielectric characteristics, diffuse phase transitions, and strong piezoelectricity. Understanding the structure and dynamics of relaxors has been one of the long-standing challenges in solid-state physics, with the current model of polar nanoregions in a non-polar matrix providing only a qualitative description of the relaxor phase transitions. In this paper, we investigate the local structure and dynamics in 75%PbMg$_{1/3}$Nb$_{2/3}$O$_3$-25%PbTiO$_3$ (PMN-PT) using molecular dynamics simulations and the dyna…
Barkhausen current noise is used to probe the slow field-driven conversion of the glassy relaxor ferroelectric state to an ordered ferroelectric (FE) state. The frequent presence of distinct micron-scale Barkhausen events well before the polarization current starts to speed up shows that the process is not a conventional nucleation-limited one. The prevalence of reverse switching events near the onset of the rapid part of the transition strongly indicates that electric dipole interactions play a key role. The combination of Barkhausen noise changes and changes in the complex dielectric respons…
We report high $q$-resolution neutron scattering data on PMN-$x$PT single crystals with $x=20%$ and 27%. No rhombohedral distortion occurs in the 20PT sample for temperatures as low as 50 K. On the other hand, the 27PT sample transforms into a rhombohedral phase below T$_C \sim 375$ K. Our data provide conclusive evidence that a new phase with an average cubic lattice is present in the bulk of this relaxor system at low PT concentration, in which the ferroelectric polarization and lattice distortion are decoupled. The rhombohedral distortion is limited to the outermost tens of microns of the c…
The radiofrequency dielectric response of the lead-free Ba(Zr$_{0.5}$Ti$_{0.5}$)O$_3$ relaxor ferroelectric is simulated using a coarse-grained Hamiltonian. This concept, taken from Real-Space Renormalization Group theories, allows depicting the collective behavior of correlated local modes gathered in blocks. Free-energy barriers for their thermally activated collective hopping are deduced from this {\it ab-initio}-based approach, and used as input data of Kinetic Monte Carlo simulations. The resulting numerical scheme allows to simulate the dielectric response for external field frequencies …
A novel lead-free tungsten bronze type ceramic Sr5BiTi3Nb7O30, was prepared by a conventional solid-state reaction route. The room-temperature crystal structure shows an average structure with centro-symmetric space group P4/mbm identified by synchrotron XRD. Temperature dependence of dielectric permittivity indicates that Sr5BiTi3Nb7O30 is a ferroelectric relaxor with Tm near 260 K. The ceramic displays stronger frequency dispersion and lower phase-transition temperature compared with Sr6Ti2Nb8O30. A macroscopic and phenomenological statistical model was employed to describe the temperature d…
We present first-principles supercell calculations on BaTi0.74Zr0.26O3, a prototype material for relaxors with a homovalent substitution. From a statistical analysis of relaxed structures, we give evidence for four types of Ti-atom polar displacements: along the < 1 1 1 >, < 1 1 0 >, or < 1 0 0 > directions of the cubic unit cell, or almost cancelled. The type of a Ti displacement is entirely determined by the Ti/Zr distribution in the adjacent unit cells. The underlying mechanism involves local strain effects that ensue from the difference in size between the Ti4+ and Zr4+ cations. These resu…
For the first time we proposed the model for the calculations of the relaxor ferroelectrics films properties in the framework of the random field theory. We took into account the misfit strain between film and substrate as well as surface piezoelectric effect that causes built-in electric field in the strained films. In the statistical theory framework we calculated random field distribution function with the electric dipoles and monopoles as the field sources. It was shown that with thickness decrease the mean field decreases, while the width of the distribution function increases. This leads…
Ferroelectric relaxors are complex materials with distinct properties. The understanding of their dielectric susceptibility, which strongly depends on both temperature and probing frequency, have interested researchers for many years. Here we report a macroscopic and phenomenological approach based on statistical modeling to investigate and better understand how the dielectric response of a relaxor depends on temperature. Employing the Maxwell-Boltzmann distribution and considering temperature dependent dipolar orientational polarizability, we propose a minimum statistical model and specific e…
Spatial variability of polarization relaxation kinetics in relaxor ferroelectric 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 is studied using time-resolved Piezoresponse Force Microscopy. Local relaxation attributed to the reorientation of polar nanoregions is shown to follow stretched exponential dependence, exp(-(t/tau)^beta), with beta~~0.4, much larger than the macroscopic value determined from dielectric spectra (beta~~0.09). The spatial inhomogeneity of relaxation time distributions with the presence of 100-200 nm "fast" and "slow" regions is observed. The results are analyzed to map the Vogel-Fulcher…
The complex interaction between transverse acoustic (TA) phonon, transverse optic (TO) phonon and polar nano-domains (PND) in the relaxor ferroelectric KTa1-xNbxO3 (KTN) is studied by means of high resolution diffuse and inelastic neutron scattering. The experimental results and a comparison with lead relaxors, suggest a new scattering mechanism of the TA phonon by localized modes in PNDs. A theoretical model is developed, which accurately predicts the evolution of the TA damping with temperature and wavevector. Such a mechanism suggests the possible use of high frequency acoustic modes for th…
Neutron scattering measurements of the lowest-energy TO phonons in the relaxor Pb(Mg1/3Nb2/3)O3 (PMN) are reported for 10<=T<=750 K. The soft mode, which is overdamped by the polar nanoregions below the Burns temperature T_d = 620 K, surprisingly recovers below 220 K. The square of the soft mode energy hw0^2 increases linearly with decreasing temperature, and is consistent with the behavior of a ferroelectric soft mode. At 10 K, hw0 reaches 11 meV, the same value observed in ferroelectric Pb(Zn1/3Nb2/3)O3 at low-T. An unusual broadening of the TA phonon starts at T_d and disappears at 220 K, c…
The interactions between the dielectric polarization and the fluctuations of the strain (stress) tensor in relaxor ferroelectrics are shown to give rise to the anisotropy of the anharmonic P4-term in the Landau-type free energy, however, the harmonic P2-term is still properly described by the rigid spherical random bond--random field model. These are the essential features of the compressible spherical dipolar glass model, which is used to calculate the singularities of the specific heat near field-induced critical points. The results agree with recent high-resolution calorimetric experiments …
Relaxor ferroelectrics, which can exhibit exceptional electromechanical coupling are some of the most important functional materials with applications ranging from ultrasound imaging to actuators and sensors in microelectromechanical devices. Since their discovery nearly 60 years ago, the complexity of nanoscale chemical and structural heterogeneity in these systems has made understanding the origins of their unique electromechanical properties a seemingly intractable problem. A full accounting of the mechanisms that connect local structure and chemistry with nanoscale fluctuations in polariza…
An atomistic first-principles-based effective Hamiltonian is used to investigate energy storage in Ba(Zr$_{0.5}$Ti$_{0.5}$)O$_{3}$ relaxor ferroelectrics, both in their bulk and epitaxial films' forms, for electric fields applied along different crystallographic directions. We find that the energy density linearly increases with temperature for electric fields applied along the pseudocubic [001], [110] and [111] directions in Ba(Zr$_{0.5}$Ti$_{0.5}$)O$_{3}$ bulk. For films at room temperature, the energy density adopts different behaviors (i.e., increase versus decrease) with strain depending …
Large low-frequency noise is found in some perovskite relaxor ferreoelectrics when they are polarized, regardless of whether the polarization is accompanied by an applied electric field. The noise appears both in the ferroelectric and relaxor states, including in the nominally ergodic paraelectric state at temperatures above the susceptibility peak. Since it is present whenever the samples are microphonic due to piezoelectricity but not evident when they are not microphonic, it appears to be a response to mechanical strain changes. Dependence of the noise on sample thermal history indicates th…
We study the free energy landscape of a minimal model for relaxor ferroelectrics. Using a variational method which includes leading correlations beyond the mean-field approximation as well as disorder averaging at the level of a simple replica theory, we find metastable paraelectric states with a stability region that extends to zero temperature. The free energy of such states exhibits an essential singularity for weak compositional disorder pointing to their necessary occurrence. Ferroelectric states appear as local minima in the free energy at high temperatures and become stable below a coex…
