Long-time relaxation of the dielectric response in lead magnoniobate

Supercritical Relaxor Nanograined Ferroelectrics for Ultrahigh-Energy-Storage Capacitors

Supercritical relaxor nanograined ferroelectrics are demonstrated for high-performance dielectric capacitors, showing record-high overall properties of energy density ≈13.1 J cm^-3 and field-insensitive efficiency ≈90% at ≈74 kV mm^-1 and superior charge-discharge performances of high power density ≈700 MW cm^-3 , high discharge energy density ≈6.67 J cm^-3 , and ultrashort discharge time <40 ns at 55 kV mm^-1 . Ex/in situ transmission electron microscopy, Raman spectroscopy, and synchrotron X-ray diffraction provide clear evidence of the supercritical behavior in (Na,K)(Sb,Nb)O₃ -SrZrO₃ -(Bi_0.5 Na_0.5 )ZrO₃ ceramics, being achieved by engineering the coexistence of multiple local symmetries within the ergodic relaxor zone. The vanished difference between the ground relaxor state and the high-field supercritical state eliminates polarization hysteresis. The supercritical evolution with electric field enables a highly delayed polarization saturation with continuously increased polarization magnitudes. The results demonstrate that such a design strategy of compositionally induced and field-manipulated supercritical behavior can be generalizable for developing desirable energy-storage dielectrics for applications in ceramic/film capacitors.

Ultrafast Neuromorphic Dynamics Using Hidden Phases in the Prototype of Relaxor Ferroelectrics

Materials possessing multiple states are promising to emulate synaptic and neuronic behaviors. Their operation frequency, typically in or below the GHz range, however, limits the speed of neuromorphic computing. Ultrafast THz electric field excitation has been employed to induce nonequilibrium states of matter, called hidden phases in oxides. One may wonder if there are systems for which THz pulses can generate neuronic and synaptic behavior, via the creation of hidden phases. Using atomistic simulations, we discover that relaxor ferroelectrics can emulate all the key neuronic and memristive synaptic features. Their occurrence originates from the activation of many hidden phases of polarization order, resulting from the response of nanoregions to THz pulses. Such phases further possess different dielectric constants, which is also promising for memcapacitor devices.

An Ultrathin Single Crystalline Relaxor Ferroelectric Integrated on a High Mobility Semiconductor

The epitaxial growth of multifunctional oxides on semiconductors has opened a pathway to introduce new functionalities to semiconductor device technologies. In particular, the integration of gate materials that enable nonvolatile or hysteretic functionality in field-effect transistors could lead to device technologies that consume less power or allow for novel modalities in computing. Here we present electrical characterization of ultrathin single crystalline SrZr_xTi_1-xO₃ (x = 0.7) films epitaxially grown on a high mobility semiconductor, Ge. Epitaxial films of SrZr_xTi_1-xO₃ exhibit relaxor behavior, characterized by a hysteretic polarization that can modulate the surface potential of Ge. We find that gate layers as thin as 5 nm corresponding to an equivalent-oxide thickness of just 1.0 nm exhibit a ∼2 V hysteretic window in the capacitance-voltage characteristics. The development of hysteretic metal-oxide-semiconductor capacitors with nanoscale gate thicknesses opens new vistas for nanoelectronic devices.

First-principles-based effective Hamiltonian simulations of bulks and films made of lead-free Ba(Zr,Ti)O3 relaxor ferroelectrics

A review of the recent development and application of a first-principles-derived effective Hamiltonian technique to the study of lead-free Ba(Zr,Ti)O3 (BZT) relaxor ferroelectrics is provided. In addition to the computation and analysis of macroscopic properties (such as different types of dielectric responses and electric polarization) and their connections to previous published works, particular emphasis is given to microscopic insights arising from this atomistic technique. These include (i) the numerically-found determination of the physical origin of the relaxor behavior in BZT; and (ii) the prediction of polar nanoregions and the evolution of their morphology as a response to temperature, electric fields and epitaxial misfit strain. Other striking phenomena that were predicted in BZT compounds, such as Fano resonance and field-driven percolation, are also documented and discussed. Finally, a brief perspective of possible remaining computational studies to be conducted in relaxor ferroelectrics, in order to further understand them, is attempted.

Properties of epitaxial films made of relaxor ferroelectrics

Finite-temperature properties of epitaxial films made of Ba(Zr,Ti)O3 relaxor ferroelectrics are determined as a function of misfit strain, via the use of a first-principles-based effective Hamiltonian. These films are macroscopically paraelectric at any temperature, for any strain ranging between ≃-3% and ≃+3%. However, original temperature-versus-misfit strain phase diagrams are obtained for the Burns temperature (Tb) and for the critical temperatures (Tm,z and Tm,IP) at which the out-of-plane and in-plane dielectric response peak, respectively, which allow the identification of three different regions. These latter differ from their evolution of Tb, Tm,z, and/or Tm,IP with strain, which are the fingerprints of a remarkable strain-induced microscopic change: each of these regions is associated with its own characteristic behavior of polar nanoregions at low temperature, such as strain-induced rotation or strain-driven elongation of their dipoles or even increase in the average size of the polar nanoregions when the strength of the strain grows.

Field-induced percolation of polar nanoregions in relaxor ferroelectrics

A first-principles-based effective Hamiltonian is used to investigate low-temperature properties of Ba(Zr,Ti)O(3) relaxor ferroelectrics under an increasing dc electric field. This system progressively develops an electric polarization that is highly nonlinear with the dc field. This development leads to a maximum of the static dielectric response at a critical field, E(th), and involves four different field regimes. Each of these regimes is associated with its own behavior of polar nanoregions, such as shrinking, flipping, and elongation of dipoles or change in morphology. The clusters propagating inside the whole sample, with dipoles being parallel to the field direction, begin to form at precisely the E(th) critical field. Such a result, and further analysis we perform, therefore, reveal that field-induced percolation of polar nanoregions is the driving mechanism for the transition from the relaxor to ferroelectric state.

High-resolution calorimetric study of Pb(Mg(1/3)Nb(2/3))O3 single crystal

Motivated by the long-standing unresolved enigma of the relaxor ferroelectric ground state, we performed a high-resolution heat capacity and polarization study of the field-induced phase transition in the relaxor ferroelectric single crystal Pb(Mg(1/3)Nb(2/3))O3 (PMN) oriented along the [110] direction. We show that the discontinuous evolution of polarization as a function of the electric field or temperature is a consequence of a true first order transition from a glassy to ferroelectric state, which is accompanied by an excess heat capacity anomaly and released latent heat. We also find that in a zero field there is no ferroelectric phase transition in bulk PMN at any temperature, indicating that the nonergodic dipolar glass phase persists down to the lowest temperatures.

Finite-temperature properties of Ba(Zr,Ti)O3 relaxors from first principles

A first-principles-based technique is developed to investigate the properties of Ba(Zr,Ti)O(3) relaxor ferroelectrics as a function of temperature. The use of this scheme provides answers to important, unresolved and/or controversial questions such as the following. What do the different critical temperatures usually found in relaxors correspond to? Do polar nanoregions really exist in relaxors? If yes, do they only form inside chemically ordered regions? Is it necessary that antiferroelectricity develop in order for the relaxor behavior to occur? Are random fields and random strains really the mechanisms responsible for relaxor behavior? If not, what are these mechanisms? These ab initio based calculations also lead to deep microscopic insight into relaxors.

Relaxor Pb(Mg(1/3)Nb(2/3))O3: a ferroelectric with multiple inhomogeneities

Despite intensive studies on Pb(Mg(1/3)Nb(2/3))O(3) (PMN) relaxor, understanding the exact nature of its giant dielectric response and of its physical ground state is a fundamental issue that has remained unresolved for decades. Here, we report a comprehensive study of PMN relaxor crystal, and show that (i) its anomalous dielectric behavior in a broad temperature range results from the reorientation of polarization in the crystal, and (ii) the PMN relaxor is essentially a nanosized ferroelectric material with multiscale inhomogeneities of domain structure in addition to the well-known inhomogeneities of chemical composition and local symmetry. Such inhomogeneities are believed to play a crucial role in producing the huge and enigmatic physical effects in relaxor system, and may be used to design other new systems with giant effects such as a relaxor system.

Structural properties of 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 relaxor ferroelectric thin films on SrRuO3 conducting oxides

Coating of 0.65Pb(Mg(1/3)Nb(2/3))O(3)-0.35PbTiO(3) (PMN-PT) relaxor ferroelectrics by a sol-gel method is followed by growth of epitaxial SrRuO(3) (SRO) metallic oxide electrodes on SrTiO(3) (STO) single-crystal substrate by pulsed laser deposition. High-quality PMN-PT films on SRO with preferred growth orientation were successfully fabricated by controlling the operation parameters. Structural properties of relaxor ferroelectric PMN-PT thin films on SRO/STO substrates have been studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and atomic force microscopy (AFM). In-plane and out-of-plane alignments of the heterostructure are confirmed and the structural twinning of the materials are also revealed.

Direct observation of the formation of polar nanoregions in Pb(Mg1/3Nb2/3)O3 using neutron pair distribution function analysis

Using neutron pair distribution function analysis over the temperature range from 1000 to 15 K, we demonstrate the existence of local polarization and the formation of medium-range, polar nanoregions (PNRs) with local rhombohedral order in a prototypical relaxor ferroelectric Pb(Mg(1/3)Nb(2/3))O3. We estimate the volume fraction of the PNRs as a function of temperature and show that this fraction steadily increases from 0% to a maximum of approximately 30% as the temperature decreases from 650 to 15 K. Below T approximately 200 K the volume fraction of the PNRs becomes significant, and PNRs freeze into the spin-glass-like state.

Barkhausen noise in a relaxor ferroelectric

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)O3, driven by a periodic electric field. The temperature dependences of both the amplitude and spectral form show that the size of the coherent changes in the dipole moment shrink as the relaxor regime is entered, contrary to expectations based on some simple models.

Aging in a relaxor ferroelectric: scaling and memory effects

A crossover as a function of temperature is found in the zero-field aging properties of the relaxor ferroelectric Pb(Mg(1/3)Nb(2/3))O3. Below a temperature near which nonlinear susceptibility has indicated a suspected phase transition, the time-frequency dependence shows simple scale-independent behavior resembling that for spin glasses. As in spin glasses, high temperature aging is stable as further aging occurs at lower temperature, but not vice versa, indicating hierarchical state arrangement. A more general interpretation of such effects is briefly discussed.

Phenomenological model of dynamic nonlinear response of relaxor ferroelectrics

A phenomenological model was proposed which describes frequency dispersion of nonlinear dielectric response of relaxor ferroelectrics (relaxors) as a result of dispersion of their linear dielectric permittivity. The model was applied to Pb(Mg(1/3)Nb(2/3))O3 (PMN) relaxor. It provided a good qualitative description of temperature and frequency dependence of the third harmonics of PMN. Analysis within the model yielded a frequency independent nonlinear coefficient corresponding to static nonlinear dielectric susceptibility. The model explained the recently reported for PMN data on the Vogel-Fulcher law for frequency dependence of the temperature at which the third harmonics passes a maximum.

Crossover from glassy to inhomogeneous-ferroelectric nonlinear dielectric response in relaxor ferroelectrics

The temperature dependence of the dielectric nonlinearities in a PMN single crystal and in 9/65/35 PLZT ceramics has been determined by measuring the first and third harmonic response as well as the dielectric behavior as a function of the dc electric field. In zero field a paraelectric-to-glass, and, in a high enough dc field, a glass-to-ferroelectriclike crossover in the temperature dependence of the nonlinear response have been observed. Both crossovers agree with the predictions of the spherical random-bond-random-field model. Relaxors thus undergo in zero field a transition to a spherical glass, while above the critical field a transition into a ferroelectric state occurs.

Soft phonon anomalies in the relaxor ferroelectric Pb(Zn(1/3)Nb(2/3))0.92Ti0.08O3

Neutron inelastic scattering measurements of the polar transverse optic phonon mode dispersion in the cubic relaxor Pb(Zn(1/3)Nb(2/3))0.92Ti0.08O3 at 500 K 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 A(-1) measured from the zone center. We speculate that this behavior is the result of nanometer-sized polar regions in the crystal.