Incipient Ferroelectrics Research Articles

Electrical properties of bismuth ferrites: Bi2Fe4O9 and Bi25FeO39

Bi2Fe4O9 was prepared by solid-state reaction and the electrical properties measured by impedance spectroscopy. After annealing in O2 at 900 °C, Bi2Fe4O9 is an electrically-homogeneous insulator. Its high frequency permittivity is constant (∼14.1) over the temperature range 300–400 °C and shows no evidence of incipient ferroelectricity at lower temperatures. On annealing in N2 at 900 °C, the pellets gradually decompose.Bi25FeO39 was prepared by both solid-state reaction and mechanosynthesis. It showed a modest amount of mixed conduction of both oxide ions and holes. Impedance analysis showed a complex response that best fitted an equivalent circuit consisting of a parallel combination of long-range conduction and short range dielectric relaxation elements.The electrical conductivity of both Bi2Fe4O9 and Bi25FeO39 is less than that of BiFeO3 prepared by solid-state reaction, which indicates that any leakage conductivity of BiFeO3 is not due to the possible presence of small amounts of these secondary phases.

Continuum thermodynamics of unusual domain evolution-induced toughening effect in nanocracked strontium titanate

This work is concerned with the analysis of both unusual mechanical and electric behaviors of nanoscale pre-cracked incipient ferroelectric of SrTiO3 at room temperature. A nonlinear thermodynamic model based on the Ginzburg-Landau theory is thus constructed and employed, which takes into account the appropriate mechanical boundary conditions, the electromechanical coupling between the polarization and the mechanical strain, and the self-strains of the ferroelastic and ferroelectric phase transformations. A large toughening effect is explored as a consequence of a softening nonlinear mechanical behavior, which characterizes for nanoscale SrTiO3, despite of the brittleness of bulk SrTiO3 ceramic. Depending upon applied strain, the large toughening and nonlinear mechanical behavior are attributed to unusual domain evolution of strain-induced polarization from the crack tip, in which a local-to-global transition of ferroelectric phase takes place from polarization vortex to hybrid structure of vortex and stripe domains, and finally to stripe domain structure with Néel type domain walls. Here, we also show the great importance of cross-coupling between the ferroelectric polarization and mechanical strain as it intrinsically leads to the local-to-global transition of ferroelectric phase. More interestingly, such cross-coupling additionally scatters the concentrated stress near the crack tip to Néel type domain walls. The present findings shed light on the crucial role of mechanical strain in controlling both the size and topology of polarization structures in nanoscale SrTiO3. Our numerical results also provide an insight into the vital role of strain-induced polarization for the toughness and strength of SrTiO3 material. Based on our study, the applicability of the present results to other incipient ferroelectrics such as CaTiO3, EuTiO3, and KTaO3 with the same mechanism would be interesting and possible.

ZnO‐enhanced electrical properties of Bi0.5Na0.5TiO3‐based incipient ferroelectrics

AbstractBi0.5Na0.5TiO3‐based incipient ferroelectrics with pseudocubic structure generally show weak ferro‐/piezoelectricity but giant field‐induced strains. It is difficult to artificially and smoothly improve the electrical property based on conventional chemical doping or substituting without changing the crystal structure and suppressing the strain. Here, by introducing the semiconductor ZnO into the lead‐free incipient ferroelectric ((Bi0.5(Na0.84K0.16)0.5)0.96Sr0.04)(Ti0.975Nb0.025)O3 (BNT–2.5Nb) to form 0‐3 type composites (BNT–2.5Nb:xZnO), we experimentally illustrate that the resistance and ferro‐/piezoelectric properties can be enhanced significantly with an unchanged crystal structure and only slightly suppressed strains. For example, the remanent polarization and piezoelectric coefficient increase from 4.6 μC/cm2 and 8 pC/N for x=0 to 9.0 μC/cm2 and 31 pC/N for x=0.3. At the same time, the total strain only decreases from 0.140% for x=0 to 0.108% for x=0.3, whereas the negative strain increases from −0.003% for x=0 to −0.010% for x=0.3. And the thermal stability of d33 is enhanced. The corresponding mechanism is attributed to that ZnO can form a local field, preventing the depolarization of field induced macroscopic ferroelectric domains. Our results not only provide a feasible way to tune electrical properties of BNT‐based incipient ferroelectrics, but also may stimulate further work on artificially structured high‐performance ferroelectrics.

Incipient ferroelectric to a possible ferroelectric transition in Te4+ doped calcium copper titanate (CaCu3Ti4O12) ceramics at low temperature as evidenced by Raman and dielectric spectroscopy

Partial replacement of Ti4+ by Te4+ ions in calcium copper titanate lattice improved its dielectric behaviour mostly due to cubic-to-tetragonal structural transformation and associated distortion in TiO6 octahedra. The relative permittivity values (23–30 x 103) of Te4+ doped ceramics is more than thrice that of un-doped ceramics (8 x 103) at 1 kHz. A decreasing trend in relative permittivity with increasing temperature (50–300 K) is observed for all the samples. Barrett’s formula, as a signature of incipient ferroelectricity, is invoked to rationalize the relative permittivity variation as a function of temperature. A systematic investigation supported by temperature dependent Raman studies reveal a possible ferroelectric transition in Te4+ doped ceramic samples below 120 K. The possible ferroelectric transition is attributed to the interactions between quasi-local vibrations associated with the micro-clusters comprising TiO6 and TeO6 structural units and indirect dipole-dipole interactions of off-center B–cations (Ti4+ and Te4+) in double perovskite lattice.

A Unified View of the Substitution-Dependent Antiferrodistortive Phase Transition in SrTiO3

The cubic-to-tetragonal antiferrodistortive transition at 105 K in the most widely studied perovskite, SrTiO3, is perhaps the preeminent example of a second-order structural phase transition. Extensive investigations since the 1960s have tracked the softening of the phonon mode associated with this transition, the development of octahedral rotations, and the interplay with incipient ferroelectricity and superconductivity. It is less well known that modest ionic substitutions alter the transition temperature (Ta) over a remarkable range in SrTiO3, from 0 K to above ambient. Here, we first study the thermodynamics of the transition via specific heat and determine a Ta shift of +10.9 K/atomic % in Nb-substituted crystals, the most heavily studied in terms of electronic transport. We then present the first quantitative rationalization of the trends in Ta with substitution in SrTiO3. We demonstrate that the apparently complex behavior versus tolerance factor occurs simply due to a nonlinear dependence of the rate of change of Ta with substituent concentration. We then connect this to ionic valence mismatch, using bond valence concepts to establish a new parameter, ⟨ε4⟩, which exhibits a universal linear dependence with Ta for all known substitutions. This provides the first unified view of the substituent-dependent Ta in SrTiO3, deepens our understanding of the phase transition (including a theoretical maximum in the rate of Ta suppression), and demonstrates predictive power via a simply computed parameter.

Far infrared and terahertz spectroscopy of ferroelectric soft modes in thin films: A review

ABSTRACTFar-infrared and terahertz spectroscopy of ferroelectric soft and central modes in thin films on substrates is reviewed. In addition to classical displacive proper ferroelectrics, also incipient and relaxor ferroelectrics and multiferroics are discussed. Special attention is paid to differences between the soft-mode behavior in thin films and bulk materials (ceramics and single crystals) and their influence on the low-frequency permittivity. Particularly the effects of the thin film strains and depolarizing electric fields of the probing waves on the grain boundaries are emphasized. The soft-mode spectroscopy is shown to be a very sensitive tool to reveal the thin film quality.

Incipient ferroelectricity of water molecules confined to nano-channels of beryl.

Water is characterized by large molecular electric dipole moments and strong interactions between molecules; however, hydrogen bonds screen the dipole–dipole coupling and suppress the ferroelectric order. The situation changes drastically when water is confined: in this case ordering of the molecular dipoles has been predicted, but never unambiguously detected experimentally. In the present study we place separate H2O molecules in the structural channels of a beryl single crystal so that they are located far enough to prevent hydrogen bonding, but close enough to keep the dipole–dipole interaction, resulting in incipient ferroelectricity in the water molecular subsystem. We observe a ferroelectric soft mode that causes Curie–Weiss behaviour of the static permittivity, which saturates below 10 K due to quantum fluctuations. The ferroelectricity of water molecules may play a key role in the functioning of biological systems and find applications in fuel and memory cells, light emitters and other nanoscale electronic devices.

The origin of incipient ferroelectricity in lead telluride.

The interactions between electrons and lattice vibrations are fundamental to materials behaviour. In the case of group IV–VI, V and related materials, these interactions are strong, and the materials exist near electronic and structural phase transitions. The prototypical example is PbTe whose incipient ferroelectric behaviour has been recently associated with large phonon anharmonicity and thermoelectricity. Here we show that it is primarily electron-phonon coupling involving electron states near the band edges that leads to the ferroelectric instability in PbTe. Using a combination of nonequilibrium lattice dynamics measurements and first principles calculations, we find that photoexcitation reduces the Peierls-like electronic instability and reinforces the paraelectric state. This weakens the long-range forces along the cubic direction tied to resonant bonding and low lattice thermal conductivity. Our results demonstrate how free-electron-laser-based ultrafast X-ray scattering can be utilized to shed light on the microscopic mechanisms that determine materials properties.

Hardening of the soft phonon in bulkSrTiO3interfaced withLaAlO3andSrRuO3

The low-temperature softening of the TO1 phonon of SrTiO3 (STO), which determines its incipient ferroelectricity, is known to be partially hindered either in the bulk under strong electric fields, or in thin STO films. Here we show, by terahertz (THz) reflectivity measurements, that a similar effect is produced in bulk STO and at zero static field by ultrathin metallic films on its surface, like a 10-nm-thick film of SrRuO3 (SRO), or the two-dimensional electron system (2DES) present at the interface with LaAlO3. In SRO/STO, the observed hardening is well explained by the depolarizing action of the SRO free electrons which follow adiabatically the ion motion. In LAO/STO, a weaker TO1 hardening could be detected by patterning the 2DES in the form of microstripes and using a polarized THz field parallel (E-) or orthogonal (E-) to the stripes. At 10 K, when TO1 is excited together with the free electrons by E-, its absorbance is harder by about 7 cm-1 than that measured when TO1 is coupled to the plasmon-polariton confined within the stripes, being excited by E-.

Incipient ferroelectricity and conductivity relaxations in Dy2Ti2O7

Dy2Ti2O7 ceramic samples were prepared via the solid-state reaction route. The dielectric properties of the sample were systematically investigated in the temperature range from 4 K to 973 K and the frequency range of 100 Hz to 1 MHz. Both Raman and low-temperature (down to 4 K) dielectric measurements indicate incipient ferroelectricity in the Dy2Ti2O7 sample. In addition, two relaxations were observed in the temperature range above 450 K. Our results indicate that the two relaxations are related to conductivity relaxation associated with singly and doubly ionized oxygen vacancies.

Softening of infrared-active mode of perovskite BaZrO3 proved by terahertz time-domain spectroscopy

The low-frequency infrared-active optical modes were studied in a barium zirconate, BaZrO3, single crystal with the perovskite structure using terahertz (THz) time-domain spectroscopy (TDS). The real and imaginary parts of the dielectric constants were accurately determined in the frequency range between 0.2 and 2.7 THz. Upon cooling from room temperature to 8 K, the lowest-frequency TO1 mode at 2.32 THz showed a pronounced softening to 1.94 THz. The real part of the dielectric constant at 0.5 THz determined by THz-TDS obeys Barrett's relation, and the existence of a plateau confirms that the quantum effects lead to saturation of the soft mode frequencies of the TO1 and TO2 modes below ≈20 K. This is reminiscent of incipient ferroelectrics with the perovskite structure such as CaTiO3.

Interplay between antiferrodistortive, ferroelectric, and superconducting instabilities inSr1−xCaxTiO3−δ

$\phantom{\rule{4.pt}{0ex}}{\text{SrTiO}}_{3}$ undergoes a cubic-to-tetragonal phase transition at 105 K. This antiferrodistortive transition is believed to be in competition with incipient ferroelectricity. Substituting strontium by isovalent calcium induces a ferroelectric order. Introducing mobile electrons to the system by chemical nonisovalent doping, on the other hand, leads to the emergence of a dilute metal with a superconducting ground state. The link between superconductivity and the other two instabilities is a question gathering momentum in the context of a popular paradigm linking unconventional superconductors and quantum critical points. We present a set of specific-heat, neutron-scattering, dielectric-permittivity, and polarization measurements on $\phantom{\rule{4.pt}{0ex}}{\text{Sr}}_{1\ensuremath{-}x}\phantom{\rule{4.pt}{0ex}}{\text{Ca}}_{x}\phantom{\rule{4.pt}{0ex}}{\text{TiO}}_{3}$ $(0<x<0.009)$ and a study of low-temperature electric conductivity in $\phantom{\rule{4.pt}{0ex}}{\text{Sr}}_{0.9978}\phantom{\rule{4.pt}{0ex}}{\text{Ca}}_{0.0022}\phantom{\rule{4.pt}{0ex}}{\text{TiO}}_{3\ensuremath{-}\ensuremath{\delta}}$. Calcium substitution was found to enhance the transition temperature for both antiferrodistortive and ferroelectric transitions. Moreover, we find that $\phantom{\rule{4.pt}{0ex}}{\text{Sr}}_{0.9978}\phantom{\rule{4.pt}{0ex}}{\text{Ca}}_{0.0022}\phantom{\rule{4.pt}{0ex}}{\text{TiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ has a superconducting ground state. The critical temperature in this rare case of a superconductor with a ferroelectric parent is slightly lower than in $\phantom{\rule{4.pt}{0ex}}{\text{SrTiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ of comparable carrier concentration. A three-dimensional phase diagram for $\phantom{\rule{4.pt}{0ex}}{\text{Sr}}_{1\ensuremath{-}x}\phantom{\rule{4.pt}{0ex}}{\text{Ca}}_{x}\phantom{\rule{4.pt}{0ex}}{\text{TiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ tracking the three transition temperatures as a function of $x$ and $\ensuremath{\delta}$ results from this study, in which ferroelectric and superconducting ground states are not immediate neighbors.

Dielectric Behavior of Low Microwave Loss Unit Cell for All Dielectric Metamaterial

With a deep study of the metamaterial, its unit cells have been widely extended from metals to dielectrics. The dielectric based unit cells attract much attention because of the advantage of easy preparation, tunability, and higher frequency response, and so forth. Using the conventional solid state method, we prepared a kind of incipient ferroelectrics (calcium titanate, CaTiO3) with higher microwave permittivity and lower loss, which can be successfully used to construct metamaterials. The temperature and frequency dependence of dielectric constant are also measured under different sintering temperatures. The dielectric spectra showed a slight permittivity decrease with the increase of temperature and exhibited a loss of 0.0005, combined with a higher microwave dielectric constant of ~167 and quality factorQof 2049. Therefore, CaTiO3is a kind of versatile and potential metamaterial unit cell. The permittivity of CaTiO3at higher microwave frequency was also examined in the rectangular waveguide and we got the permittivity of 165, creating a new method to test permittivity at higher microwave frequency.

High-temperature dielectric properties of incipient ferroelectric Gd0.5Na0.5TiO3

Gd0.5Na0.5TiO3 ceramic samples were prepared by the solid-state reaction method. Incipient ferroelectricity was confirmed in the sample. Dielectric properties were investigated at temperatures from room temperature to 1073K and the frequency range of 20 Hz–10 MHz. By means of electric modulus and impedance analysis, the sample was found to show two thermally activated relaxations with the high-frequency (low-temperature) one being the relaxation caused by hopping motion of oxygen vacancies and the low-frequency (high-temperature) one being the interfacial relaxation.

Thickness Dependence of Phase Transition Temperature in Disordered Monodomain Ferroelectric Thin Films

We propose a theoretical approach to calculate the thermodynamic properties of thin films fabricated from monodomain disordered ferroelectrics. We do that for the incipient ferroelectrics with dipole impurities like KTaO3: Li, Nb, Na or SrTiO3: Ca, although the proposed approach can be also applied to ferroelectric relaxors. In our calculations we use the method of random fields, modified for the case of confined geometry. The essence of modification is the altering of the interaction between impurity dipoles by geometrical confinement. We show that in thin films the ferroelectric phase transition is inhibited as compared to the case of bulk samples.

Local electrostatic imaging of striped domain order in LaAlO3/SrTiO3

The emerging field of complex oxide interfaces is generically built on one of the most celebrated substrates--strontium titanate (SrTiO3). This material hosts a range of phenomena, including ferroelasticity, incipient ferroelectricity, and most puzzlingly, contested giant piezoelectricity. Although these properties may markedly influence the oxide interfaces, especially on microscopic length scales, the lack of local probes capable of studying such buried systems has left their effects largely unexplored. Here we use a scanning charge detector--a nanotube single-electron transistor--to non-invasively image the electrostatic landscape and local mechanical response in the prototypical LaAlO3/SrTiO3 system with unprecedented sensitivity. Our measurements reveal that on microscopic scales SrTiO3 exhibits large anomalous piezoelectricity with curious spatial dependence. Through electrostatic imaging we unravel the microscopic origin for this extrinsic piezoelectricity, demonstrating its direct, quantitative connection to the motion of locally ordered tetragonal domains under applied gate voltage. These domains create striped potential modulations that can markedly influence the two-dimensional electron system at the conducting interface. Our results have broad implications to all complex oxide interfaces built on SrTiO3 and demonstrate the importance of microscopic structure to the physics of electrons at the LaAlO3/SrTiO3 interface.

Ferroic properties of nanosized SnO2

Undoped nanoparticles of incipient ferroelectrics and binary oxides without any magnetic ions can become ferromagnetic even at room temperatures due to the inherent presence of a new type of magnetic defect, namely oxygen vacancies. The surface-induced magnetic states of the oxygen vacancies have shown to exist at the surface of SnO2 and under the surface up to the distances 5–10 nm. Oxygen vacancies are the sources of long-range ferromagnetic order experimentally observed in thin films of otherwise nonmagnetic SnO2. Thus, we report about surface-induced ferroic phase in otherwise nonmagnetic and nonferroelectric SnO2.

Precursor dynamics, incipient ferroelectricity and huge anharmonicity in antiferroelectric lead zirconate PbZrO3

To better understand the phase transition mechanism of PbZrO3 (PZO), the lattice dynamics of this antiferroelectric compound are investigated within the polarizability model, with emphasis on the cubic to orthorhombic phase transition. Similarly to ferroelectric phase transitions in ABO3 perovskites, polar dynamical clusters develop and grow in size upon approaching TC from the high temperature side and never form a homogeneous state. Simultaneously, elastic anomalies set in and compete with polar cluster dynamics. These unusual dynamics are responsible for precursor effects that drive the PZO lattice towards an incipient ferroelectric state. Comparison of the model calculations with the temperature dependences of elastic coefficients measured on PZO single crystals reveals a striking similarity.

Oxide nanomaterials with properties absent in bulk (Author Review)

A ferroelectric phase can be induced in otherwise nonferroelectric binary oxides like BaO, EuO, and Er2O3 by a strong enough intrinsic surface stress present under the curved surface in nanowires. Our analytical calculations performed within the Landau–Ginzburg–Devonshire theory prove that BaO nanowires can be ferroelectric at room temperatures. Undoped nanoparticles of incipient ferroelectrics SrTiO3 and KTaO3 without any magnetic ions can become ferromagnetic even at room temperatures due to the presence of a new type of magnetic defects, such as oxygen vacancies, inherent to nanostructures. Nanosize Eu x Sr1–x TiO3 wires and tubes are shown to be the new multiferroics with “triple” the antiferrodistortive–ferroelectric–ferromagnetic phase, which is of interest for fundamental study and can be important for their potential applications. We hope that our predictions will stimulate experimental studies of the nanosized binary oxides and perovskites.

Ionically-Mediated Electromechanical Hysteresis in Transition Metal Oxides

Nanoscale electromechanical activity, remanent polarization states, and hysteresis loops in paraelectric TiO(2) and SrTiO(3) thin films are observed using scanning probe microscopy. The coupling between the ionic dynamics and incipient ferroelectricity in these materials is analyzed using extended Landau-Ginzburg-Devonshire (LGD) theory. The possible origins of electromechanical coupling including ionic dynamics, surface-charge induced electrostriction, and ionically induced ferroelectricity are identified. For the latter, the ionic contribution can change the sign of first order LGD expansion coefficient, rendering material effectively ferroelectric. The lifetime of these ionically induced ferroelectric states is then controlled by the transport time of the mobile ionic species and well above that of polarization switching. These studies provide possible explanation for ferroelectric-like behavior in centrosymmetric transition metal oxides.