Polar Instability Research Articles

Accelerating ferroelectric materials discovery through high-throughput first-principles screening and experimental validation

We conducted high-throughput screening of ferroelectrics using first-principles calculations based on an existing crystal structure database. We focused on nonpolar structures with polar instability, to efficiently screen materials for their potential to undergo ferroelectric phase transitions from oxide materials in crystal structure databases. Our screening criteria included computational feasibility (excluding partial occupation), the absence of hazardous elements, and a maximum of 250 atoms in the conventional cell. Through this screening, we identified 47 ferroelectric candidates, 8 of which have already been reported as ferroelectrics. To validate our screening approach, we synthesized and evaluated several candidate materials with Dion–Jacobson-type structures, and measured their dielectric and ferroelectric properties. Although the ferroelectric behavior was not initially identified in these materials, our experiments confirmed their properties. Finally, we discovered a new ferroelectric material, CsCa2Nb3O10, which exhibited a ferroelectric phase transition at 28 K, clearly demonstrating the effectiveness of our screening strategy.

Thermal cycle stability and microstructure of Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals

The Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) ferroelectric single crystals have been commercially available as important components in medical ultrasound transducers due to their excellent piezoelectric and electromechanical coupling performance. The variation in piezoelectric and dielectric properties of PMN-PT single crystals with ambient temperature is an important application indicator. In this work, the PMN-PT single crystals after direct current poling (DCP) and alternating current poling (ACP) were subjected to the cyclic thermal treatment process. The thermal cycling stability and microstructural changes in PMN-PT single crystals were investigated. The ACP single crystals exhibit a higher dielectric constant ε33T/ε0 (6500–7600) and piezoelectric coefficient d33 (2100–2500 pC N−1) compared to the DCP single crystals (ε33T/ε0 of 4100–5000, d33 of 1200–1300 pC N−1). Under thermal cycling at 60 °C, the DCP and ACP single crystals exhibit good thermal cycling stability after 150 cycles. Microstructural observations show that the domain structure of the DCP single crystals exhibits “staggered domain walls, inhomogeneous domain size, variety of domain structure,” while the relatively homogeneous stripe-like domains were observed in the ACP single crystals. After thermal cycling, new fine striped domains appear in both the DCP and ACP single crystals due to the instability of rotated polarization, but the piezoelectric and dielectric properties are not greatly affected. This work provides an intensive understanding of the effects of thermal cycling on the domain structure, which is useful for applications.

Pressure-dependent phase transitions in hybrid improper ferroelectric Ruddlesden-Popper oxides

The temperature-dependent phase transitions in Ruddlesden-Popper oxides with perovskite bilayers have been under increased scrutiny in recent years due to the so-called hybrid improper ferroelectricity that some chemical compositions exhibit. However, little is currently understood about the hydrostatic pressure dependence of these phase transitions. Herein we present the results of a combined high-pressure powder synchrotron x-ray diffraction experiment and abinitio study on the bilayered Ruddlesden-Popper phases Ca3Mn2O7 and Ca3Ti2O7. In both compounds we observe a first-order phase transition, that in combination with our density functional theory calculations, we can confidently assign as being between polar A21am and nonpolar Acaa structures. Interestingly, we show that while the application of pressure ultimately favors a nonpolar phase, as is commonly observed for proper ferroelectrics, regions of response exist where pressure actually acts to increase the polar mode amplitudes. The reason for this can be untangled by considering the varied response of octahedral tilts and rotations to hydrostatic pressure and their trilinear coupling with the polar instability. Published by the American Physical Society 2024

Molecular beam epitaxy of KTaO3

Strain-engineering is a powerful means to tune the polar, structural, and electronic instabilities of incipient ferroelectrics. KTaO3 is near a polar instability and shows anisotropic superconductivity in electron-doped samples. Here, we demonstrate growth of high-quality KTaO3 thin films by molecular-beam epitaxy. Tantalum was provided by either a suboxide source emanating a TaO2 flux from Ta2O5 contained in a conventional effusion cell or an electron-beam-heated tantalum source. Excess potassium and a combination of ozone and oxygen (10% O3 + 90% O2) were simultaneously supplied with the TaO2 (or tantalum) molecular beams to grow the KTaO3 films. Laue fringes suggest that the films are smooth with an abrupt film/substrate interface. Cross-sectional scanning transmission electron microscopy does not show any extended defects and confirms that the films have an atomically abrupt interface with the substrate. Atomic force microscopy reveals atomic steps at the surface of the grown films. Reciprocal space mapping demonstrates that the films, when sufficiently thin, are coherently strained to the SrTiO3 (001) and GdScO3 (110) substrates.

Modeling polar order in compressively strained SrTiO3

Strontium titanate is an incipient ferroelectric in which superconductivity emerges at exceptionally low doping levels. Remarkably, stabilizing the polar phase through strain or chemical substitution has been shown to significantly enhance the superconducting critical temperature, and the polar instability plays a pivotal role in the majority of proposed superconducting pairing mechanisms. A rigorous understanding of ferroelectricity is therefore essential to elucidate the electron pairing mechanism in this material. To investigate the nature of the polar order in strontium titanate, we develop a simplified free energy model that only includes the degrees of freedom necessary to capture the relevant physics in a biaxially compressively strained system. Our model is able to calculate the energies of large, disordered systems with near DFT-level accuracy. We simulate the ferroelectric and antiferrodistortive phase transitions using the Monte Carlo method and discuss the coupling between various order parameters. Finally, we assess the character of the polar transition, which we find to be neither strictly displacive nor order-disorder.

Terahertz time-domain spectroscopic ellipsometry of metallic strontium titanate

The coexistence of the polar instability and metallic conduction has attracted much attention. Ferroelectric soft optic modes and carrier electrons were investigated in A- and B-site heterovalent doped strontium titanate SrTiO3 crystals by the terahertz time-domain spectroscopic ellipsometry. The observed real and imaginary parts of a complex dielectric constant were fitted by the damped harmonic oscillator model for the ferroelectric soft optic mode and the Drude-Smith model for carrier electrons. The observed soft mode frequency increases as the La and Nb content increases. It is found that the square of the ferroelectric soft mode frequency linearly changes to the carrier electron concentration. This fact indicates that the carrier electrons suppress the ferroelectric instability and the ferroelectric soft mode frequency increases as the carrier concentration increases.

Piezoelectricity of Sm-doped Pb(Zn1/3Nb2/3)O3–PbTiO3 single crystals by enhancement of local disorder polarization

Relaxor-based ferroelectrics are a kind of ideal material, which are widely applied in transducers, medical ultrasound probes, sensors and a series of electromechanical instruments. Rare dopants can enhance the piezoresponse by introducing local structural heterogeneity. However, the origination of the giant piezoresponse is still controversial due that the instability of polarization results from the shift of phase transition temperature and component inconsistency. Herein, we grew high homogeneity Sm-doped Pb(Zn1/3Nb2/3)O3–5%PbTiO3 (Sm-PZNT) single crystals, which the d33 values is of ∼3200 pC/N and variation of phase transition temperatures is less than 7 K. The local polarization is characterized by atomic-scale level transmission microscopy (HRTEM). The macro-irregular ferroelectric domain is observed by piezoforce microscopy (PFM). These results identify that the Sm3+ dopant enhances the local structural heterogeneity and improves the piezoelectric properties of PZNT with slightly effect on phase transition temperatures. These results pave the way towards a new generation of high-performance relaxor ferroelectrics design.

Coupling between tilts and charge carriers at polar-nonpolar perovskite interfaces

The phenomenological theory for the polar instability giving rise to a two-dimensional electron gas at perovskite interfaces is hereby extended to include the coupling to perovskite tilts. A Landau theory for homogeneous tilts is first explored, setting the scene for the further, more realistic Landau-Ginzburg theory describing varying tilt amplitudes across a thin film. The theory is also generalized to account for the response to an applied electric field normal to the interface, which allows a finer control on phase transitions. The conventionally described physics of a single metal-insulator transition is substantially enriched by the coupling, the model describing various scenarios with one or two transitions, possibly continuous or discontinuous. First-principles calculations permit the estimation of the parameters defining the model, which have been calculated for the interface between lanthanum aluminate and strontium titanate.

Systematic search of the rotation-compatible polar phases in perovskite oxides: A first-principles study

In many perovskite oxides, ferroelectricity is intimately related to octahedral rotation patterns, which can suppress or enhance polar distortions. Using first-principles density functional theory, we investigate the relation between octahedral rotation patterns and polar instability. Based on the rotation patterns commonly observed in perovskite oxides, we present a workflow that allows to systematically and efficiently search for the unstable polar phonon modes and identify metastable polar structures. We apply the workflow to investigate rotation-pattern-dependent polar phases of CaSnO3 and find metastable polar structures by changing rotation patterns. Further investigation of the polar R3c structures shows sizable polarization comparable to the conventional ferroelectrics. We discuss substrate materials having the potential to stabilize the polar structure. Our work provides an efficient way to identify new polar phases by changing the octahedral rotation patterns, which will be useful to design new functional materials in the thin-film/substrate configuration utilizing interfacial coupling.

Effect of Cu\(^{2+}\) Dopant on the Formation of Zinc Oxide Microrod Fabricated by a Hydrothermal Method

Reconstruction and stabilization of polar oxide surfaces, such as ZnO, contribute a significant role in photocatalysis, chemical sensing, and optoelectronic applications, however their physical chemistry insight is still a puzzle in the surface science. In this work, the polar surface instability induced the morphological evolution of hydrothermally synthesized micro-rod ZnO doped with various contents of Cu2+ ion (1-10 at.%) was investigated. The transformation of micro-rod morphology from the high aspect ratio flower-like shape of the pure ZnO to the hexagonal prism-like shape of the doped ZnO was characterized by X-ray diffractometry, scanning electron microscopy and micro Raman spectroscopy. The chemically active Zn-terminated polar surface in doped samples was less positive charge density which was the main reason to cancel the electrostatic instability for the dominant growing direction. Furthermore, the schematic models of the electron transferring from the conduction band region to the electron trap centre of Cu2+, and the Zn-terminated polar surface reconstruction were proposed for the morphological evolution mechanism.

Causes of the Record-Breaking Pacific Northwest Heatwave, Late June 2021

The extreme heat event that hit the Pacific Northwest (Oregon, Washington, southern British Columbia) at the end of June 2021 was 3 °C greater than the previous Seattle record of 39 °C; larger extremes of 49 °C were observed further inland that were 6 °C above previous record. There were hundreds of deaths over the region and loss of marine life and forests. At the large scale prior to the event, the polar vortex was split over the Arctic. A polar vortex instability center formed over the Bering Sea and then extended southward along the west coast of North America. The associated tropospheric trough (low geopotential heights) established a multi-day synoptic scale Omega Block (west-east oriented low/high/low geopotential heights) centered over the Pacific Northwest. Warming was sustained in the region due to subsidence/adiabatic heating and solar radiation, which were the main reasons for such large temperature extremes. The seasonal transition at the end of spring suggests the possibility of a southern excursion of a polar vortex/jet stream pair. Both the Pacific Northwest event in 2021 and the Siberian heatwave climax in June 2020 may be examples of crossing a critical state in large-scale atmospheric circulation variability.

Ordynacja wyborcza do włoskiej Izby Deputowanych z 4 sierpnia 1993 roku

The political instability of Italian governments, excessive fragmentation and party polarization, political corruption characterized Italy's political system from the unification of the state in the second half of the nineteenth century. Successive amendments to the parliamentary election law were an attempt to solve the above problems. Such a process also took place in 1993. The aim of this article is to present the legal provisions of the electoral law to the Chamber of Deputies in Italy of August 4th, 1993. The shape of the changes introduced, the mixed election formula and the original method of distributing seats (scorporo totale and scorporo pro quota) were an example of searching for new solutions in electoral engineering on the way to finding the perfect electoral law. The article uses mainly legal-dogmatic and legal-institutional methods.

Factors influencing flexoelectric polarization in liquid crystals

The work experimentally investigated the factors affecting the formation and physical parameters of flexoelectric polarization in liquid crystals (LC) with nematic as well as smectic A- and C-phases. Factors such as the heating temperature of the sample, orientation effects near the nematic – isotropic state (N-I) phase transition, and the initial orientation of LC molecules were studied. The thickness of the studied layers is from 10 to 100 μm, the frequency of exposure is 1000 Hz. It was found that the flexosignal takes maximum value in homeotropically oriented samples in the nematic phase. When approaching the N-I phase transition, the magnitude of the flex signal increases, which is associated with an increase in the deviation angle of the director of the LC. As the dipole moment grows, the quadrupole moment also increases. This is due to the tendency of LC molecules to form dipole pairs with antiparallel orientation of electric moments. The estimate of the flexural value U1ω yielded (for example, for MBBA) with a planar orientation 3 times less than with a homeotropic one. The presence of the second harmonic U2ω is a consequence of the orientational instability of surface polarization. The rapid growth of U2ω is associated with an increase in the square of the director deviation angle .

Probing the Polar Instability of Strained SrTiO3 with HAADF-STEM

Probing the Polar Instability of Strained SrTiO₃ with HAADF-STEM

Origin of the persistence of soft modes in metallic ferroelectrics

Metallicity and ferroelectriclike polar distortion are mutually noncompatible, and their coexistence in the same system is an intriguing subject of fundamental interest in the field of structure phase transition. However, it is unclear what mechanism may extend the limit of metallicity that allows polar distortion. We investigate the polar instability and soft modes in electron-doped ${\mathrm{PbTiO}}_{3}$ using linear-response density functional calculations. We find that ferroelectric instability can remarkably sustain up to an electron concentration of ${n}_{e}=0.7$ per unit cell, which is beyond the limit that causes the polar catastrophe in ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$. Our study further reveals two discoveries: (i) electron doping can turn nonsoft mode into soft mode, which leads to different microscopic mechanism for ferroelectricity when the system is strongly metallic; (ii) the frequency change $\mathrm{\ensuremath{\Delta}}\ensuremath{\omega}/\mathrm{\ensuremath{\Delta}}{n}_{e}$ is surprisingly flat at large ${n}_{e}$, which is pivotal for the persistence of soft mode and polar distortion at high metallicity. We also provide an interesting physical origin---which is caused by the strong mode-mode interaction---to explain these phenomena, and the finding of this origin may further extend the limit where metallicity and polar distortion coexist.

Some features of the day-to-day MLT wind variability in winter 2017–2018 as seen with a European/Siberian meteor radar network

We present results of wind measurements near the mesopause carried out with meteor radars (MRs) at Collm (51°N, 13°E), Obninsk (55°N, 37°E), Kazan (56°N, 49°E), Angarsk (52°N, 104°E) and Anadyr (65°N, 178°E) from October 1, 2017 till March 31, 2018. The Collm and Kazan MRs are SKiYMET radars with vertical transmission and radio echo height finding, while the other radars operate with horizontal transmission and without height finding. We paid particular attention to the meridional wind variability with periods of 4–6 days and 9–11 days. The waves with these periods are seen as spots of the wave activity in the wavelet spectra and include oscillations with different periods and different discrete zonal wavenumbers. These wave packets successively propagate as a group of waves from one site to another one in such a way that they are observed at one site and almost disappear at the previous one. The 4–6 wave group includes planetary-scale oscillations (individual spectral components) which have eastward phase velocities and mostly zonal wavenumbers 2 and 3, and the vertical wavelength exceeds 70 km at middle latitudes. The source of the oscillations is the polar jet instability. The wave group itself propagates westward, and the amplitudes of wind oscillations are approximately 5–6 m/s as obtained from the wind data averaged over the meteor zone. The 9–11 day wave set propagates westward as a group and mainly consists of spectral components which have westward phase velocity and zonal wavenumber 1. Amplitudes of these wind perturbations strongly vary from station to station and can reach, approximately, 8 m/s. The vertical wavenumber is 0.014 km−1 as taken from the Kazan and 0.05 km−1 according to the Collm data. We obtained a global view on the waves by using the AURA MLS geopotential data. We found a good correspondence between wave features obtained from the MR wind measurements and the MLS data. To our knowledge, such a wave propagation of planetary wave in the mesosphere/lower thermosphere (MLT) region has so far not obtained much attention.

Strain-engineered interaction of quantum polar and superconducting phases

The pairing mechanism of unconventional superconductivity in strontium titanate is hotly debated. Here, using a multisensor experimental apparatus with a mechanical strain cell, an optical microscope, and with transport and magnetic probes all contained in a closed-cycle dilution refrigerator, we determined that the superconducting transition temperature of strontium titanate increases dramatically even for very small strains induced by application of uniaxial tension. These results imply that superconductivity is controlled by very small atomic shifts; the only strain-sensitive pairing channel candidate is the one linked to quantum ferroelectric (polar) instability. This investigation, therefore, uncovers additional constraints on the debated theories of superconductivity in this low carrier concentration material near the ferroelectric quantum phase transition.

Quantum paraelectricity to dipolar glass transition in Sc doped BaFe12O19 single crystals

We report a quantum paraelectric (QP) to dipolar glass (DG) phase transition induced by Sc substitution in ferrimagnetic hexaferrite BaFe12−xScxO19 (0≤x≤2.4) single crystals. The DG behavior is revealed via anisotropic dielectric spectroscopy, polarization, and specific heat measurements. The phase evolution from QP to DG (0.5≤x≤1.2) and finally to the nonpolar state (x≥ 2.4) with increasing x is found to be due to the competition between the enhanced polar instability by lattice volume expansion and disordered/frustrated dipole-dipole interaction induced by the partial replacement of Sc3+ for Fe3+ in the bipyramids. Our results suggest further experimental clues to realize possible multiferroicity with strong displacive ferroelectricity in BaFe12O19 and other magnetic materials.

Observation of oxygen pyramid tilting induced polarization rotation in strained BiFeO3 thin film

AbstractOxygen octahedral tilting has been recognized to strongly interact with spin, charge, orbital, and lattice degrees of freedom in perovskite oxides. Here, we observe a strain‐driven stripe‐like morphology of two supertetragonal (monoclinic Cc and Cm) phases in the strained BiFeO3/LaAlO3 thin films. The two supertetragonal phases have a similar giant axial ratio but differences in oxygen pyramid tilting mode. Especially, the competition between polar instability and oxygen pyramid tilting is identified using atomically resolved scanning transmission electron microscopy, leading to the polarization rotation across the phase boundary. In addition, microtwins are observed in the Cc phase. Our findings provide new insights of the coupling between ferroelectric polarization and oxygen pyramid tilting in oxide thin films and will help to design novel phase morphology with desirable ferroelectric polarization and properties for new applications in perovskite oxides.

Giant enhancement of cryogenic thermopower by polar structural instability in the pressurized semimetal MoTe2

We found that a high mobility semimetal 1T'-MoTe2 shows a significant pressure-dependent change in the cryogenic thermopower in the vicinity of the critical pressure, where the polar structural transition disappears. With the application of a high pressure of 0.75 GPa, while the resistivity becomes as low as 10 {\mu}{\Omega}cm, thermopower reached the maximum value of 60 {\mu}VK-1 at 25 K, leading to a giant thermoelectric power factor of 300 {\mu}WK-2cm-1. Based on semiquantitative analyses, the origin of this behavior is discussed in terms of inelastic electron-phonon scattering enhanced by the softening of zone center phonon modes associated with the polar structural instability.