Ferroelectric Distortion Research Articles

The origin of ferroelectricity in HfO2 from orbital hybridization and covalency

Fluorite-structured HfO2 ferroelectrics exhibit remarkable ferroelectricity owing to the robust thickness scalability, rendering them promising for next-generation ferroelectric memories. Unlike the well-understood perovskite structured ferroelectrics, such as Pb(Zr,Ti)O3, the origin of ferroelectricity in HfO2 remains elusive, which impedes the experimental fabrication of pure and stable ferroelectric orthorhombic phase in films. This study seeks to elucidate its origin by analyzing the covalent nature of local chemical bonding and changes in orbital hybridization and by catching the typical feather of the half-unit-cell spacer/polar layer in the orthorhombic phase. Notably, we find that the differences in the hybridization intensity of the 2s orbitals of two types of O atoms (OI and OII) and 5d orbitals of Hf atoms play a crucial role in inducing ferroelectric distortion. Furthermore, we demonstrate that the intrinsic mechanisms of stress in enhancing the ferroelectricity of HfO2 originate from the modulation of orbital hybridization intensity of the Hf–O bonds. These insights provide a vital theoretical foundation for further exploration of ferroelectric phase transitions and property modulation in HfO2 and similar fluorite-structured ferroelectrics.

Chiral multiferroicity in two-dimensional hybrid organic-inorganic perovskites.

Chiral multiferroics offer remarkable capabilities for controlling quantum devices at multiple levels. However, these materials are rare due to the competing requirements of long-range orders and strict symmetry constraints. In this study, we present experimental evidence that the coexistence of ferroelectric, magnetic orders, and crystallographic chirality is achievable in hybrid organic-inorganic perovskites [(R/S)-β-methylphenethylamine]2CuCl4. By employing Landau symmetry mode analysis, we investigate the interplay between chirality and ferroic orders and propose a novel mechanism for chirality transfer in hybrid systems. This mechanism involves the coupling of non-chiral distortions, characterized by defining a pseudo-scalar quantity, ( represents the ferroelectric displacement vector and denotes the ferro-rotational vector), which distinguishes between (R)- and (S)-chirality based on its sign. Moreover, the reversal of this descriptor's sign can be associated with coordinated transitions in ferroelectric distortions, Jahn-Teller antiferro-distortions, and Dzyaloshinskii-Moriya vectors, indicating the mediating role of crystallographic chirality in magnetoelectric correlations.

First‐Principles Investigations of the Structural Phases of Low‐Spin State of BiCoO3

Herein, from first‐principles calculations, the phase diagram of the low‐spin (LS) state of BiCoO3 is scrutinized. The phonon‐dispersion curves of the cubic phase are analyzed to identify all the possible unstable modes and assessed the energy gain of the resulted distorted phases. In the findings, the presence of similar phases is revealed in both LS and high‐spin (HS) states, including ferroelectric and octahedra rotations distortions. However, the relative energy ordering of these phases differs significantly between the two states. Notably, the energy gain from mode condensation is considerably less pronounced in the LS case compared to HS state. Furthermore, it is identified that the common Pnma phase is the ground state of the LS state of BiCoC3, closely followed by the Imma octahedra rotation phase and the R3c ferroelectric phase, hence, different from the P4mm ferroelectric ground state of the HS state.

First-principles study of the structural, mechanical, dynamical, and transport properties of Cs2NaInX6[X = Br,I] for thermoelectric applications

Thermoelectricity generation techniques utilized for conversion of waste heat into electric power are currently gaining prominence. It is necessary that for improved efficiency, the materials used for such applications should have a high thermoelectric figure of merit (ZT) and appropriate stability. In this study, the properties of Cs2NaInX6 double perovskites (DPs) were examined using a first-principles approach to ascertain their suitability as thermoelectric materials. The findings revealed that the materials are mechanically stable according to the Born stability criteria for cubic crystals. They were also observed to be anisotropic given isotropic values of 0.702 and 0.662 for Cs2NaInBr6 and Cs2NaInI6 DPs respectively. Phonon calculations indicate soft modes at the Brillouin zone boundary and center (Γ) for Cs2NaInI6 DP, which suggests ferroelectric and anti-ferroelectric distortions. Similarly for Cs2NaInBr6 DP, the soft modes were exclusively found at the BZ center (Γ), suggesting ferroelectric distortion. The transport coefficient properties calculated are used for the determination of the ZT values. The calculated values of ZT at ambient temperature are of the order 1.0 for all the DPs which is high enough for thermoelectric applications suggesting that they may be potential thermoelectric materials

Structural insights into electric field induced polarization and strain responses inK0.5Na0.5NbO3modified morphotropic phase boundary compositions ofNa0.5Bi0.5TiO3-based lead-free piezoelectrics

${\mathrm{K}}_{0.5}{\mathrm{Na}}_{0.5}\mathrm{Nb}{\mathrm{O}}_{3}$ (KNN)-modified morphotropic phase boundary (MPB) compositions of the two ${\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3}$-based lead-free piezoelectrics, namely, $0.94{\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3}\text{\ensuremath{-}}0.06\mathrm{BaTi}{\mathrm{O}}_{3}$ (NBT-6BT) and $0.80{\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3}\text{\ensuremath{-}}0.20{\mathrm{K}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3}$ (NBT-20KBT) are model systems exhibiting large $(>0.4%)$ electric-field-driven strain. There is a general perception that (i) increasing KNN concentration monotonically weakens the direct piezoelectric response $({d}_{33})$, and (ii) maximum electrostrain occurs when KNN pushes the system in the fully ergodic relaxor state. We have examined these issues using various complementary techniques involving electrostrain, piezoelectric coefficient $({d}_{33})$, ferroelectric switching-current measurements, and field-driven structural studies on the global and local scales using laboratory and synchrotron x-ray diffraction, neutron powder diffraction, and ${\mathrm{Eu}}^{+3}$ photoluminescence techniques. Our investigations revealed the following important features: (i) In the low-concentration regime, KNN induces a tetragonal ferroelectric distortion, which improves the weak signal piezoresponse. (ii) Beyond a threshold concentration, in-phase octahedral tilt sets in and weakens the long-range ferroelectric order to partially stabilize an ergodic state. (iii) The maximum electrostrain (\ensuremath{\sim}0.6%) is achieved in the mixed (nonergodic + ergodic) state. (iv) The mixed state invariably exhibits a less-known phenomenon of field-driven ferroelectric-to-relaxor transformation during bipolar field cycling. (v) The enhanced electrostrain in the mixed state is associated with the electric field increasing the correlation lengths of the short-ranged tetragonal and rhombohedral ferroelectric regions without overall transformation of one phase to the other. We summarize the findings of this work in a comprehensive electric field composition (E-x) phase diagram. The findings reported here are likely to be true for other NBT-based MPB systems.

Contrasting the Roles of Cu Interstitials and Sb Substitutions in Regulating Ferroelectric Distortions and Thermoelectric Properties of α-GeTe

Contrasting the Roles of Cu Interstitials and Sb Substitutions in Regulating Ferroelectric Distortions and Thermoelectric Properties of α-GeTe

In search of Pca21 phase ferroelectrics

In recent years, hafnia-based ferroelectrics have attracted enormous attention due to their capability of maintaining ferroelectricity below 10 nm thickness and excellent compatibility with microelectronics flow lines. However, the physical origin of their ferroelectricity is still not fully clear, although it is commonly attributed to a polar Pca21 orthorhombic phase. The high-temperature paraelectric phases (the tetragonal phase or the cubic phase) do not possess a soft mode at the Brillouin zone center, thus the ferroelectric distortion has to be explained in terms of trilinear coupling among three phonon modes in the tetragonal phase. It is necessary to explore new materials with possible ferroelectricity due to the polar Pca21 phase, which in turn should be very helpful in evaluating the microscopic theory for ferroelectric hafnia. In this work, based on the idea of the Materials Genome Engineering, a series of hafnia-like ferroelectrics have been found, exemplified by LaSeCl, LaSeBr, LuOF and YOF, which possess adequate spontaneous polarization values and also relatively favorable free energies for the polar phase. Their common features and individual differences are discussed in detail. In particular, a promising potential ferroelectric material, Pca21 phase LuOF, is predicted and recommended for further experimental synthesis and investigation.

Intrinsic ferroelectrics and carrier doping-induced metallic multiferroics in an atomic wire

Low-dimensional multiferroic metals characterized by the simultaneous coexistence of ferroelectricity, conductivity, and magnetism hold tremendous potential for scientific and technological endeavors. However, the mutually exclusive mechanisms among these properties impede the discovery of multifunctional conducting multiferroics, especially at the atomic-scale. Here, based on first-principles calculations, we design and demonstrate intrinsic one-dimensional (1D) ferroelectrics and carrier doping-induced metallic multiferroics in an atomic WOF4 wire. The WOF4 atomic wire that can be derived from a 1D van der Waals crystal exhibits pronounced ferroelectricity manifested in the form of large cooperative atomic displacements. By performing Monte Carlo simulations with an effective Hamiltonian method, we obtain the nanowire that can sustain a high Curie temperature, indicating its potential for room-temperature applications. Moreover, doping with electrons is found to induce magnetism and metallic conductivity that coexists with the ferroelectric distortion in the nanowire. These appealing properties in conjunction with the experimental feasibility enable the doped WOF4 nanowire to act as a promising atomic-scale multifunctional material.

Engineering Thermoelectric Performance of α‐GeTe by Ferroelectric Distortion

The rhombohedral α‐GeTe can be approximated as a slightly distorted rock‐salt structure along its [1 1 1] direction and possesses superb thermoelectric performance. However, the role of such a ferroelectric‐like structural distortion on its transport properties remains unclear. Herein, we performed a systematic study on the crystal structure and electronic band structure evolutions of Ge1‐xSnxTe alloys where the degree of ferroelectric distortion is continuously tuned. It is revealed that the band gap is maximized while multiple valence bands are converged at x = 0.6, where the ferroelectric distortion is the least but still works. Once undistorted, the band gap is considerably reduced, and the valence bands are largely separated again. Moreover, near the ferro‐to‐paraelectric phase transition Curie temperature, the lattice thermal conductivity reaches its minima because of significant lattice softening enabled by ferroelectric instability. We predict a peak ZT value of 2.6 at 673 K in α‐GeTe by use of proper dopants which are powerful in suppressing the excess hole concentrations but meanwhile exert little influence on the ferroelectric distortion.

Investigation of local distortion effects on X-ray absorption of ferroelectric perovskites from first principles simulations.

Understanding the role of ferroelectric polarization in modulating the electronic and structural properties of crystals is critical for advancing these materials for overcoming various technological and scientific challenges. However, due to difficulties in performing experimental methods with the required resolution, or in interpreting the results of methods therein, the nanoscale morphology and response of these surfaces to external electric fields has not been properly elaborated. In this work we investigate the effect of ferroelectric polarization and local distortions in a BaTiO3 perovskite, using two widely used computational approaches which treat the many-body nature of X-ray excitations using different philosophies, namely the many-body, delta-self-consistent-field determinant (mb-ΔSCF) and the Bethe-Salpeter equation (BSE) approaches. We show that in agreement with our experiments, both approaches consistently predict higher excitations of the main peak in the O-K edge for the surface with upward polarization. However, the mb-ΔSCF approach mostly fails to capture the L2,3 separations at the Ti-L edge, due to the absence of spin-orbit coupling in Kohn-Sham density functional theory (KS-DFT) at the generalized gradient approximation level. On the other hand, and most promising, we show that application of the GW/BSE approach successfully reproduces the experimental XAS, both the relative peak intensities as well as the L2,3 separations at the Ti-L edges upon ferroelectric switching. Thus simulated XAS is shown to be a powerful method for capturing the nanoscale structure of complex materials, and we underscore the need for many-body perturbation approaches, with explicit consideration of core-hole and multiplet effects, for capturing the essential physics in these systems.

Ferroelectricity in HfO2 from a Coordination Number Perspective

Ferroelectricity observed in thin-film HfO2, either doped with Si, Al, and so forth or in the Hf0.5Zr0.5O2 form, has gained great technical significance. While a trilinear coupling between phonon modes could explain its ferroelectric distortion, from a practical perspective, one may be concerned with a theory that is more straightforward to predict similar ferroelectric candidates through some apparent features of HfO2 and ZrO2. In this work, we propose that the 7 cation coordination number of HfO2/ZrO2 lies at the heart of this ferroelectricity, which stems from the proper ionic radii of Hf/Zr compared with O. Among the numerous compounds with a non-centrosymmetric nature, for example, the mm2 point group, HfO2 and ZrO2 are special in that they are close to the border of 7 and 8 cation coordination, such that the 8-coordination tetragonal intermediate phase could greatly reduce the switching barrier. Other 7-coordination candidates, including SrI2, TaON, YSBr, and YOF, are also studied in comparison to HfO2/ZrO2, and six switching paths are analyzed in detail for the Pca21 phase. A rule of the preferred switching path in terms of the ionic radii ratio and coordination number has been established. We also show the possible route from the ferroelectric Pca21 phase to the monoclinic P21/c phase in HfO2, which is relevant to the fatigue phenomenon.

Ferroelectric and antiferroelectric distortions coupling of nitride perovskite LaWN3 under epitaxial strain using first-principles calculations

LaWN3, a novel nitride perovskite, have been synthesized and its piezoelectric properties have been investigated (Talley K. R. et al., Science, 374 (2021) 1488). However, the understanding of its ferroelectric properties under the external strain is still lacking. Here, the misfit-strain–dependent coupling between antiferrodistortions (AFD) and ferroelectric (FE) distortions has been studied by using a first-principle approach. It can be observed that AFD and FE distortions can work cooperatively with each other as the compressive strain increases, and the coupling energy between them is found to work in different ways under various strains. Our results show that the coupling tends to stabilize the ground structure when the compressive strain is smaller than −1.9%, it works oppositely when the compressive strain becomes larger than 1.9%. Our results can provide us with more hints about the influence of the epitaxial strains on the intrinsic coupling behavior in the perovskite ferroelectric compounds, which is very important for us to design and fabricate new kinds of functional materials.

Synthesis, structural and dielectric properties of Ca3Mn2O7 thin films prepared by pulsed laser deposition

Multiferroic Ruddlesden-Popper Ca3Mn2O7 thin films were prepared by laser ablation on SrTiO3 substrates. For low laser fluences and high oxygen pressures, the films assumed the CaMnO3 Pnma orthorhombic structure. However, by increasing fluence and decreasing the oxygen pressure, the Ca3Mn2O7 phase is achieved through a post-deposition annealing treatment. Furthermore, the ferroelectric phase A21am was observed in the films, along with the orthorhombic Acaa phase, with a preferential [111] growth orientation and substrate-induced enhancement of the polar ferroelectric distortion. The dielectric permittivity shows dispersion described by the Havriliak-Negami function, and from fits to the curves, a different behavior was observed in the antiferromagnetic region. Also, the Kohlrausch-Williams-Watts stretched exponential parameter showed an abrupt decrease below ∼110 K, near the Néel temperature. This indicates the presence of magnetoelectric interactions and magnetically induced enhancement of dipolar correlations in the samples, favoring substrate-induced strain to enhance multiferroicity in these films.

Finite-size effects on ferroelectricity in rhombohedral HfO2

In this work we analyze the finite-size effects on the structural properties and on the polarization of the rhombohedral phase of HfO$_2$ subjected to a biaxial compressive strain. We show how the presence of surface charges affects the polarization, leading to a strong reduction with respect to its bulk value. This reduction can be ascribed to two mechanisms: the coupling between compressive strain and the phase-transition order parameter; the changes in the ferroelectric distortion. We give two alternative explanations of this phenomenon: from an atomistic point of view, analyzing the evolution of the bond lengths, and from a symmetry-analysis point of view, considering the changes in the amplitude of the symmetry-allowed distortions, when a slab configuration is considered. These results are independent on the slab-thickness in the considered range, suggesting the absence of a critical thickness for ferroelectricity in HfO$_2$, in agreement with the proposed improper nature of hafnia ferroelectricity.

Surface chemistry on a polarizable surface: Coupling of CO with KTaO3(001).

Polarizable materials attract attention in catalysis because they have a free parameter for tuning chemical reactivity. Their surfaces entangle the dielectric polarization with surface polarity, excess charge, and orbital hybridization. How this affects individual adsorbed molecules is shown for the incipient ferroelectric perovskite KTaO3. This intrinsically polar material cleaves along (001) into KO- and TaO2-terminated surface domains. At TaO2 terraces, the polarity-compensating excess electrons form a two-dimensional electron gas and can also localize by coupling to ferroelectric distortions. TaO2 terraces host two distinct types of CO molecules, adsorbed at equivalent lattice sites but charged differently as seen in atomic force microscopy/scanning tunneling microscopy. Temperature-programmed desorption shows substantially stronger binding of the charged CO; in density functional theory calculations, the excess charge favors a bipolaronic configuration coupled to the CO. These results pinpoint how adsorption states couple to ferroelectric polarization.

Competing electronic states emerging on polar surfaces

Excess charge on polar surfaces of ionic compounds is commonly described by the two-dimensional electron gas (2DEG) model, a homogeneous distribution of charge, spatially-confined in a few atomic layers. Here, by combining scanning probe microscopy with density functional theory calculations, we show that excess charge on the polar TaO2 termination of KTaO3(001) forms more complex electronic states with different degrees of spatial and electronic localization: charge density waves (CDW) coexist with strongly-localized electron polarons and bipolarons. These surface electronic reconstructions, originating from the combined action of electron-lattice interaction and electronic correlation, are energetically more favorable than the 2DEG solution. They exhibit distinct spectroscopy signals and impact on the surface properties, as manifested by a local suppression of ferroelectric distortions.

Mechanism for switchability in electron-doped ferroelectric interfaces

With the recent experimental verification that ferroelectric lattice distortions survive in the metallic phase of some materials, there is a desire to create devices that are both switchable and take advantage of the novel functionalities afforded by polar interfaces. In this work, we explore a simple model for such an interface and demonstrate a mechanism by which a metallic ferroelectric substrate may be switched by a bias voltage. This finding is in contrast to the reasonable expectation that hysteresis is prevented by screening of external fields in ferroelectric metals. Instead, the electron gas binds to polarization gradients to form a compensated state. Uncompensated electrons, which may screen external fields, are generated either when the electron density exceeds the ferroelectric polarization or when the bias voltage exceeds a "spillover" threshold. We propose that switchable thin films may be optimized by choosing an electron density that is slightly less than the lattice polarization. In addition to the high-polarization states, we find that thin metallic films also have a low-polarization state with average polarization near zero. Unlike in insulating films, where the polarization is small everywhere in this state, the low-polarization state in the metallic films consists of two head-to-head domains of opposite polarization. This domain formation is enabled by the screening of depolarizing fields by the electron gas.

Structural, magnetic and ferroelectric properties of VOBr<sub>2 </sub>monolayer: A first-principles study

On the basis of first-principles calculations, the structure, magnetism and ferroelectricity of VOBr<sub>2</sub> monolayer are studied systematically in the present work. The calculation results indicate that a spontaneous ferroelectric distortion takes place at low temperature, causing the structure of VOBr<sub>2</sub> to transform from a centrosymmetric paraelectric phase to a ferroelectric one. In contrast with its sister compound VOI<sub>2</sub>, the dimerization of V is unstable in VOBr<sub>2</sub> and may quench the local magnetic moment on V ions. Additionally, the easy magnetization axis of VOBr<sub>2</sub> monolayer is in-plane along the <i>a</i>-axis, and the magnetic coupling between adjacent local moments is antiferromagnetic both along the <i>a</i>-axis and along the <i>b</i>-axis. Moreover, the ferroelectric displacement of V ions occurs in the <i>a</i>-axis, along the V—O—V chains direction, resulting in a polarization of about 40 μC/cm<sup>2</sup>. Comparing with the ferro-to-paraelectric reversal pathway, the energy barrier can be effectively reduced for ferroelectric switching on partial or individual V—O—V chains. It is reasonable to believe that the dipole moment flipping on specific chain can be achieved through a moderate external field, thereby providing new direction for designing the low-energy-consumption and high-density ferroelectric memory device.

The Local Structure of the BiS2 Layer in RE(O,F)BiS2 Determined by In-Plane Polarized X-ray Absorption Measurements

We have investigated the local structure of BiS2-based layered materials by Bi L3-edge extended X-ray absorption fine structure (EXAFS) measurements performed on single crystal samples with polarization of the X-ray beam parallel to the BiS2 plane. The results confirm highly instable nature of BiS2 layer, characterized by ferroelectric like distortions. The distortion amplitude, determined by the separation between the two in-plane (Bi-S1) bonds, is found to be highest in LaO0.77F0.23BiS2 with ΔR∼0.26 Å and lowest in NdO0.71F0.29BiS2 with ΔR∼0.13 Å. Among the systems with intrinsic doping, CeOBiS2 shows smaller distortion (ΔR∼0.15 Å) than PrOBiS2 (ΔR∼0.18 Å) while the highest distortion appears for EuFBiS2 revealing ΔR∼0.22 Å. It appears that the distortion amplitude is controlled by the nature of the RE(O,F) spacer layer in the RE(O,F)BiS2 structure. The X-ray absorption near edge structure (XANES) spectra, probing the local geometry, shows a spectral weight transfer that evolves systematically with the distortion amplitude in the BiS2-layer. The results provide a quantitative measurements of the local distortions in the instable BiS2-layer with direct implication on the physical properties of these materials.

Electric field-induced photoluminescence quenching in Pr-doped BNT ceramics across the MPB region

Piezophotonics is a great interesting field of physics that has led to a number of important technologies, such as light source, smart sensors, and mechatronics. In this work, we reported Pr-doped (Bi0·5Na0.5)TiO3-based lead-free ceramics with strong red photoluminescence emission and large strain response (d33∗ = 460 pm/V, S = 0.32 %). The PL emission can be quenched by decreasing the intensity by 93 % after electrical polarization (E = 50 kV/cm). The local structure and electric field-induced structural changes were systematically investigated to reveal the significant distinction in photoluminescence properties caused by electrical polarization. The results indicated that polarization treatment eliminates the structural inhomogeneities and establishes a long-range ferroelectric tetragonal and rhombohedral distortion. The crystal structure transformed irreversibly from a non-ergodic to a normal ferroelectric state. PL quenching originated from the decreased distortion of octahedral due to the transition from a non-ergodic state to a highly ordered symmetrical structure. Meanwhile, the enlarged domain structure contributed to the photoluminescence quenching effect. Our findings demonstrate that an electric field can be a robust tool for adjusting the photoluminescence property and provide insights into the relationship between the structure and PL properties of BNT-based ceramics under an external stimulus.