Low-temperature Ferroelectric Phase Research Articles

Significantly enhanced energy-storage properties in NaNbO3-based relaxor ferroelectric ceramics via introducing a wide band-gap linear dielectric

The achievement of simultaneous high energy-storage density and efficiency is a long-standing challenge for dielectric ceramics. Herein, a wide band-gap lead-free ceramic of NaNbO3–BaZrO3 featuring polar nanoregions with a rhombohedral local symmetry, as evidenced by piezoresponse force microscopy and transmission electron microscopy, were reported to exhibit a giant energy-storage density of ∼5.0 J/cm3 and a high efficiency of ∼85 % as well as excellent charge-discharge properties simultaneously. Enhanced breakdown strength from a synergistic effect of wider band gap and decreased grain size provides a prerequisite for improving energy-storage performance. Meanwhile, the substitution of linear dielectric BaZrO3 can not only adjust the low-temperature ferroelectric rhombohedral phase of NaNbO3 into near room temperature, but also significantly reduce the FE domain size owing to the increased local random fields. Therefore, suppressed polarization hysteresis and delayed polarization saturation can be observed with increasing BaZrO3 content, leading to the significantly enhanced overall energy-storage properties.

Temperature-driven structural phase transitions in 0.05(Na0.50Bi0.50)TiO3-0.95NaNbO3 solid solution via amplitude mode analysis

Structural phase transitions in the ABO3-type lead-free eco-friendly piezoelectric (05NBT) have been studied as a function of temperature using dielectric and X-ray diffraction techniques. Below 300 K, we observed dispersion in dielectric permittivity with the frequency, which confirmed relaxor behaviour in 05NBT. Using detailed analysis of temperature-dependent powder X-ray diffraction data of 05NBT, four phase transitions were identified across 600 K, 773 K, 823 K and 923 K. We found that on doping 5% NBT in pure sodium niobate, the low-temperature rhombohedral ferroelectric (N) phase and the high-temperature (S) phase of the sodium niobate get suppressed. To get a better insight about the temperature-driven structural phase transitions, various antiferrodistortive and additional distortive mode as well as the mode amplitude relative to high-symmetry cubic phase were computed and results are discussed.

Stochastic theory of ferroelectric domain structure formation dominated by quenched disorder

A self-consistent stochastic model of domain structure formation in a uniaxial ferroelectric, quenched from a high-temperature paraelectric phase to a low-temperature ferroelectric phase, is developed with an account of the applied electric field and the feedback effect via local depolarization fields. Both polarization and field components are considered as Gauss random variables. A system of integro-differential equations for correlation functions of all involved variables is derived and solved analytically and numerically. Phase diagram in terms of the average value and dispersion of polarization reveals different possible equilibrium states and available final single-domain and multi-domain states. The time-dependent evolution of the average polarization and dispersion discloses a bifurcation behavior and the temperature-dependent value of the electric field, deciding between the single-domain and multi-domain final states, which can be interpreted as the coercive field. Analytical and numerical results for the time-dependent correlation length and correlation functions exhibit plausible agreement with available experimental data.

Phase transitions study of the liquid crystal DIO with a ferroelectric nematic, a nematic, and an intermediate phase and of mixtures with the ferroelectric nematic compound RM734 by adiabatic scanning calorimetry.

High-resolution calorimetry has played a significant role in providing detailed information on phase transitions in liquid crystals. In particular, adiabatic scanning calorimetry (ASC), capable of providing simultaneous information on the temperature dependence of the specific enthalpy h(T) and on the specific heat capacity c_{p}(T), has proven to be an important tool to determine the order of transitions and render high-resolution information on pretransitional thermal behavior. Here we report on ASC results on the compound 2,3',4',5'-tetrafluoro[1,1'-biphenyl]-4-yl 2,6-difluoro-4-(5-propyl-1,3-dioxan-2-yl) benzoate (DIO) and on mixtures with 4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (RM734). Both compounds exhibit a low-temperature ferroelectric nematic phase (N_{F}) and a high-temperature paraelectric nematic phase (N). However, in DIO these two phases are separated by an intermediate phase (N_{x}). From the detailed data of h(T) and c_{p}(T), we found that the intermediate phase was present in all the mixtures over the complete composition range, albeit with strongly decreasing temperature width for that phase with decreasing mole fraction of DIO (x_{DIO}). The x_{DIO} dependence on the transition temperatures for both transitions could be well described by a quadratic function. Both these transitions were weakly first order. The true latent heat of the N_{x}-N transition of DIO was as low as L=0.0075±0.0005J/g and L=0.23±0.03J/g for the N_{F}-N_{x} transition, which is about twice the previously reported value of 0.115 J/g for the N_{F}-N transition in RM734. In the mixtures both transition latent heats decrease gradually with decreasing x_{DIO}. At all the N_{x}-N transitions pretransition fluctuation effects are absent and these transitions are purely but very weakly first order. As in RM734 the transition from the N_{F} to the higher-temperature phase exhibits substantial pretransitional behavior, in particular, in the high-temperature phase. Power-law analysis of c_{p}(T) resulted in an effective critical exponent α=0.88±0.1 for DIO and this value decreased in the mixtures with decreasing x_{DIO} toward α=0.50±0.05 reported for RM734. Ideal mixture analysis of the phase diagram was consistent with ideal mixture behavior provided the total transition enthalpy change was used in the analysis.

Single E -type collinear spin state in orthorhombic YMnO3

We studied the ferroelectricity and magnetic ordering of multiferroic orthorhombic perovskite ${\mathrm{YMnO}}_{3}$ using magnetic and dielectric measurements and polarized/unpolarized neutron diffraction experiments with a single crystal. The ferroelectric polarization of a relatively large value $P\ensuremath{\simeq}0.55\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{cm}}^{2}$ was observed below $T=29\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The magnetic $\mathbit{k}$ vector was identified as $\mathbit{k}=(0,0.487,0)$ in the low-temperature ferroelectric phase. Based on the spherical neutron polarimetry experiment, the magnetic structure was determined to be of collinear $E$-type ordering with spins along the $b$ axis. The $E$-type state did not coexist with other states in the low-temperature ferroelectric phase. A simple domain structure model indicated that a single collinear $E$-type structure with a small incommensurability can be realized as the magnetic ground state of ${\mathrm{YMnO}}_{3}$.

Observation of room-temperature ferroelectricity in spark-plasma sintered GdCrO3

A spark-plasma sintered ${\mathrm{GdCrO}}_{3}$ (SPS-GCO) is found to stabilize in its ferroelectric phase beyond room temperature. The intrinsic nature of this room-temperature ferroelectricity is established using ferroelectric positive-up--negative-down measurements and supported through piezoresponce force microscopy measurements. The SPS-GCO undergoes antiferromagnetic ordering at much lower temperatures, only below $\ensuremath{\approx}170\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Thus, any role of magnetism to the observed room temperature ferroelectricity in SPS-GCO can be ruled out. This is contrast to the concomitant antiferromagnetic and ferroelectric ordering observed below $\ensuremath{\approx}170\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ in ${\mathrm{GdCrO}}_{3}$ (GCO) (prepared using standard solid-state synthesis technique). Using detailed Rietveld refinements of room-temperature x-ray diffraction patterns, SPS-GCO is found to stabilize in the noncentrosymmetric orthorhombic $Pna{2}_{1}$ space group (the reported low-temperature ferroelectric phase in GCO), while GCO stabilizes in the centrosymmetric $Pbnm$ space group at room temperature. Using first-principles calculations, we investigated the relative energies among various possible structures of ${\mathrm{GdCrO}}_{3}$ and found that the orthorhombic $Pna{2}_{1}$ and $Pbnm$ space groups are the most stable structures. The ferroelectric $Pna{2}_{1}$ phase of SPS-GCO (stabilized at room temperature using the high-pressure and high-temperature spark-plasma sintering process) undergoes transition to the paraelectric centrosymmetric phase upon heating beyond $\ensuremath{\approx}450\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ (as confirmed using dielectric and calorimetric measurements), which on subsequent cooling to room temperature does not undergo a transition back to the ferroelectric phase and remains in the centrosymmetric $Pbnm$ phase.

Structure of ferroelectric low-temperature phase of yttrium manganate YMn2O5 revisited

Structure of ferroelectric low-temperature phase of yttrium manganate YMn₂O₅ revisited

Role of A and B site doping on antiferrodistortive phase transition in sodium bismuth titanate

We report the results of temperature-dependent powder X-ray diffraction studies on lead-free piezoelectric sodium bismuth titanate (NBT) modified by partially substituting Ba at the A site or Nb at the B site. We observed the disappearance of certain reflections and a change in the main perovskite peaks of the rhombohedral phase suggesting a structural phase transition. Detailed Rietveld refinements revealed that both the ferroelectric and the antiferrodistortive phase transitions occur together at high temperature leading to a change in crystal structure from the low-temperature ferroelectric rhombohedral phase (space group: R3c) to the high-temperature paraelectric cubic phase (space group:Pm3¯m). This suggests the suppression of the intermediate tetragonal phase of pure NBT. We found that substituting Ba onto the A site enhances the lattice strain and lowers the antiferrodistortive phase transition temperature.

Soft mode theory of ferroelectric phase transitions in the low-temperature phase

Historically, the soft mode theory of ferroelectric phase transitions has been developed for the high-temperature (paraelectric) phase, where the phonon mode softens upon decreasing the temperature. In the low-temperature ferroelectric phase, a similar phonon softening occurs, also leading to a bosonic condensation of the frozen-in mode at the transition, but in this case the phonon softening occurs upon increasing the temperature. Here we present a soft mode theory of ferroelectric and displacive phase transitions by describing what happens in the low-temperature phase in terms of phonon softening and instability. A new derivation of the generalized Lyddane–Sachs–Teller (LST) relation for materials with strong anharmonic phonon damping is also presented which leads to the expression ɛ 0/ɛ ∞ = |ω LO|2/|ω TO|2. The theory provides a microscopic expression for T c as a function of physical parameters, including the mode specific Grüneisen parameter. The theory also shows that , and again specifies the prefactors in terms of Grüneisen parameter and fundamental physical constants. Using the generalized LST relation, the softening of the TO mode leads to the divergence of ϵ 0 and to a polarization catastrophe at T c. A quantitative microscopic form of the Curie–Weiss law is derived with prefactors that depend on microscopic physical parameters.

A combinatory ferroelectric compound bridging simple ABO3 and A-site-ordered quadruple perovskite

The simple ABO3 and A-site-ordered AA′3B4O12 perovskites represent two types of classical perovskite functional materials. There are well-known simple perovskites with ferroelectric properties, while there is still no report of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here we report the high pressure synthesis of an A-site-ordered perovskite PbHg3Ti4O12, the only known quadruple perovskite that transforms from high-temperature centrosymmetric paraelectric phase to low-temperature non-centrosymmetric ferroelectric phase. The coordination chemistry of Hg2+ is changed from square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHg3Ti4O12. Thus PbHg3Ti4O12 appears to be a combinatory link from simple ABO3 perovskites to A-site-ordered AA′3Ti4O12 perovskites, sharing both displacive ferroelectricity with former and structure coordination with latter. This is the only example so far showing ferroelectricity due to symmetry breaking phase transition in AA′3B4O12-type A-site-ordered perovskites, and opens a direction to search for ferroelectric materials.

Lead-free molecular ferroelectric [N,N-dimethylimidazole]3Bi2I9 with narrow bandgap

Lead-based organic-inorganic hybrid perovskite materials are widely applied in solar cells due to their excellent optoelectronic properties. These materials usually appear ferroelectricity with the built-in electric field, which can be used to improve the power conversion efficiency (PCE) of solar cells. However, the toxicity of lead has severely hampered their applications. Hence, it is of utmost significance to explore novel organic-inorganic hybrid perovskite materials with non-toxicity, ferroelectricity, and narrow bandgap at room temperature to enhance the PCE for broad practical applications. Herein, we reported a lead-free hybrid molecular ferroelectric material, i.e. (C5N2H9)3Bi2I9, with zero-dimensional perovskite-like structure. (C5N2H9)3Bi2I9 undergoes first-order ferroelectric phase transition with a Curie temperature of 327 K. Moreover, the dielectric constants of (C5N2H9)3Bi2I9 exhibit a step-like anomaly varied between the high-temperature paraelectric phase and the low-temperature ferroelectric phase. Furthermore, (C5N2H9)3Bi2I9 demonstrates a narrow bandgap of 2.1 eV. Hence, the as-reported novel member of the organic-inorganic hybrid family renders great promise for optoelectronic devices.

Phase transitions in Bi4Ti3O12.

Bi4Ti3O12 is a representative of the Aurivillius family of layered perovskites. These are high-temperature ferroelectric materials with prospects for applications in random-access memory and are characterized by an extremely confused interaction of their structural degrees of freedom. Using group-theoretical methods, structural distortions in the Bi4Ti3O12 high-symmetry phase, caused by rotations of rigid octahedra and their displacements as a single unit, have been investigated, taking into account the connections between them. Within the Landau theory, a stable thermodynamic model of phase transitions with three order parameters has been constructed. It is shown that, according to the phenomenological phase diagram, the transition between the high-temperature tetragonal phase and the low-temperature ferroelectric can occur both directly and through intermediate states, including those observed experimentally. The role of improper ordered parameters and possible domain configurations in the structure formation of the low-temperature ferroelectric phase are discussed.

Topological Semimetals in the SnTe Material Class: Nodal Lines and Weyl Points.

We theoretically show that IV-VI semiconducting compounds with low-temperature rhombohedral crystal structure represent a new potential platform for topological semimetals. By means of minimal k·p models, we find that the two-step structural symmetry reduction of the high-temperature rocksalt crystal structure, comprising a rhombohedral distortion along the [111] direction followed by a relative shift of the cation and anion sublattices, gives rise to topologically protected Weyl semimetal and nodal line semimetal phases. We derive general expressions for the nodal features and apply our results to SnTe, showing explicitly how Weyl points and nodal lines emerge in this system. Experimentally, the topological semimetals could potentially be realized in the low-temperature ferroelectric phase of SnTe, GeTe, and related alloys.

Dielectric functions and phase transitions of MgO modified Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 ceramics revealed by spectroscopic ellipsometry

The dielectric functions and phase transitions of MgO modified Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 (PZTN-x wt% MgO, x = 0, 0.1, 0.2, 0.5) ceramics have been investigated using temperature-dependent spectroscopic ellipsometry. With the aid of standard critical-point (SCP) model, four typical interband transitions can be observed from the second derivative of dielectric functions. Three transition process are observed and can be linked to antiferroelectric (AFE)-ferroelectric (FE) (T1 ∼ 250 K), low temperature ferroelectric phase [FER(LT)]to high temperature one [FER(HT)] (T2 ∼ 300 K), and ferroelectric-paraelectric phase (TC ∼ 500 K).

A crystal optical study of short range polar order in the ferroelectric phase: doped incipient ferroelectrics

The temperature dependence of the principal values of the refraction of light (specific optical retardation) in Sr1-xCaxTiO3 with x = 0.014, SrTi(16O0.0318O0.97)3 and KTa1–xNbxO3 with x = 0.018 single crystals have been measured and the magnitudes and temperature variations of polar contributions to the refraction were derived. From polar contributions to the refraction on the basis of the method developed by the authors the magnitude of the root mean square polarization fluctuations characterizing the short-range polar order in the low temperature ferroelectric phase were determined in the crystals under study.

Ferroelectric domain dynamics and stability in graphene oxide-P(VDF-TrFE) multilayer films for ultra-high-density memory application

The rapid growth of miniaturized electronic devices has raised the demand for compact, flexible, wearable, and non-volatile memory units. However, integration into nanoelectronic devices requires a scaled-down data-storage component, but this often results in the deterioration of the ferroelectric switching performance. Herein, we demonstrate a simple and scalable fabrication of poly (vinylidene fluoride trifluoroethylene) [P(VDF-TrFE)] film with graphene oxide (GO) nanosheets. Using piezoresponse force microscopy (PFM), the storage features of this multilayer film were investigated, including establishment of two stable memory states, ferroelectric switching dynamics in the point-polarization and linear-polarization modes, and time and thermal stability of information storage. Remarkably, the GO-P(VDF-TrFE) film favored formation of low-temperature (LT) ferroelectric phase with much more ordered sequences of trans conformations relative to pristine P(VDF-TrFE) due to the presence of electrostatic interaction between GO nanosheets and CF dipoles of P(VDF-TrFE), thus affording improved ferroelectric properties. The GO-P(VDF-TrFE) film showed several excellent storage features, such as ultra-high density of more than 300 Gbits in−2, good writing & erasing repeatability, long data retention time, and elevated device operation temperature. In-depth understanding and utilization of the excellent non-volatile memory performance of this new GO-ferroelectric system will open new avenues for the next generation of nanoelectronic devices.

Disorder–order transitions in the perovskite metal–organic frameworks [(CH3)2NH2][M(HCOO)3] at high pressure

High-pressure single-crystal X-ray diffraction was performed on dimethylammonium metal formates (DMAMF), [(CH3)2NH2][M(HCOO)3] where M = Mn2+, Fe2+, and Cu2+, in order to compare the high-pressure phases with the known low-temperature ferroelectric (for DMAMnF) and multiferroic (for DMAFeF) phases. The ambient phases of dimethylammonium metal formates were stable up to 5.53(8), 5.7(3), and 7.3(2) GPa for DMAMnF, DMACuF, and DMAFeF, respectively. At higher pressures, phase transitions occurred that were initiated by the structural distortions of the metal formate framework. The distortions are accompanied by loss of dynamic disorder of dimethylammonium (DMA) in the DMAMnF and DMAFeF compounds, resulting in an antiferroelectric arrangement of the DMA cations. For DMACuF, the Jahn–Teller distortion is reduced upon compression up to ∼3.5 GPa; the inability to continue reducing the Cu–O bonding distances beyond this pressure could be responsible for triggering a distortive transition of the copper formate framework at 5.5 GPa. In the case of DMAFeF, the experiment was in addition conducted with a penetrating pressure-transmitting medium (PTM) that resulted in a disorder–order transition of the DMA cation at lower pressures with p = 4.0(6) GPa due to the PTM inclusion.

Magnetoelectric Effect in a Spin-State Transition System

Magnetic, dielectric, and magnetoelectric properties in a spin-state transition system are examined, motivated by the recent discovery of a multiferroic behavior in a cobalt oxide. We construct an effective model Hamiltonian based on the two-orbital Hubbard model, in which the spin-state degrees of freedom in magnetic ions couple with ferroelectric-type lattice distortions. A phase transition occurs from the high-temperature low-spin phase to the low-temperature high-spin ferroelectric phase with accompanying an increase of the spin entropy. The calculated results are consistent with the experimental pressure-temperature phase diagram. We predict the magnetic-field induced electric polarization in the low-spin paraelectric phase near the ferroelectric phase boundary.

The unique physical properties of the hydrogen bonded in dimers liquid crystals

The dimerization of aromatic carboxylic acids, is the base of the structure formation of hydrogen bonded in dimers liquid crystals (HBDLCs), that exhibit non-conventional mesomorphism. The structural units of these LCs are amphiphilic-type molecules, which after suitable functionalization, induce supramolecular complexes, nanocomposites based on HBDLCs. The liquid crystalline character of the nanocomposites strongly dependent on intermolecular hydrogen bonds between symmetric, where the H-donors and H acceptors are contained in similar and non-symmetric HBDLCs, where the H-donors and H acceptors are contained in unlike molecules. The strength and non-covalent character of the hydrogen bonds provides both sufficient HBDLCs complex stability and bonding flexibility with a possibility to design and drive the supramolecular geometry. We investigated a series of nanocomposites produced by mixture of HBDLC (p-n-alkyloxybenzoic acid - nOBA), serving as matrices, with non-mesogenic (single walled carbon nanotubes - SWCNTs, hydroxypiridine – HOPY) and mesogenic (cholesteryl benzoate - ChB) nano-particles in various shapes and sizes. A set of new chiral ferroelectric phases were found in the nanocomposites, otherwise do not appearing in the pristine achiral HBDLCs materials. A molecular model of an unique low-temperature ferroelectric smectic phase C based on the molecular dimer ring symmetry reduction (bent dimer formation) towards to the lowest triclinic one is presented for both symmetric and nonsymmetric supramolecular liquid crystal complexes.

Emergence of a fluctuating state in the stuffed tridymite-type oxides Ba1−xSrxAl2O4

We investigated the crystal structures and dielectric properties of an improper ferroelectric ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Al}}_{2}{\mathrm{O}}_{4} (x\ensuremath{\le}0.1)$ and revealed that suppressing the condensation of the $M$-point soft mode involves the emergence of a ``fluctuating'' state. In the low-Sr-concentration region of $x\ensuremath{\le}0.06$, crystals exhibit a ferroelectric phase transition at ${T}_{\mathrm{C}}$ from a paraelectric phase with a space group $P{6}_{3}22$ (PE phase) to a low-temperature ferroelectric phase with a $P{6}_{3}$ structure with doubled $a$ and $b$ axes (FE phase). Additionally, the temperature dependence of the dielectric constant ${\ensuremath{\varepsilon}}^{\ensuremath{'}}$ exhibits a peak at ${T}_{\mathrm{C}}$. As $x$ increases, ${T}_{\mathrm{C}}$ decreases substantially, and the peak at ${T}_{\mathrm{C}}$ becomes small. For $x\ensuremath{\ge}0.07$, the peak is barely noticeable and becomes an anomaly at ${T}^{*}\ensuremath{\approx}200$ K, indicating that the system possesses another state (FL state) below the ${T}^{*}$. The PE phase has been reported to possess two energetically competing soft modes at the $M$ and $K$ points. Electron diffraction (ED) experiments revealed that the superlattice reflections of the FE phase become diffuse scatterings originating from the $M$-point soft mode as the FE-FL boundary is approached. The $K$-point soft mode disappears in the FL state, whereas the $M$-point soft mode survives and fluctuates without condensation. Dark-field (DF) images revealed that the $M$-point soft mode exhibits coherent motion in nanodomains with sizes of approximately 10 nm in the FL state. The emergence of the FL state is ascribed to enhanced vibration of the ${\mathrm{AlO}}_{4}$ tetrahedra resulting from the substitution of Sr, which has a smaller ionic radius than Ba.