Antiparallel Displacements Research Articles

Microstructure and dielectric properties of high-entropy Sr0.9La0.1MeO3 (Me: Zr, Sn, Ti, Hf, Mn, Nb) perovskite ceramics

Entropy engineering has attracted extensive attention, which is applied to induce controllable changes in structures through lattice distortion effects to optimize certain properties. In this work, a series of pure phase high-entropy dielectric ceramics, Sr0.9La0.1MeO3 (Me = Zr, Sn, Ti, Hf, Mn, Nb) were synthesized at the sintering temperature of 1550 °C via a traditional solid phase method. The internal relationship between phase stability and dielectric properties with respect to the variation of mixing entropy were investigated. The distortion of oxygen octahedron (in-phase and antiphase) and antiparallel cation displacement were produced in Sr0.9La0.1(Zr0.25Sn0.25Ti0.25Hf0.25)O3 (4M) ceramic system after the introduction of four cations into B-site. Finally, 4M ceramic possesses excellent dielectric permittivity frequency stability and dielectric-temperature stability (εr/ε25°C<5% within −100 to 300 °C) with a low dielectric loss (<0.01). This work provides a valuable reference for entropy engineering to control dielectric performance and phase stability, and may facilitate the discovery and design of novel electronic ceramics.

Large piezoelectricity in NaNbO3-based lead-free ceramics via tuning oxygen octahedral tilt.

The development of high-performance lead-free piezoceramics for replacing Pb-based perovskites has attracted lots of attention, and comparable room-temperature piezoresponse has been realized in (Na,K)NbO3 and BaTiO3-based ceramics with local structure heterogeneity via adjusting polymorphic phase boundary. In this work, a new method of oxygen octahedron tilt design is used in NaNbO3-based lead-free ceramics, which usually shows complex oxygen octahedron tilt information inherited from NaNbO3. The substitution of Ba(Fe0.5Nb0.5)O3 and BaTiO3 with high tolerance factor into NaNbO3 leads to a gradual elimination of anti-parallel cation displacement and anti-phase tilt along the three axes; as a result, a single tetragonal phase with P4bm space group and only in-phase tilt along c axis is achieved in 0.88NaNbO3-0.04Ba(Fe0.5Nb0.5)O3-0.08BaTiO3 ceramic. Accompanying the decreased oxygen octahedral tilt degree, a drastically improved d33 up to 367 pC N-1 (over ten times that of NaNbO3 ceramic) is obtained, reaching a record high in NaNbO3-based lead-free ceramics. Together with temperature insensitive piezoresponse originating from thermally stable oxygen octahedral tilt structure, the studied NaNbO3-based materials show large potential for replacing Pb-based ceramics in some electronic devices. The oxygen octahedron tilt engineering would be used for designing more high-performance lead-free ceramics with high piezoresponse and excellent thermal stability simultaneously.

Mode Coupling at around M-Point in PZT.

The question of the microscopic origin of the M-superstructure and additional satellite peaks in the Zr-rich lead zirconate-titanate is discussed for nearly 50 years. Clear contradiction between the selection rules of the critical scattering and the superstructure was found preventing unambiguous attributing of the observed superstructure either to the rotation of the oxygen octahedra or to the antiparallel displacements of the lead cations. Detailed analysis of the satellite pattern explained it as the result of the incommensurate phase transition rather than antiphase domains. Critical dynamics is the key point for the formulated problems. Recently, the oxygen tilt soft mode in the PbZrTiO (PZT2.4) was found. But this does not resolve the extinction rules contradiction. The results of the inelastic X-ray scattering study of the phonon spectra of PZT2.4 around M-point are reported. Strong coupling between the lead and oxygen modes resulting in mode anticrossing and creation of the wide flat part in the lowest phonon dispersion curves is identified. This flat part corresponds to the mixture of the displacements of the lead and oxygen ions and can be an explanation of the extinction rules contradiction. Moreover, a flat dispersion surface is a typical prerequisite for the incommensurate phase transition.

Oxygen octahedral coupling mediated ferroelectric-antiferroelectric phase transition based on domain wall engineering

Exotic domain-wall phenomena make ferroelectrics the candidates for nanoelectronics. The local symmetry and structure anomalies at domain walls have raised interest in the unusual functionalities, such as domain wall chirality and conductivity. Especially, the spontaneous lattice distortion and symmetry breaking at domain walls activate them as the location of structural transformation. However, the routes to achieve ferroelectric-antiferroelectric phase transition via ferroelectric domain walls remain challenging, which are important to develop materials for energy storage and conversion. Here, we have observed stepwise antiferroelectric phase transition in strained pure BiFeO3 ultrathin films derived from ferroelectric domain walls. Aberration-corrected transmission electron microscopy observation reveals that the resultant phase transition is mediated by dense 180° domain walls via providing the antiparallel cation displacement, cooperating with the enhanced interfacial oxygen octahedral clamping. First-principles calculations further confirm the critical role of interfacial oxygen octahedral coupling during this transition. These findings report an alternative route for antiferroelectric phase transition. Besides, our results provide fresh insights into functionalities of ferroelectric domain walls and open a venue for developing energy-storage materials based on domain engineering.

Incommensurate instability and diffuse scattering at Brillouin zone boundary in Zr-rich lead zirconate titanate

Detailed study of the X-ray diffuse scattering in the paraelectric phase of the PbZr0.985Ti0.015O3 was carried out. We concentrated on the analysis of the scattering peculiarities in the vicinities of M- and R- points of the Brillouin zone. We have observed a critical increase of the diffuse scattering at the position Q ≈ (0.47 + h 0.53 + k l) slightly aside of M point on cooling to the intermediate phase. We attribute observed critical scattering to a mixed mode containing both oxygen octahedral tilts and antiparallel cationic displacements. Tentative conclusion is made describing earlier reported complicated set of the satellite peaks around M-point as the result of the soft-mode driven incommensurate phase transition.

Structure and synergy performance of (1-x)Sr0.25Ce0.5TiO3 -xLa(Mg0.5Ti0.5)O3 based microwave dielectric ceramics for 5G architecture

The crystal structure and microwave (MW) dielectric characteristics of the compound in the perovskite type series (1-x) (Sr0.25Ce0.5TiO3)-xLa(Mg0.5Ti0.5)O3, x = 0.0 to 0.6 have been studied. The structural phase evolutions from orthorhombic (for x = 0.0) to cubic (for x ≥ 0.15) as a function of La(Mg0.5Ti0.5)O3 have been investigated and are related to variation in the dielectric characteristics. For x = 0.0, the electron diffraction (TEM) revealed that Sr0.25Ce0.5TiO3 compound were tilted in both anti-phase and anti-parallel displacement of A-site cation. The nonlinear trend of the temperature coefficient of resonant frequency (τf) vs dielectric constant resulted from the tilted (for x = 0.0) to nontilted (for x ≥ 0.15) structure transition. The value of τf decreases markedly from +194 ppm/°C (x = 0.0) to + 0.8 ppm/°C (x = 0.6), but the Q × f value (Q-factor) increases from 19,238 GHz to 37,508 GHz. Low loss microwave (MW) dielectric properties: εr = 47.0, Q × f = 37508 GHz and τf = +15 ppm/°C and εr = 39, Q × f = 34098 GHz and τf = +0.8 ppm/°C were achieved for compositions with x = 0.45 and 0.6.

Spontaneous shear flow in confined cellular nematics.

In embryonic development or tumor evolution, cells often migrate collectively within confining tracks defined by their microenvironment 1,2. In some of these situations, the displacements within a cell strand are antiparallel 3, giving rise to shear flows. However, the mechanisms underlying these spontaneous flows remain poorly understood. Here, we show that an ensemble of spindle-shaped cells plated in a well-defined stripe spontaneously develop a shear flow whose characteristics depend on the width of the stripe. On wide stripes, the cells self-organize in a nematic phase with a director at a well-defined angle with the stripe’s direction, and develop a shear flow close to the stripe’s edges. However, on stripes narrower than a critical width, the cells perfectly align with the stripe’s direction and the net flow vanishes. A hydrodynamic active gel theory provides an understanding of these observations and identifies the transition between the non-flowing phase oriented along the stripe and the tilted phase exhibiting shear flow as a Fréedericksz transition driven by the activity of the cells. This physical theory is grounded in the active nature of the cells and based on symmetries and conservation laws, providing a generic mechanism to interpret in vivo antiparallel cell displacements.

Activation of B1 silent Raman modes and its potential origin as source for phonon-assisted replicas in photoluminescence response in N-doped ZnO nanowires

ZnO nanowires are grown by metal organic chemical vapor deposition using two different zinc precursors, i.e., dimethylzinc-triethylamine which contains nitrogen, and diethylzinc which does not. The growth conditions are varied using different oxygen/zinc pressure ratios (RO/Zn). Temperature dependent Raman spectroscopy shows that the additional Raman modes are related to B1 modes which are activated because of translational symmetry breaking resulting from the nitrogen substitution on oxygen sites and/or Zn-O bond breaking caused by complex defects. Simultaneously, the antiparallel atomic displacements which are at the origin of B1 phonon vibrations are no more compensated, allowing B1 modes to acquire a polar character. The resulting polar phonons, and especially B12 located at 580 cm−1 (i.e., 72 meV), are therefore believed to strongly couple to photogenerated electrons through a Fröhlich mechanism and could lead or contribute to the phonon-assisted replicas observed in the photoluminescence (PL) spectrum. Finally, we also discuss the possible defects involved in the Raman and PL responses including native donor and acceptor defects and their interaction with the N-dopant, depending on the growth conditions.

Complex biphase nature of the superconducting dome of the FeSe phase diagram

Single crystal synchrotron X-ray diffraction as a function of temperature and pressure has revealed a complex biphase mixture in superconducting FeSe. Based on our experimental results we construct a phase diagram where structural behavior and superconducting properties of FeSe are found to be correlated. We show that below 6 GPa, where pressure promotes the superconducting critical temperature, the FeSe structure is composed of 2D layers of edge-shared FeSe4 tetrahedra, while above 6 GPa the superconductivity is strongly suppressed on formation of a new orthorhombic polymorph characterized by a 3D network of face sharing FeSe6 octahedra. Therefore changes in topology and connectivity of the FeSe structure are found to be detrimental for superconductivity to exist. This previously controversial crystal structure of the high pressure polymorph of FeSe was also unambiguously determined. High pressure FeSe adopts an orthorhombic MnP-type structure (Pnma) which corresponds to a slightly distorted hexagonal NiAs-type arrangement (P63/mmc). The structural transformation from the low- to high-pressure FeSe polymorph is first order in nature and is manifested as antiparallel displacements within the Fe and Se sublattices.

Ferrielectricity in the metal-organic ferroelectric tris-sarcosine calcium chloride

We report ferrielectricity in a single-phase crystal, TSCC -- tris-sarcosine calcium chloride [(CH3NHCH2COOH)3CaCl2]. Ferrielectricity is well known in smectic liquid crystals but almost unknown in true crystalline solids. Pulvari reported it in 1960 in mixtures of ferroelectrics and antiferroelectrics, but only at high fields. TSCC exhibits a second-order displacive phase transition near Tc = 130 K that can be lowered to a Quantum Critical Point at zero Kelvin via Br- or I-substitution, and phases predicted to be antiferroelectric at high pressure and low temperatures. Unusually, the size of the primitive unit cell does not increase. We measure hysteresis loops and polarization below T = 64 K and clear Raman evidence for this transition, as well of another transition near 47-50 K. X-ray and neutron studies below Tc = 130K show there is an antiferroelectric displacement out of plane of two sarcosine groups; but these are antiparallel displacements are of different magnitude, leading to a bias voltage that grows with decreasing T. A monoclinic subgroup C2 may be possible at the lowest temperatures (T<64K or T<48K), but no direct evidence exists for a crystal class lower than orthorhombic.

New carbon structures obtained from C70 at high pressures and high temperatures

The structures of two newly discovered C70 high-pressure polymerized phases are described in a common basis. Both structures involve anti-parallel displacements of basal compact layers occurring, for each structure, along a particular unit cell axis. These displacements are crucial in the establishment of intermolecular covalent bonds and, thus, in the formation of the new phases. Pressure-temperature conditions for the synthesis of both phases are also described.

Monolayer AgBiP2Se6: an atomically thin ferroelectric semiconductor with out-plane polarization.

Using first-principles calculations, we designed a two-dimensional material, monolayer AgBiP2Se6, with the thickness of only 6 Å, which exhibited out-plane ferroelectricity. The ground state of the monolayer AgBiP2Se6 was not purely ferroelectric since the out-plane ferroelectricity originated from the compensated ferrielectric state: the off-centering antiparallel displacements of Ag+ and Bi3+ ions. The compensated ferrielectric ordering has superiority on reducing the depolarization field to stabilize the ferroelectricity. Furthermore, together with strong visible-light adsorption and suitable band edge alignments, we proposed the monolayer AgBiP2Se6 as a visible-light photocatalyst for water-splitting as the out-plane polarization could enhance the electron-hole separation. Our results offer a new way to overcome the critical thickness limitation of nanoscale ferroelectrics. The out-plane ferroelectricity in monolayer AgBiP2Se6 has great potential for developing various devices with desirable applications.

Structural and Electrical Characteristics of (1−x)Pb(Lu1/2Nb1/2)O3–xPbTiO3 Ceramics with Low PbTiO3

A solid solution of (1−x)Pb(Lu1/2Nb1/2)O3–xPbTiO3 with composition of 0.01 ≤ x ≤ 0.08 have been prepared successfully. XRD analysis indicates the crystal structure adopts an orthorhombic (O) phase in 0.01 ≤ x ≤ 0.06 interval and becomes the coexistence of O and rhombohedral (R) phase at x = 0.07, then turns into R phase mostly at x = 0.08. In addition, two sets of superlattice reflections due to B‐site ordering and antiparallel cation displacement are distinguished by XRD and the superstructures which arise from antiparallel cation displacement disappear gradually with the increasing x. The grain size increases gradually with the increasing x, and then becomes the bimodal microstructure at x ≥ 0.06 due to the coexistence of O and R phase. The dielectric spectra exhibit Curie temperature decreases from 248°C to 147°C with increasing x from 0.01 to 0.08. As 0.01 ≤ x ≤ 0.04, the samples display typical double hysteresis loops, suggesting antiferroelectric nature, then turn into ferroelectric gradually at x = 0.05. Finally, it exhibit typical ferroelectric hysteresis loops in 0.06 ≤ x ≤ 0.08 interval.

Antiferroelectric single crystal of La0.011Pb0.984(Lu1/2Nb1/2)O3 with high energy storage density

High quality La0.011Pb0.984(Lu1/2Nb1/2)O3 (La-PLN) antiferroelectric (AFE) single crystal was successfully obtained by a top-seed solution growth technique. The crystal structure adopts an orthorhombic phase and two sets of superlattice reflections due to B-site ordering and anti-parallel cation displacement are observed clearly and distinguished by the X-ray diffraction patterns (XRD). The Curie temperature TC is near 221°C with a low dielectric constant (about 371). The energy storage density decreases along with the temperature increasing and the maximum energy storage density is up to 5.03J/cm3 at 25°C. The results indicate that the La-PLN single crystal is a promising candidate for the AFE energy storage application.

Bi deficiency-tuned functionality in multiferroic Bi1-δFe0.95Mn0.05O3 films.

Structural evolution and ferroelectric (FE)-to-antiferroelectric (AFE) transition behaviors were observed in Bi1-δFe0.95Mn0.05O3 (100)-textured films with a carefully controlled Bi deficiency concentration δ. Raman spectra revealed an orthorhombic structural transition induced by Mn substitution. The polarization-electric field hysteresis loops and capacitance-voltage loops of Bi1-δFe0.95Mn0.05O3 films clearly demonstrated antiferroelectric behavior with increasing δ. The responses of the domain structure of the Bi1-δFe0.95Mn0.05O3 film under positive and negative applied voltages directly suggested the coexistence of FE and AFE phases. The existence of (100) superstructure reflections and antiparallel displacements of the Bi atoms along the [100] direction observed by transmission electron microscopy unambiguously reveal the AFE phase. The chemical substitution-induced orthorhombic structural transition in BiFe0.95Mn0.05O3 film implies that as the δ concentration increases, the changes in Bi-O bonding and the stereochemical activity of Bi 6s lone pair affect both the ferroelectric distortion and the antiferrodistortive rotation and therefore drive the Bi1-δFe0.95Mn0.05O3 crystal lattice to form a PbZrO3-type orthorhombic phase with an AFE order. A continuing increase in Bi deficiency creates defect dipole complexes which produce an internal field leading to a preferred direction of the ferroelectric domain. The Bi deficiency in multiferroic BiFeO3 provides a new route by which to tune functionality.

Ferrielectric phase in the (Bi1/2Na1/2)TiO3–Ba(Mg1/3Nb2/3)O3 system

We have investigated the crystal structure of (1 − x)(Bi1/2Na1/2)TiO3–xBa(Mg1/3Nb2/3)O3 (BNT–BMN) by high-resolution neutron powder diffraction (NPD) analysis. Our NPD study combined with polarization hysteresis measurements for ceramics reveals that a boundary between rhombohedral R3c and tetragonal P4bm phases exists at x = 4–5%. The Rietveld analysis of the NPD data shows that the crystal structure with x = 10% has a single-phase tetragonal structure in space group P4bm, as has been reported for BNT–7%BaTiO3. The P4bm structure exhibits antiparallel displacements of A- and B-site cations, which results in a small spontaneous polarization (Ps) of 13.3 µC/cm2 (estimated from the refined structural parameters). The results obtained for BNT–10%BMN ceramics strongly suggest ferrielectric features; the application of electric fields induces a transition from the P4bm phase with a small Ps to other ferroelectric phases with a large Ps.

Near-infrared trapped mode magnetic resonance in an all-dielectric metamaterial

Optical responses in conventional metamaterials based on plasmonic metal nanostructures are inevitably accompanied by Joule losses, which obstruct practical applications by limiting resonance quality factors and compromising the efficiency of metamaterial devices. Here we experimentally demonstrate a fully-dielectric metamaterial that exhibits a 'trapped mode' resonance at optical frequencies, founded upon the excitation by incident light of anti-parallel displacement currents in meta-molecules comprising pairs of parallel, geometrically dissimilar dielectric nano-bars. The phenomenon is demonstrated in the near-infrared part of the spectrum using silicon, showing that in principle strong, lossless resonant responses are possible anywhere in the optical spectral range.

Phase Transition Domains in Ca‐based Complex Perovskite Dielectric Ceramics

Phase transition‐induced lamellar domain structures together with the phase transition process in Ca [( Mg 1/3 Ta 2/3)0.9 Ti 0.1] O 3 complex perovskite ceramics have been investigated by high‐resolution transmission electron microscopy. As the orthorhombic and monoclinic structures coexist in the present ceramics and even in a single grain, groups of lamellar domains have been observed and analyzed in the three characteristic zone axes of the parent cubic perovskite structure: 〈100〉, 〈110〉, and 〈111〉. zone axes. The antiparallel displacement of A‐site cations, in‐phase, and antiphase oxygen octahedra tilting and B‐site 1:2 cation ordering cause superstructure reflections to destroy the symmetry of the parent cubic structure and subsequently induce domain structures. Groups of (010), (100) reflection twin domains and 90° rotation domains along the [100], [010], and [001] directions in the 〈100〉 and 〈110〉 zone axes are refined to be caused by the loss of reflection planes and fourfold rotation symmetry, whereas the loss of threefold rotoinversion and gain of twofold rotation symmetry lead to 120° and 180° rotation domains in the [111] zone axis. Rhombohedral m and tetragonal 4/mmm structures are revealed to act as the bridging intermediate phase to make the first‐order phase transitions from the parent cubic m m structure to the orthorhombic mmm structure and monoclinic 2/m structure without group‐subgroup relations continuous and diffusionless, and thus could occur via second order to form domain structures.

Polymorphism, structural characterisation and electrical properties of Na2Nb4O11

Structural characterisation of Na2Nb4O11 powder has been carried out by Rietveld refinement of neutron powder diffraction data in the temperature range 10–573 K and measurements of second harmonic generation in the range 75 to 450 K. The space group changes from Rc to C2/c on cooling through a phase transition at ∼380 K. The origin of the phase transition in Na2Nb4O11 is mainly related to distortion of some Nb(1)O7 pentagonal bipyramids within the equatorial plane whereas other Nb(2)O7 pentagonal bipyramids become almost regular. Although Na2Nb4O11 ceramics show a maximum permittivity of ∼150 at ∼380 K and frequency-independent permittivity below this temperature, the room temperature polymorph is not ferroelectric. Comparison of the high and room temperature structures shows clear evidence of antiparallel atomic displacements in the room temperature structure and, therefore, a tendency towards low temperature antiferroelectric behaviour. A comparison is also made with the structurally-related ferroelectric Ag2Nb4O11.

ChemInform Abstract: 31P MAS NMR Study of the Ferrielectric-Paraelectric Transition in Layered CuInP2S6.

The order−disorder ferrielectric−paraelectric transition in lamellar CuInP2S6 is studied using 31P solid-state MAS NMR spectroscopy. Spectra from a powder sample were recorded at various temperatures between 255 and 355 K with a probe precalibrated for heating or cooling due to magic angle spinning. Two center bands are observed at the lowest measured temperature while only one is detected at the highest temperature. The former two represent the inequivalent positions for the P atoms of the P2S6 group which reflect the antiparallel displacements of the polar CuI and InIII sublattices in the ferrielectric phase. The latter corresponds to the appearance of a 2-fold axis through the P−P bond as the CuI ions undergo double-well hopping motions, and the InIII ions occupy on-center sites in the paraelectric phase. At intermediate temperatures, both ferrielectric and paraelectric type resonances contribute to the spectrum at ratios which are T-dependent, indicating a transition temperature Tc = 312 ± 1 K (310 ± ...