Tetragonal Tungsten Bronze Research Articles

Spontaneous electric polarization of high-valance-ion-doped lead-free ferroelectric KSr2Nb5O15 by first-principles calculations

The polarizability and spontaneous polarization of high-valent cations (La3+, Bi3+, Y3+, Ti4+, Ta5+ and W6+) doped KSr2Nb5O15 lead-free tetragonal tungsten bronze ferroelectrics are calculated by using the first principles and experimental results. The cation dopants give different results, mainly due to the shortage of the additivity rule that ignores the ion polarizability of the missing atoms and the polarizability caused by the interaction of the atoms. The polarizability based on the dielectric constants is in good agreement with experimental values, especially for the structures with trivalent cation doping. And the spontaneous polarization calculated by the polar moment by using first principles result is consistent with that based on the polar moment from experiment. This shows the reliability of the prediction, and the spontaneous polarization of high-valent cations doped KSr2Nb5O15 are confirmed. This work paves a way for the spontaneous polarization of the ferroelectric materials with tetragonal tungsten bronze structure.

Synthesis, structural and microstructural properties of CBN ferroelectric ceramics

Ca x Ba1- x Nb2O6 (CBN) ceramics with tetragonal tungsten bronze (TTB) structures are very attractive from academic and technological fronts due to their stupendous ferroelectric properties. Polycrystalline samples of Ca x Ba1- x Nb2O5 were prepared by a high temperature solid state reaction technique. The phase formation, microstructure and optical properties of the prepared samples were investigated by X-ray diffraction, scanning electron microscope and Raman spectrometer, respectively. X-ray analysis confirms the partially filled tetragonal tungsten bronze (TTB) structure. Scanning electron micrographs provide information related to the morphology and grain size distribution of the samples. Detailed analysis of the structural and optical properties suggests that these samples have undergone a phase transition well above the room temperature.

Crystal structure, dielectric, and ferroelectric characteristics of zirconate tantalate ceramics with tungsten bronze structure

In the present work, dielectric and ferroelectric characteristics of Ba6−pRpZr2+pTa8−pO30 (p = 1, 2, R = La, Nd, Sm) tungsten bronze ceramics were investigated systematically, and the effects of Zr4+ and Ta4+ occupation upon the polar order were emphasized. X-ray diffraction patterns confirmed the tetragonal tungsten bronze structure with P4/mbm space group at ambient temperature. Relaxor ferroelectric behavior was determined for all compounds, even for those who had order A1- and A2-site occupations, and this was quite different from other tungsten bronze systems. Compared with the Ti- and Nb-based analogue, these Zr- and Ta-based compounds exhibited stronger relaxor nature and obvious polar order disruption. Raman spectra and X-ray photoelectron spectra demonstrated distinct local crystal environment for these compounds with different B-site substitutions. Smaller A-site cation displacement inside the pentagonal channels, frustrating BO6 deformation, and weaker B–O-bond covalency were the main reasons for the severe disruption of ferroelectric polar order in these filled tungsten bronzes.

BiNb5.4O15: A new Li+-storage material with a tetragonal tungsten bronze crystal structure

Niobates with tungsten bronze crystal structures are regarded as prospective Li[Formula: see text]-storage anode candidates due to the rich chemistry of niobium and structural openness. However, the exploration of this type of materials is still insufficient. Here, BiNb[Formula: see text]O[Formula: see text] with a tetragonal tungsten bronze crystal structure (Cmmm space group) is explored. Abundant large-sized tunnels and a large interlayer spacing of [Formula: see text]3.97 Å are found in this open structure, enabling fast Li[Formula: see text] diffusivity (3.25 × 10[Formula: see text]/1.68 × 10[Formula: see text] cm2 ⋅ s[Formula: see text] during lithiation/delithiation). Additionally, this structure is very stable after the first lithiation process, resulting in excellent cyclability (95.4%/[Formula: see text]100% capacity retention after 100/1000 cycles at 1C/10C). BiNb[Formula: see text]O[Formula: see text] further exhibits a safe Li[Formula: see text]-storage potential ([Formula: see text]1.55 V) and a large reversible capacity (309.7 mAh ⋅ g[Formula: see text]@ 0.1C). Therefore, BiNb[Formula: see text]O[Formula: see text] can be a practical Li[Formula: see text]-stora ge anode compound for large-scale energy-storage applications.

Disorder in Strontium Barium Niobate: Local Structure, Phonons, and Polarization Dynamics.

The main representative of the tetragonal tungsten bronze family, strontium barium niobate, has been recently studied by the scattering and spectroscopic techniques [diffuse scattering (DS), Raman, infrared, and high-frequency dielectric spectroscopies] to decipher the nature of its ferroelectric-relaxor transformation tuned by composition. DS revealed that mesoscopic range ordered correlations along the polar axis are presented in the crystal, being less organized in the relaxor compositions with higher Sr content. For these compositions, transverse polar correlations are found to be anisotropic. Pair distribution function experiments and first-principle calculations in several compositions showed a concomitant new peak around ~6.5 Å, which develops on cooling and cannot be explained by the current structural models with long-range incommensurate oxygen tilts but with a short-range ordered tilt arrangement. This relates with the development of the macroscopic polarization, tightly linked to the structure, and shows two different contributions related to the two different crystallographic sites for the ferroactive Nb atom, recalling a picture of two independent polarization networks showing disparate dynamics, temperature behavior, and correlation lengths. Cationic substitution in different channels affects mainly the relaxations. Above the MHz range, all members of the solid solutions show three main relaxations, whose temperature dependence is unalike, due to the distinct correlation lengths and nature of the processes. On increasing Sr content, the frequencies are higher, and the contribution to the permittivity of excitations at GHz-THz ranges gets stronger. A soft anharmonic central mode at THz range is the characteristic for all the compositions, together with the slowing down of a relaxation at GHz-MHz ranges, revealing the coexistence of displacive and order-disorder mechanisms of the ferroelectric phase transition in these solid solutions.

On the structural complexity of tetragonal tungsten bronze type niobium tungsten oxides

AbstractIn this comparative study, two samples with a slightly different oxygen : metal ratio have been characterized by various electron microscopy methods and by single‐crystal and powder X‐ray diffraction (XRD). A reasonably good fit to the experimental powder XRD patterns could be achieved by Rietveld refinement for both samples Nb8W9O47 and Nb7W10O47.5 using the structural models of Nb6.7W10.3O47 and Nb8W9O47 both obtained from X‐ray crystal structure analysis. HRTEM investigations confirm that Nb8W9O47 crystallizes as a pure phase in the well‐known threefold superstructure of the tetragonal tungsten bronze type (TTB). In contrast, a more complex real structure was found for the sample Nb7W10O47.5 with small domains of Nb4W7O31 appearing besides large ones of Nb8W9O47 and in addition extended TTB‐based areas with short‐range order only. The diffuse scattering caused by this disorder is easily detected in the electron diffraction pattern but does not affect the Rietveld refinement of the powder XRD patterns. However, on the basis of powder diffraction experiments, it is clearly possible to distinguish between the above compounds and the hypothetical, but non‐existing compound “Nb18W16O93”, a composition recently postulated for battery materials.

Resistance Switching Effect in Octahedral framework oxide

Resistive Random-Access Memories (ReRAM) are an alternative way to create new memory devices. This is physically possible due to the existence in the material, of two resistive states clearly discreditable, as a function of voltage value and polarity first parameter under control to pass from one state to another one. However, the mechanism of the resistance switching is not simple and is under debate. We present in the present chapter all the factors entering in the switching process in tetragonal tungsten bronze (TTB) type structure oxide thin films deposited by PLD technique onto MgO or STO substrates. Results show that GdK2Nb5O15 (GKN) thin films deposited on MgO and STO substrates are resistively switchable. It was found that the nature of the substrate strongly affects the resistance ratio: GKN on SRO/LSCO/MgO showed a large hysteresis compared to GKN on SRO/STO. Substrate effect and oxygen vacancy on resistance switching in GKN thin film were studied in the same experimental conditions. The study of resistance switching in the GKN/MgO and GKN/STO thin films has confirmed that for low voltages, below the threshold value of 1.3 V, the electric transport is dominated by the formation of a Schottky type barrier, which allows a minimum leakage current. Resistance switching in GKN is attributed to the oxygen vacancies migration which can be controlled by the substrate or the frequency sweep.

Framework structure crystalline materials and Rigid Unit Modes (RUMs). Introducing the new concept of MLRUMs and skeletions Authors

This article reviews Framework Structures (FWSs), defined as crystalline materials built of rigid AXn polyhedra sharing vertices (like perovskites, tungsten bronzes, Dion-Jacobson, Ruddlesden-Popper, and Aurivillius phases, quartz, silicates, and others), and their pecularities resulting from this linkage. The situation of rigid units linked by common vertices may allow the units to accomplish concordant rotations without deformation, which gives rise to soft phonon modes called “Rigid Unit Modes” (RUMs). The condensation of a RUM can trigger structural phase transitions to a structure of lower symmetry, with tilted polyhedra, at the origin of spontaneous ferroic or multiferroic properties. We overview results precedently obtained on RUMs in perovskites, tetragonal tungsten bronzes, and quartz, and detail new results on “maximally localized RUMs” (MLRUMs), a fundamental new concept in the physics of RUMs. We introduce also the related new concept of “skeletions” that allows to generate all ferroelastic phases found in these systems, and generalizes the Glazer's tilt-system approach.

Phonons and relaxations in unfilled tetragonal tungsten-bronzes

The lead-free unfilled tetragonal tungsten-bronzes SBN and CBN are investigated by Raman, infrared and high-frequency dielectric spectroscopies. The substitution in the different channels affects phonons as well as relaxations. Relaxations in SBN show similar qualitative behaviour on increasing Sr content and relaxor behaviour, but for the extreme relaxor samples all characteristic frequencies are higher with stronger contribution to the permittivity in the GHz-THz range is stronger. The presence of a soft anharmonic central mode in the THz range together with the slowing down of a relaxation from GHz to MHz ranges reveals the coexistence of displacive and order-disorder scenarios for the ferroelectric phase transition in this family.

Ferroelectric to relaxor crossover in Li solid solutions derived from Ba2Nd□2FeNb4O15

Three solid solutions of Tetragonal Tungsten Bronze structure containing Li were prepared by solid state route starting from the parent compound Ba2NdFeNb4O15. These solid solutions involve Li insertion compensated by cationic removal. All cationic sites were involved to investigate their influence on the dielectric transitions observed in Ba2NdFeNb4O15, i.e. from relaxor (R) to ferroelectric (FE) and finally to paraelectric (PE). All solid solutions display the same pattern, which is an evolution from a relaxor to ferroelectric crossover, leading to a single transition (relaxor to paraelectric). This evolution is accompanied by a sudden increase of the ordering temperature (Tm). A focus on structure refinements shows the slight differences induced by the insertion of Li and points towards the role of anionic framework as responsible for the dielectric properties of these materials. This study completes the work performed on other cationic substitutions performed in the title material, and represents a new example of Li substituted TTB materials.

Effects of Gd-substitution on structural, and impedance spectroscopic study of Sr2Sm1−xGdxTi2Nb3O15 tungsten bronze ceramics

In the present work, we have studied the effect of Gd-substitution on the structural, dielectric and electrical properties of Sr2Sm1−xGdxTi2Nb3O15 (x = 0 and 0.5) tungsten bronze ceramics prepared by a high-temperature solid-state reaction technique. The formation of the phase of tetragonal tungsten bronze was verified by the Rietveld refinement using X-ray diffraction data. With increasing Gd concentration, the crystal structure of the Sr2Sm1−xGdxTi2Nb3O15 ceramics has slightly deformed from the tetragonal P4bm phase to the paraelectric P4/mbm phase. Scanning electron microscopy (SEM) of ceramics shows high densification and homogeneous distribution of grains of different sizes over the total surface. The crystallite size increases with Gd-substitution from 6.19 nm for Sr2SmTi2Nb3O15 to 10.83 nm for Sr2Sm0.5Gd0.5Ti2Nb3O15. The sample Sr2SmTi2Nb3O15 shows a dielectric anomaly of ferroelectric paraelectric type at 332 °C, and has non-relaxor type of diffuse phase transition. In addition, the dielectric properties deteriorated and even disappeared for the sample Sr2Sm0.5Gd0.5Ti2Nb3O15, because the ceramic presented a P4/mbm phase structure as a result of the decrease in Curie temperature (Tc) below room temperature. The electrical property of Sr2Sm1−xGdxTi2Nb3O15 ceramic has been investigated by non-destructive complex impedance spectroscopy (CIS) as a function of frequency at different temperatures.

A new class of upconversion luminescence tuning materials based on non-photochromic reaction: Er3+-activated Ba0.7Sr0.3Nb2O6 ferroelectrics

The upconversion (UC) luminescence tuning of phosphors has attracted increasing interest as optical switches, optical sensing, and high-density optical data storage devices. However, the design and synthesis of photo-switchable inorganic luminescent materials were still at primary stage. In this work, a new type of UC luminescence regulation materials, Er3+-activated tetragonal tungsten bronze Ba0.7Sr0.3Nb2O6 ferroelectrics, showing strong susceptibility to visible light irradiation, was reported based on non-photochromic (N-PC) effect. Under visible light irradiation, the maximum luminescent modulation ability reached up to 79.4%. The tuning of UC luminescence was disclosed to tightly rely on the irradiation time and thermal treatment processes. Meanwhile, excellent reproducibility was also demonstrated. Besides, the luminescent readout almost had no any influence on the UC emission, indicating the extremely low destruction to the stored luminescent states. The mechanisms related to the UC luminescence and manipulation were also discussed. In contrast to the previous tuning way based on PC reaction, it is believed that this work shed new light for the understanding of UC modulation fundamentals.

Relaxor Nature and Energy Storage Properties of Sr2–xMxNaNb5–xTixO15 (M = La3+ and Ho3+) Tungsten Bronze Ceramics

A series of lead- and bismuth-free tetragonal tungsten bronze ceramics with chemical formula given as Sr2–xMxNaNb5–xTixO15 (M = La3+ and Ho3+) were prepared. Through codoping of A- and B-sites, swi...

Antiferroelectricity and robust dielectric response owing to competing polar and antipolar instabilities in tetragonal tungsten bronze K2RNb5O15 ( R : rare-earth)

Antiferroelectricity (AFE) and its robust dielectric response under a high electric field were experimentally demonstrated in tetragonal tungsten bronze ${\mathrm{K}}_{2}R{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ (R: rare-earth). Electrical resistivity and density of ceramics samples were sufficiently improved by optimizing chemical compositions and processes, and the phase transition temperatures are widely controlled by changing R ions. Typical features of AFE, i.e., a double-hysteresis loop and a dielectric peak under a DC electric field, were demonstrated at room temperature in ${\mathrm{K}}_{2}{\mathrm{Pr}}_{0.75}{\mathrm{La}}_{0.25}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$. Notably robust relative permittivity against DC fields, which might be applicable to ceramics capacitors for high-voltage usage, is realized owing to AFE. Its dielectric constant under a high DC electric field, \ensuremath{\epsilon} \ensuremath{\sim} 800 at 10 MV/m, exceeds that of conventional ${\mathrm{BaTiO}}_{3}$, \ensuremath{\epsilon} \ensuremath{\sim} 620 at 10 MV/m. First-principles calculations suggested competing polar and antipolar instabilities underlying the successive phase transitions in this system. Two different types of antipolar displacement patterns $({{\mathrm{\ensuremath{\Gamma}}}_{2}}^{\ensuremath{-}} \mathrm{and} {{M}_{1}}^{+}{{M}_{4}}^{+})$ were found and are considered to be the structural origin of AFE. These results clearly demonstrate that ${\mathrm{K}}_{2}R{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ offers a new antiferroelectric materials platform that operates around room temperature.

Tetragonal tungsten bronze/barium hexaferrite room-temperature multiferroic composite ceramics

The spontaneously formed multiferroic composite based on tetragonal tungsten bronze structure was successfully synthesized in the form of ceramics by the solid-state reaction. The crystallization of the ceramics in the tetragonal tungsten bronze structure Ba2SmFeNb4O15 and the presence of a secondary magnetic phase of barium hexaferrite BaFe12O19 were established by structural investigation. The hysteresis behavior describing the polarization as a function of an applied electric field and the high ferroelectric Curie temperature ≈ 417 K evidenced the room-temperature ferroelectric properties of the synthesized material. While the piezoelectric coefficients were determined to be around 1.3 pm/V, the microscopic polar domains of the composite ceramics were established by microelectromechanical study. The hysteresis loop of the magnetization versus magnetic field and the high magnetic transition temperature ≈ 590 K evidenced the ferromagnetic properties of the secondary phase and its presence at room temperature. Moreover, the spatial distribution of the magnetic domains was determined microscopically. In this study, not only the multiferroic properties of the Ba2SmFeNb4O15/BaFe12O19 composites have been studied and presented, but the distribution of the polar and magnetic properties was locally investigated as well.

Intergrowth of niobium tungsten oxides of the tetragonal tungsten bronze type

Abstract Since the 1970s, high-resolution transmission electron microscopy (HRTEM) is well established as the most appropriate method to explore the structural complexity of niobium tungsten oxides. Today, scanning transmission electron microscopy (STEM) represents an important alternative for performing the structural characterization of such oxides. STEM images recorded with a high-angle annular dark field (HAADF) detector provide not only information about the cation positions but also about the distribution of niobium and tungsten as the intensity is directly correlated to the local scattering potential. The applicability of this method is demonstrated here for the characterization of the real structure of Nb7W10O47.5. This sample contains well-ordered domains of Nb8W9O47 and Nb4W7O31 besides little ordered areas according to HRTEM results. Structural models for Nb4W7O31 and twinning occurring in this phase have been derived from the interpretation of HAADF-STEM images. A remarkable grain boundary between well-ordered domains of Nb4W7O31 and Nb8W9O47 has been found that contains one-dimensionally periodic features. Furthermore, short-range order observed in less ordered areas could be attributed to an intimate intergrowth of small sections of different tetragonal tungsten bronze (TTB) based structures.

High Oxygen Evolution Activity of Tungsten Bronze Oxides Boosted by Anchoring of Co2+ at Nb5+ Sites Accompanied by Substantial Oxygen Vacancy.

The participation of lattice oxygen in the oxygen evolution reaction (OER) process has been proved to be faster in kinetics than the mechanisms where only metal is involved, although activating the lattice oxygen in the traditional rigid structures remains a big challenge. In this work, efforts are devoted to exploring a new flexible structure that is competent in providing large amounts of oxygen vacancies as well as offering the freedom to manipulate the electronic structure of metal cations. This is demonstrated by anchoring low valence state Co at high valence state Nb sites in the tetragonal tungsten bronze (TTB)‐structured Sr0.5Ba0.5Nb2‐ xCoxO6‐δ, with different ratios of Co to Nb to optimize the Co substitution proportion. It is found that the occupation of Co in the Nb5+ sites gives rise to the generation of massive surface oxygen vacancies (Ovac), while Co itself is stabilized in Co2+ by adjacent Ovac. The coexistence of Ovac and LS Co2+ enables an oxygen intercalation mechanism in the optimal SBNC45 with specific activity at 1.7 V versus reversible hydrogen electrode that is 20 times higher than for the commercial IrO2. This work illuminates an entirely new avenue to rationally design OER electrocatalysts with ultrafast kinetics.

Local Structure and Order–Disorder Transitions in “Empty” Ferroelectric Tetragonal Tungsten Bronzes

The “empty” tetragonal tungsten bronze Ba4La0.67□1.33Nb10O30 displays both relaxor-like and normal dielectric anomalies as a function of temperature; the former is associated with loss of ferroelec...

Luminescence modulation behavior upon in-situ electric field of Sm3+ doped KSr2Nb5O15 ferroelectric ceramics

Sm3+ doped KSr2Nb5O15 (KSN-Sm) ceramics exhibiting the electro-photoluminescent coupling effect were prepared by two-step sintering technology. The microstructures, electrical and luminescent properties were characterized. In addition, the luminescence modulation behavior upon the application of an in-situ electric field was investigated. It was found that the luminescent emission intensity decreased with increasing in-situ dc bias, and a high modulation ratio of approximately 20%@20 kV/cm was achieved. This work demonstrates that KSN-based ferroelectric ceramics with the tetragonal tungsten bronze structure are promising optical materials with good luminescence modulation behavior.

Phase transformations and dielectric characteristics of FE-relaxor Sr0.75Ba0.25Nb2O6 ceramics and thin films

In the paper the results of a structure and dielectric properties study of ferroelectric-relaxor Sr0.75Ba0.25Nb2O6 (SBN-75) ceramics and thin films with the structure of an unfilled tetragonal tungsten bronze have been presented. The existence of orientation domains in the SBN-75 film with rotation in the interface plane only by angles of ± 18.4° relative to the axes of the MgO substrate is revealed. In the tetragonal approximation, the unit cell parameters of the film (c = 3.948 Å and a = 12.49 Å) and its strain in comparison with the cell parameters of the ceramic sample have been determined.