Tetragonal Tungsten Bronze Structure Research Articles

Iron-based fluorides of tetragonal tungsten bronze structure as potential cathodes for Na-ion batteries

A iron-based fluoride of tetragonal tungsten bronze structure prepared by topotactically densifying a HTB framework is used as a cathode for Na-ion batteries. K-ion stuffing enables the pristine fluoride to achieve an initial charge capacity of 125 mA h g−1 and reversible capacity of 100–150 mA h g−1.

Superconductivity and crystal structural origins of the metal-insulator transition inBa6−xSrxNb10O30tetragonal tungsten bronzes

${\mathrm{Ba}}_{6\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Nb}}_{10}{\mathrm{O}}_{30}$ solid solution with $0\ensuremath{\le}$ $x$ $\ensuremath{\le}6$ forms the filled tetragonal tungsten bronze (TTB) structure. The Ba-end member crystallizes in the highest symmetry $P4/mbm$ space group $(a=b=12.5842(18)\AA{}$ and $c=3.9995(8)\AA{})$ and so do all the compositions with $0\ensuremath{\le}$ $x$ $\ensuremath{\le}5$. The Sr-end member of the solid solution crystallizes in the tentatively assigned $Amam$ space group $(a{}^{*}=17.506(4)\AA{}$, $b{}^{*}=34.932(7)\AA{}$, and $c{}^{*}=7.7777(2)\AA{})$. The latter space group is related to the parent $P4/mbm$ TTB structure as $a{}^{*}$ $\ensuremath{\approx}$ $\sqrt{2}a,b{}^{*}$ $\ensuremath{\approx}2\sqrt{2}a,c{}^{*}=2c$. Low-temperature specific heat measurements indicate that the Ba-rich compositions with $x$ $\ensuremath{\le}2$ are conventional BCS superconductors with ${T}_{C}$ $\ensuremath{\le}1.6$ K and superconducting energy gaps of $\ensuremath{\le}0.38$ meV. The values of the ${T}_{C}$ in the cation-filled Nb-based TTBs reported here are comparable with those of the unfilled ${\mathrm{K}}_{x}{\mathrm{WO}}_{3}$ and ${\mathrm{Na}}_{x}{\mathrm{WO}}_{3}$ TTBs having large alkali ion deficiency. As the unit cell volume decreases with increasing $x$, an unexpected metal-insulator transition (MIT) in ${\mathrm{Ba}}_{6\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Nb}}_{10}{\mathrm{O}}_{30}$ occurs at $x$ $\ensuremath{\ge}3$. We discuss the possible origins of the MIT in terms of the carrier concentration, symmetry break, and Anderson localization.

Structural characterization and photoluminescence behavior of pure and doped potassium strontium niobates ceramics with tetragonal tungsten–bronze structure

KSr2Nb5O15 and K1.2Sr1.6Ln0.2Nb5O15 (Ln=La, Pr, and Nd) were synthesized by the Pechini method and calcined at 1373K for 10h under air atmosphere. Rietveld refinements indicated the presence of structural defects in both KSN and KSN-Ln, especially distortions in O–Nb–O and Nb–O–Nb bonds. Modifications in bond distances of [NbO6] clusters were also confirmed by Raman spectroscopy. Ultraviolet-visible absorption spectra and photoluminescence profiles were influenced by lanthanide atoms in the band structure of modified KSN materials, creating new defects and intermediary energy levels within the band gap.

Microstructures and dielectric properties of Sr0.6Ba0.4Nb2O6 ceramics with BaCu (B2O5) addition for energy storage

The Sr0.6Ba0.4Nb2O6 ceramics with BaCu(B2O5) (abbreviated as BCB) additive have been prepared via the solid state reaction route. The effects of BCB content on the sintering behavior, crystal structure, microstructure, dielectric property and energy storage property of Sr0.6Ba0.4Nb2O6 ceramics have also been investigated. XRD results show that Sr0.6Ba0.4Nb2O6 ceramics adopt the tetragonal tungsten bronze structure and the secondary phase SrB2O4 emerges when BCB content exceeds 4 wt%. SEM results reveal that the addition of BCB lowers the sintering temperature and improves the densification and microstructure of the Sr0.6Ba0.4Nb2O6 ceramics. An analysis of the dielectric and ferroelectric properties indicates that the values of dielectric constant and saturation polarization decrease with the increase of BCB additive. The breakdown strength (BDS) is notably improved due to the reduction of the grain size and dense uniform microstructure. And the highest BDS of 187 kV/cm can be achieved for the sample with 4 wt% BCB. Complex impedance spectra exhibit that the BDS is closely related to the activation energy of interface polarization. The sample with 4 wt% BCB addition possesses the maximum energy-storage density of 1.63 J/cm3, which is 1.9 times higher than that of pure Sr0.6Ba0.4Nb2O6 ceramics. This study provided the BCB added ceramic as an attractive candidate for making high-energy density capacitors.

Investigation of Dielectric Relaxation Processes in Ba2NdFeNb4-xTaxO15 Ceramics

Dielectric response of Ba2NdFeNb4-xTaxO15 (x = 0.3, 0.6 and 2) solid solutions with Tetragonal Tungsten Bronze structure has been investigated by means of broadband dielectric spectroscopy in the frequency range from 20 Hz to 37 GHz. Pure Ba2NdFeNb4O15 compound displays ferroelectric behavior. Substitution of Nb5+ by Ta5+ considerably alters dielectric response of this system. Ferroelectric state was observed only for x = 0.3 compound with slightly lowered ferroelectric phase transition temperature. Further increase of x causes formation of relaxor state in these solid solutions and considerably lowers corresponding dielectric permittivity dispersion temperature region.

Dielectric and relaxor ferroelectric properties of Sr4CaLaTi3Nb7O30 tetragonal tungsten bronze ceramics

Relaxor ferroelectric Sr4CaLaTi3Nb7O30 ceramics were prepared by using the conventional solid state reaction technique. Dielectric property and ferroelectric transition were studied in detail over a broad temperature range. X-ray diffraction analysis revealed that the Sr4CaLaTi3Nb7O30 ceramics crystallizes in the tetragonal tungsten bronze structure with space group P4bm. The dielectric studies confirmed two relaxation regions due to the formation of the polar nanoregions below the deviation temperature ~405K. Relaxations I and II were attributed possibly due to the relaxational contributions from dipole reversal of polar nanoregions and fluctuation of polar nanoregion boundaries, respectively. A maximum dielectric constant of 1177 was found at a measuring frequency of 500Hz. Room temperature polarization–electric field (P–E) hysteresis loops were assessed and a remnant polarization of 0.65μC/cm2 with a coercive field of 3.33kV/cm was observed under an applied electric field of 58kV/cm. The Sr4CaLaTi3Nb7O30 ceramics exhibited a prominent dielectric and ferroelectric performance which indicates the potential application of this material in various electronic devices.

Abnormal Grain Growth Free Strontium Barium Niobate by Microwave Assisted Sintering

Strontium barium niobate (SBN) with the general formula SrxBa1-xNb2O6 having tetragonal tungsten bronze (TTB) structure is an exceptional and promising lead free ferroelectric material among the family of ferroelectrics. SBN exhibits ferroelectricity in a wide compositional range x = 0.32 – 0.82, compared to perovskite materials such as barium strontium titanate (BST) and barium titanate (BT), which exhibit ferroelectric phase in only one particular composition. Consequently, desired applications can be reached by tuning the Curie temperature (Tc). However, a serious problem encountered in densified SBN was the presence of an abnormal grain growth (AGG), associated duplex microstructure and pores. In the present work, these problems were successfully overcome by microwave assisted sintering. Structure, pure TTB phase analysis and lattice parameters of SBN were confirmed by X-Ray diffraction. AGG, associated duplex microstructure and pore free morphology of SBN were confirmed using scanning electron microscopy. The density of the microwave sintered SBN was found to be ∼94% of theoretical density which is higher than the reported value of conventionally sintered SBN. Dielectric studies of SBN were carried out to tune the Tc for all SBN compositions.

Dielectric Characterization at High Temperature of Iron Doped Potassium Strontium Niobate Ceramic by Impedance Spectroscopy

Tetragonal Tungsten Bronze structure TTB-type structure has attracted interest by the high anisotropy of the crystal structure. The dielectric characterization of iron-doped niobate of TTB-type structure, with stoichiometry KSr2(Fe0.25Nb4.75)O15-δ, prepared by Modified Polyol Method was investigated. Nanocrystalline single phase powders were obtained after calcination of the precursor powder at 1150 °C for 10 hours in an oxygen atmosphere. The dielectric characterization was performed by impedance spectroscopy, from room temperature to 600 °C, in the frequency range of 5 Hz to 13 MHz. The permittivity values obtained for KSr2(Fe0.25Nb4.75)O15-δshowed superior to the permittivity values of the KSr2Nb5O15host structure in all temperature range investigated. At room temperature, the permittivity values (2100) of KSr2(Fe0.25Nb4.75)O15-δis two times the permittivity values of KSr2Nb5O15. The substitution of niobium cation by iron cation in the KSr2Nb5O15host structure showed a suppression of the ferroelectric (P4bm)→paraelectric (P4/mbm) phase transition.

Modifying optical, electric and magnetic properties of multifunctional Gd3+-doped Ba4La0.95Er0.05TiNb9O30 ceramics

Tetragonal tungsten bronze (TTB) structure Ba4La0.95−xEr0.05GdxTiNb9O30 (BLETN:Gd3+) with different Gd3+ concentrations have been synthesized, and their structural, up-conversion photoluminescence (UC-PL), dielectric, ferroelectric (FE), and magnetic properties have been investigated in this work. With the incorporation of Gd3+, it is found that a BLETN:Gd3+ system exhibits multifold responds to the external electric, magnetic, and optical stimuli. Specially, enhanced UC-PL intensity, obvious relaxor-like FE state, and paramagnetic (PM) feature have been simultaneously realized in this single phase TTB-type compound. It is believed that the BLETN:Gd3+ system involving bright UC emission and promising electromagnetic properties may act as a potential material for future multifunctional device applications.

Vogel–Fulcher analysis of relaxor dielectrics with the tetragonal tungsten bronze structure: Ba6MNb9O30 (M = Ga, Sc, In)

In-depth analysis of the relaxor behaviour of Ba6MNb9O30 (M = Ga, Sc, In) tetragonal tungsten bronze (TTB) ceramics was carried out. Powder X-ray diffraction and scanning electron microscopy were performed in order to confirm the formation of desired phases and to determine the microstructure. Low-temperature dielectric spectroscopy was used in order to characterise the dielectric properties of these materials; the degree of relaxor behaviour was investigated in relation with the increase of ionic radius of the M cation on the B-site of the TTB structure. The dynamics of dielectric relaxation of dipoles was studied by fitting the dielectric permittivity data to the Vogel–Fulcher (VF) model in order to monitor the reproducibility and validity of the physical results. Restrictions to the VF fit were attempted besides the regular “free-fit” by constraining some of the fundamental relaxation parameters to physically sensible values. We show that VF fits are very sensitive to the fitting range resulting in a large range of fundamental parameters for the dielectric relaxation processes, and that the restriction of the frequency domain due to experimental noise or instrumentation limits has a dramatic influence on the values obtained.

Pyroelectricity in strontium barium niobate/polyurethane nanocomposites for thermal/infrared detection

Abstract This work reports improvement in electric field-induced pyroelectric properties of polyurethane (PU) elastomer, brought about by the incorporation of strontium barium niobate, Sr 0.3 Ba 0.7 Nb 2 O 6 (abbreviated SBN30) nanopowders with tetragonal tungsten bronze structure. Attempts have been made to develop highly sensitive and flexible thermal/IR detector materials with SBN30/PU nanocomposites prepared in the form of cast films with varying the volume fraction of SBN30. SBN30 with particle size in the range 40–90 nm are prepared following an aqueous organic gel route. Pyroelectric coefficients of poled samples, determined by Byer and Roundy method, increase from 81 to 385 μC m −2 K −1 as the volume fraction of SBN30 increases from 0 to 0.25. The dielectric properties of the nanocomposites are measured with an Impedance analyzer in the frequency range 100 Hz–5 MHz. Thermal properties, thermal conductivity and specific heat capacity, are measured following photopyroelectric (PPE) technique. The measured properties are compared with corresponding ones predicted by mean field theory. The figures of merit for the material to act as a thermal/infrared detector, current sensitivity F I , voltage responsivity F V and detectivity F D are found to increase respectively from 48 to 283, 6.83 to 20.99 and 191to 792, all in units of 10 −3 μC m/J, as the volume fraction of SBN30 increases from 0 to 0.25. In order to understand the corresponding change of flexibility, Shore A hardness for each composite has been measured and found to increase from 40 to 62. It is evident that improvement in pyroelectric properties is at the expense of flexibility of the composite films. These composites hold promise as potential materials to develop moldable thermal/IR detectors.

ChemInform Abstract: Synthesis, Structure and Photoluminescence Properties of Ho3+ Doped TTB—BaTa2O6 Phosphors.

AbstractPure and Ho3+ doped TTB‐BaTa2O6 phosphors (TTB: tetragonal tungsten bronze structure) are prepared by sintering stoichiometric amounts of BaCO3, Ta2O5, and 0.5—20 mol% Ho2O3 in air (alumina crucible, 1425 °C, 20 h, ≈91% yield).

On the energetics of cation ordering in tungsten-bronze-type oxides.

Oxides with the tetragonal tungsten bronze (TTB) structure are well-known ferroelectrics that show a large flexibility both with respect to chemical composition and cation ordering. Two of the simplest compounds in this family are lead metaniobate (PbNb2O6 or PN) and strontium barium niobate (SrxBa1-xNb2O6 or SBN). While PN is a classical ferroelectric, SBN goes from ferroelectric to relaxor-like with increasing Sr content, with a polar direction different from that in PN. The partially occupied sublattices in both systems give the possibility for cation order-disorder phenomena, but it is not known if or how this influences the polarization and ferroelectricity. Here, we use density functional theory (DFT) calculations to investigate how cation and cation vacancy ordering influences the energetics of these compounds, by comparing both the energy differences and the barriers for transition between different cation configurations. We extend the thermodynamic model of O'Neill and Navrotsky, originally developed for cation interchange in spinels, to describe the order-disorder phenomenology in TTB oxides. The influence of order-disorder processes on the functional properties of PN and SBN is discussed.

Phase transitions in fluoride KFe2F6 with tetragonal tungsten bronze structure

Heat capacity, thermal dilatation, structure, Mössbauer spectra and dielectric permittivity of fluoride KFe2F6 with tetragonal tungsten bronze crystal structure were studied. The as-made sample undergoes two structural phase transitions P4/mbm (T1≈340K)→Pbam (T2≈250K)→G2 and magnetic phase transformation at Tm≈133K. Heating up to 600–700K and subsequent cooling in helium atmosphere leads to a change of phase transition temperatures and diffused anomalies in thermal expansion. The results obtained and influence of thermal prehistory of the sample on its physical properties are discussed in the context of previous studies on related fluorides KFe2F6, which sometimes suggest conflicting structural details.

Synthesis and X-ray powder diffraction data of Ba2.64Ta11.25O30.81

The Ba2.64Ta11.25O30.81was prepared by conventional solid-state reaction technique as a single phase. It was found that the compound crystallizes in the tetragonal system, space groupP4/mbm(No. 127) and unit-cell parameters area = 12.508 59(8) Å,c = 3.912 81(2) Å,V = 612.218(7) Å3, andZ = 1. The crystal structure of the Ba2.64Ta11.25O30.81phase is found to be closely related to the tetragonal tungsten bronze structure type, comprising interstitial (TaO)+inclusions. Reference data were derived from the Rietveld analysis and reported here.

Crystallization and Properties of Strontium Barium Niobate-Based Glass–Ceramics for Energy-Storage Applications

The crystallization kinetics, phase development, and electric properties of Al2O3–SiO2–SrO–BaO–Nb2O5–ZnO glass–ceramics were investigated for potential application of the materials for energy storage. Strontium barium niobate (Ba x Sr1−x Nb2O6) with the tetragonal tungsten-bronze structure was the major crystalline phase formed by both surface and bulk crystallization. The presence of ZnO made the glasses less stable, and thus promoted their crystallization, but had no significant effect on the microstructure of the resulting glass–ceramics. All glass–ceramic samples had a uniform microstructure, with a crystal size of approximately 50 nm. Optimized energy storage density of approximately 6.0 J/cm3 was achieved for the sample containing 0.5% ZnO; the average dielectric constant was 150–180 and the breakdown strength was 950–870 kV/cm over the temperature range 850–950°C.

Ordered structure and thermal expansion in tungsten bronze Pb₂K(0.5)Li(0.5)Nb₅O₁₅.

The crystal structure and thermal expansion behaviors of a new tetragonal tungsten bronze (TTB) ferroelectric, Pb2K(0.5)Li(0.5)Nb5O15, were systematically investigated by selected-area electron diffraction (SAED), neutron powder diffraction, synchrotron X-ray diffraction (XRD), and high-temperature XRD. SAED and Rietveld refinement reveal that Pb2K(0.5)Li(0.5)Nb5O15 displays a commensurate superstructure of simple orthorhombic TTB structure at room temperature. The structure can be described with space group Bb2₁m. The transition to a paraelectric phase (P4/mbm) occurs at 500 °C. Compared with Pb2KNb5O15 (PKN), the substitution of 0.5K(+) with small 0.5Li(+) into PKN causes the tilting of NbO6 octahedra away from the c axis with Δθ ≈ 10° and raises the Curie temperature by 40 °C, and the negative thermal expansion coefficient along the polar b axis increases more than 50% in the temperature range 25-500 °C. We present that, by introduction of Li(+), the enhanced spontaneous polarization is responsible for the enhanced negative thermal expansion along the b axis, which may be caused by more Pb(2+) in the pentagonal caves.

Photoluminescence and electrical characterization of unfilled tetragonal tungsten bronze Ba4La1 − xEuxTiNb9O30

Unfilled tetragonal tungsten bronze (TTB) structure Ba4LaTiNb9O30 doped by Eu3+ (BLTN: Eu3+x) with different x have been prepared, and their structural, photoluminescence, dielectric, and ferroelectric properties are carefully investigated in this work. Bright red emission, originating from 5D0→7F1 and 5D0→7F2 transitions of Eu3+ ions, has been observed by naked eyes at room temperature under near ultraviolet (NUV) light excitation. Optimized emission intensity is obtained when x=1.00 for present unfilled TTB-type BLTN: Eu3+x samples. Furthermore, with increasing x, the dielectric and ferroelectric characteristics of the unfilled TTB-type BLTN: Eu3+x samples also display remarkable variation. When x≥0.50 relaxor-like ferroelectric phase transitions are detected above room temperature, it is believed that unfilled TTB-type BLTN: Eu3+x=1.00 involving bright photoluminescence and enhanced ferroelectric properties may act as a potentially multifunctional optical-electro material.

Dielectric and ferroelectric properties of Sr4CaSmTi3Nb7O30 with tetragonal tungsten bronze structure

Sr4CaSmTi3Nb7O30 ceramics are synthesized and indexed as tetragonal tungsten bronze structure. The dielectric behavior and ferroelectric nature are investigated. Three dielectric anomalies are observed. The phase transition is a displacive phase transition with some diffusive characteristics, which indicates possible compositional variations within the materials on the microscopic scale. The weak distortion disappears in cooling process for differential scanning calorimetry measurement, and the large depression of Curie—Weiss temperature T0 indicates the difficulty in forming macroferroelectric domain. The ferroelectric nature in these filled tungsten bronze niobates originates from the off-center displacement of B-site cations, but they are primarily dominated by A-site cation occupation. Both the radius and the valence of A1-site cations play an important role on ferroelectric properties of the filled tungsten bronze compounds. Existence of spontaneous polarization with a remanent polarization of 0.16 μC/cm2 a coercive field of Ec = 11.74 kV/cm confirms the room-temperature ferroelectric nature of Sr4CaSmTi3Nb7O30 ceramics.

Effect of Ce doping on the electrocaloric effect of SrxBa1−xNb2O6 single crystals

The electrocaloric effect (ECE) of SrxBa(1−x)Nb2O6 (SBN100x) single crystals, with a tetragonal tungsten bronze structure and a high ECE near room temperature, is studied by direct measurements. It is shown that although the onset of the ECE peak is closer to room temperature in SBN80 than in SBN75, the effect of the increase of the strontium content is very detrimental to the ECE performances with a decrease to ΔTEC = 0.23 K for SBN80 from the reported value of ΔTEC = 0.42 K under 10 kV/cm for SBN75 [F. Le Goupil et al., “Anisotropy of the electrocaloric effect in lead-free relaxor ferroelectrics,” Adv. Energy Mater. (published online)]. However, when 1.40% of cerium is introduced in SBN61, the temperature of depolarisation is shifted below 30 °C, while an ECE above 0.6 K is maintained over more than 70 K for a low electric field of 28 kV/cm. The maximum ECE ΔTEC = 0.85 K is measured at 61 °C. In addition to having an ECE peak close to room temperature, the ECE measured in Ce-doped SBN61 is comparable with the best reported values for lead-free materials [Y. Bai et al., J. Appl. Phys. 110, 094103 (2011); X.-S. Qian et al., Adv. Funct. Mater. 24, 1300 (2014)], when linearly extrapolated to higher electric fields.