Tetragonal Tungsten Bronze-type Structure Research Articles

Highly efficient lithium container based on non-Wadsley-Roth structure Nb18W16O93 nanowires for electrochemical energy storage

The tungsten niobium oxides that belong to the well-known Wadsley-Roth shear structure have been already studied as the high-performance anode materials over the past several years since these materials can provide the additional vacuum site for accommodating lithium ions. Now, a novel anode material Nb18W16O93 which has a tetragonal tungsten bronze type structure is firstly reported in this paper. This material exhibits the good electrochemical performance. Upon the modification of its surface morphology, the electrochemical performance for Nb18W16O93 nanowire has been greatly improved. Nb18W16O93 nanowire shows the reversible specific capacity of 195 mAh g−1 when the current density is 100 mA g−1 and it can reach 153 mAh g−1 even at 1000 mA g−1. Meanwhile, the lithiation/delithiation mechanism and its corresponding structural change during the charge and discharge process are also carried out by an in-situ XRD technique. The result shows that Nb18W16O93 nanowire has excellent electrochemical reversibility and structural stability. Consequently, Nb18W16O93 nanowire can be employed as a novel anode material for lithium ion batteries.

The local cationic configurations in the K2Sr4Nb10O30 lattice and peculiarities of its surface piezoresponse

ABSTRACTBasing on the X-ray diffraction studies, a possibility of six local cationic configurations was established for K2Sr4Nb10O30 (KSN) having a tetragonal tungsten bronze-type structure. The association of these locally charged configurations may lead to the formation of the electrically active nanoregions. The piezoresponse force microscopy studies revealed at the KSN surface the presence of a subsystem of nano- or meso-size regions, which may be electrically active. Distribution and reference size of these regions have been estimated using the Fourier filtering of the source piezoresponse image.

Structural and electrical properties of K3Li2Nb5O15 thin film grown by pulsed laser deposition

Potassium lithium niobate K3Li2Nb5O15 (KLN) with tetragonal tungsten bronze-type structure (TTB) thin film was successfully deposited on MgO substrate by pulsed laser ablation method. The substrate was coated by a perovskite conductive layer of La0.5Sr0.5CoO3. X-ray diffraction revealed (001) orientation with low mosaicity. The oriented film had smooth and droplets-free surface as evidenced by reflection high electron energy diffraction and atomic force microscopy. Using Pt top electrode, dielectric measurements indicated stable dielectric permittivity of about 400 up to 105Hz. A dielectric relaxation was also observed. Moreover, the capacitance voltage measurements of the obtained (001)KLN film show ferroelectric behavior. Our results provide useful insights into the growth of tetragonal tungsten and other complex materials. These results will be useful for the growth and integration of epitaxial TTB thin films on perovskite and Si substrates.

Bipolar resistive switching and substrate effect in GdK2Nb5O15 epitaxial thin films with tetragonal tungsten bronze type structure

Ferroelectric compound GdK2Nb5O15 with tetragonal tungsten bronze type structure thin films were grown by pulsed laser deposition on (001)SrRuO3/SrTiO3 and (001)SrRuO3/LSCO/MgO substrates respectively, in the same experimental conditions. Structural properties were investigated using X-ray diffraction in different modes. θ–2θ diffraction shows a single crystalline SRO and GKN phases, with c-axis perpendicular to the substrate. Phi-scans show for both SRO and GKN an epitaxial cube-on-cube growth. Reciprocal space mapping (RSM) was performed on GKN to determine the in-plane lattice parameters, and showed that GKN grown on MgO is more constrained comparing to STO. Room temperature electrical properties were investigated using platinum (Pt) top electrodes. Results show that GKN thin films deposited on MgO and STO substrates are resistively switchable. It was found that the nature of the substrate affects strongly the resistance ratio: GKN on SRO/LSCO/MgO showed a large hysteresis comparing to GKN on SRO/STO. Resistance switching in GKN is attributed to the oxygen vacancies migration which can be controlled by the substrate or the frequency sweep.

X-ray and dielectric studies of hot-pressed K2Sr4Nb10O30 ceramics

ABSTRACTX-ray diffraction (XRD) and dielectric studies of dense hot-pressed K2Sr4Nb10O30 ceramics with a tetragonal tungsten bronze–type structure revealed texturing of the samples and substantial anisotropy of the dielectric permittivity. XRD studies of the K2Sr4Nb10O30 powder show a diffuse phase transition in the 140 – 160°C range while a diffused and frequency-independent maximum of the dielectric permittivity is observed at about 120–125°C for ceramic samples. Substantial anisotropy of the dielectric permittivity (more than 50% in the phase transition region) as well as sample's texturing and a difference between phase transition temperatures of the powder and ceramic samples are supposed to be caused by anisotropic residual stresses.

Investigation of diffuse phase transition in ferroelectric Pb2−x K1+x Li x Nb5O15 (0 ≤ x ≤ 1.5) ceramics

Substitution of Pb with Li and K in the Pb2KNb5O15 phases leads to a new composition with chemical composition Pb2−xK1+xLixNb5O15 which crystallizes with tetragonal tungsten bronze-type structure. Ferroelectric ceramics with different compositions were synthesized using solid-state reaction and complex dielectric permittivity measurements in these compounds were performed in a frequency and temperature range of 20 Hz–1 MHz and from 25 to 550 °C, respectively. Special attention was paid to the diffuse phase transition (DPT) that occurs close to the Curie temperature. The empirical equation proposed by Santos–Eiras for a phenomenological description of the temperature dependence of the dielectric permittivity (\(\varepsilon_{\text{r}}^{{\prime }}\)) peak is used to calculate some characteristic parameters of DPT. From the results, it must be assumed that these compounds show a diffuse phase transition with non-relaxor behavior. A basic phase diagram showing the evolution of Tm function of composition x is deduced from this study.

In-depth structure characterization and properties of (1−x)(Li0.05Na0.475K0.475)(Nb0.95Sb0.05)O3−xBiFeO3 lead-free piezoceramics

(1−x)(Li0.05Na0.475K0.475)(Nb0.95Sb0.05O3)−xBiFeO3 ceramics were prepared by the conventional sintering method. The microstructure, phases and their relative volume fractions were investigated in detail by using transmission electron microscopy and X-ray diffraction. Fullprof and Topas softwares were also used to refine the X-ray diffraction data to confirm the crystal structure. The refinement results show that for x ≤ 0.008, the ceramics consists of the orthorhombic and tetragonal perovskite phases, and the fraction of the orthorhombic phase decreases from 50.66 % for x = 0.0 to 40.62 % for x = 0.008. When x = 0.01, the ceramics has a mixed microstructure with three coexisting phases: the orthorhombic, tetragonal perovskite phases and tetragonal tungsten bronze-type structure phase, and the fraction of the tetragonal tungsten bronze-type structure phase is 6.72 %. The selected area electron diffraction data is also consistent with the space groups for all of the above phases. This system also exhibits good ferroelectric and piezoelectric properties for applications, for example, at x = 0.004, the ceramic samples have a piezoelectric constant (d 33) of 272 pC/N and remanent polarization (P r) of 20.6 μC/cm2.

Resistive switching in a (0 0 ℓ)-oriented GdK2Nb5O15 thin film with tetragonal tungsten bronze type structure

We have grown a single oriented GdK2Nb5O15 ferroelectric thin film with tetragonal tungsten bronze (TTB) structure by pulsed laser deposition on SrRuO3/La0.5Sr0.5CoO3 bi-buffered MgO substrate. GKN thin film exhibits a single phase and (00ℓ)-orientation with tall and narrow asperities. We study as the first time the resistive switching in a TTB thin film. Room temperature electrical properties were investigated in the capacitance shape, using Pt top electrodes, and revealed a significant current hysteresis making GdK2Nb5O15 as a potential candidate for resistive memory devices.

Synthesis, structure and photoluminescence properties of tetragonal tungsten bronze-type Eu3+-doped K2LaNb5O15 niobate phosphor

A series of red-emitting Eu3+-doped K2LaNb5O15 niobate phosphors with tetragonal tungsten bronze-type structure were synthesized by the solid state reaction method. The structure of new tungsten bronze-type K2EuNb5O15 was identified by Rietveld refinements to crystallize in the tetragonal space group P4bm, which is isostructural to K2LaNb5O15. The relative intensity of the emission from both 5D0→7F2 and 5D0→7F1 increases with increasing Eu3+ ion concentration. The quenching mechanism of the K2La1−xNb5O15:xEu system was investigated in detail. The relationship between Photoluminescence properties and the TTB-type structure was discussed. The quantum efficiency of K2EuNb5O15 excited under near-UV light (399nm) is 132% of that of commercial Y2O3:Eu3+ (394nm). The results demonstrated K2EuNb5O15 phosphor as a potential candidate for white light emitting diode pumped by near-UV or blue chip.

Nanorods of Potassium Tantalum Niobate Tetragonal Tungsten Bronze Phase Grown by Pulsed Laser Deposition

K–Ta–Nb–O tetragonal tungsten bronze phase was grown on (1102) Al2O3 (R-plane sapphire) by pulsed laser deposition. The microstructure, structure, and chemical composition of the deposit were studied by scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectrometry, and transmission electron microscopy. The crystal structure was solved by precession electron diffraction as being tetragonal tungsten bronze-type structure with space group P4/mbm, refined cell parameters a = 12.537 ± 0.003 A, c = 3.975 ± 0.001 A, and composition K5.06(Ta0.57Nb0.43)10.99O30. The tetragonal potassium tantalum niobate growth follows two modes with respect to the substrate surface: (i) as single vertical right parallelepiped-shaped nanorods (50 to 100 nm wide and up to 1 μm in length) along the [001] direction and (ii) as in-plane attached crystals along the ⟨310⟩ direction. These two growth modes are understood as being governed by the plane termination of the substrate. This new phase is of potential in...

Removal of iron from BaTa2O6 ceramic powder produced by high energy milling

In order to remove Fe coming from the milling environment, mechanochemically produced BaTa2O6 ceramic powder was leached with 6 and 12 M HCl at 95 °C and 105 °C for time periods varying between 1 and 15 h. Amount of Fe in leached samples was determined by EDS and XRF. Fe concentration decreased from 10.36% to 1.97–3.16% after leaching indicating a 70–81% decrease in iron. Leaching temperature had a more pronounced effect on iron removal than concentration. Leaching caused the formation of porosities and disintegration of powder which accelerated the iron removal. X-ray diffraction analysis of leached milled powders heat treated between 1000 and 1450 °C for 5 h indicated that although iron related phases were formed above 1000 °C, single BaTa2O6 phase was obtained at 1425 °C having tetragonal tungsten bronze type structure. A possible liquid phase formation above 1300 °C led to an increase in grain size and density.

Incommensurate modulation of calcium barium niobate (CBN28 and Ce:CBN28)

The incommensurately modulated crystal structures of Ca(0.28)Ba(0.72)Nb(2)O(6) (CBN28) and Ce(0.02)Ca(0.25)Ba(0.72)Nb(2)O(6) (Ce:CBN28) were refined in the supercentred setting X4bm(AA0,-AA0) of the 3 + 2-dimensional superspace group P4bm(aa½,-aa½). Both compounds are isostructural with a tetragonal tungsten bronze-type structure. The modulation of CBN28 consists of a wavy distribution of Ba and Ca atoms as well as vacancies on the incompletely occupied Me2 site with 15-fold oxygen coordination. The occupational modulation is coupled with a modulation of the atomic displacement parameters and a very weak modulation of the positional parameters of Me2. The surrounding O atoms show strong displacive modulations with amplitudes up to ca 0.2 Å owing to the cooperative tilting of the rigid NbO(6) octahedra. The Me1 site with 12-fold coordination and Nb atoms are hardly affected by the modulations. Only first-order satellites were observed and the modulations are described by first-order harmonics. In Ce:CBN28 cerium appears to be located on both the Me2 and Me1 sites. Wavevectors and structural modulations are only weakly modified upon substitutional incorporation of 0.02 cerium per formula unit of calcium.

Growth, characterization and upconversion properties of erbium-doped potassium lithium tantalate niobate single crystals under 975 nm laser excitation

Potassium lithium tantalate niobate single crystals doped with erbium ions are grown by top-seeded solution growth method. The crystals are characterized by X-ray diffraction and differential thermal analysis. The refractive indices of the crystal are measured using ellipsometry method and fitted by Sellmeier equation. The as-grown crystals are tetragonal phase tungsten bronze-type structure with Curie temperature of 271.3°C. Characteristic Er3+ absorption bands are observed from 350 to 1100nm in ultraviolet–visible-near infrared absorption spectra. These crystals emit brightly green and red upconversion fluorescence under 975nm LD laser excitation, and the steady state upconversion spectra are obtained at room temperature. The red emission intensity increases as the erbium ions concentration increases in crystals. Processes of excited state absorption and energy transfer are responsible for upconversion luminescence. The emission intensities are quadratic dependences on pump power from pump power dependence analyses and deduction of transition rate equation model.

Ceramics With Tetragonal Tungsten Bronze Type Structure For Textured Ceramics-Polymer Composites

Textured Sr0.7Ba0.3Nb2O6-PVDF (SBN70-PVDF) composites were fabricated. Powders of strontium barium niobate Sr0.7Ba0.3Nb2O6 (SBN70) were prepared by sol-gel method. The composite surface images were obtained by AFM tapping mode (NT-MDT Solver P47). The pyroelectric properties have been studied. The obtained composite is expected to be promising material for pyroelectric detectors.

Spectral studies on erbium doped potassium lithium tantalate niobate single crystal grown by the step-cooling technique

An erbium doped K0.603Li0.397Ta0.428Nb0.572O3 single crystal was grown by the step-cooling technique. The crystal has a tetragonal tungsten bronze-type structure at room temperature with a Curie temperature of 303°C. There are Er ions characteristic absorption bands around 449, 485, 521, 550, and 652 nm in the visible absorption spectrum. Upconversion fluorescence spectra and power dependence centered at 527 nm, 548 nm, and 660 nm under 975 nm excitation were measured at room temperature. Decay lifetimes of the 548 nm and 660 nm emission bands are 281 μs and 420 μs, respectively. The lifetime of the 548 nm emission corresponding to the transition of 4S3/2→4I15/2 is ten times the lifetime of the same transition of Er3+ in LiNbO3 crystal and twice in KYb(WO4)2 crystal. The crystal might become a promising upconversion laser material. The upconversion mechanism of Er3+ in the sample was discussed based on decay curves and pump power dependence analyses in this work.

Negative thermal expansion change of different stoichiometric proportions calcium-doped strontium barium niobate crystals

The polarization Raman spectra were measured for the calcium-doped strontium barium niobate crystals ((Ca 0.28Ba 0.75) x(Sr 0.60Ba 0.40) 1 − x Nb 2O 6 ( x = 0.25,0.50 or 0.75, CSBN) to determine the negative thermal expansion(NTE) change induced by different stoichiometric proportions calcium-doped concentration. The different count-intensity (CI) of Raman peaks with the scattering geometry Z( XY) Z­, corresponding to the symmetry species B 2, are attributed to the different vacancy rate(VR) in A2 sites of tetragonal tungsten bronze-type structure, which can result in the change of the short-range molecular force (SRMF) between ions filled in A2 sites and Nb–O octahedron. Since the thermal properties of materials direct responding to the vibration of crystal lattices, the NTE change of CSBN crystals are considered as arising due to the geometry change for the different calcium-doped concentration.

Effect of CASP glass doping on sintering and dielectric properties of SBN ceramics

16CaO–29Al 2O 3–34SiO 2–13PbO–4B 2O 3–2ZnO–2P 2O 5 (CASP) glass doped-Sr 0.5Ba 0.5Nb 2O 6 (SBN50) ceramics have been synthesized by solid-state ceramic route. The effects of CASP glass on the firing, microstructure and dielectric characterization of SBN50 ceramics are investigated. The densities of the ceramic samples firstly increase and then slightly decrease with increasing CASP glass content. The appropriate amount of doping glass is 2%. The SBN50 ceramics doped with CASP glass can be sintered at a relatively low temperature, 1200 °C. X-ray diffraction analysis shows the single phase (tetragonal tungsten bronze type structure) is preserved for all the samples. The diffuse character of the ceramic system increases and the dielectric constant at phase transition temperature ( T c) markedly decreases as CASP glass content increases. Interestingly, the CASP glass addition drastically alters the microstructure of the sintered ceramics. The isotropic grains in the pure SBN50 ceramics transform to rod like grains after the addition of CASP glass. The grain size of SBN phase is found to obviously increase with increase in CASP glass doping level.

Selective oxidation of C 3–C 4 olefins over Mo-containing catalysts with tetragonal tungsten bronze structure

Mo–V–Nb–P–O-based catalysts with a tetragonal tungsten bronze-type (TTB) structure have been prepared hydrothemally from a H 3PMo 12O 40 Keggin-type heteropolyacid. These catalysts have been tested in the oxidation of C 3–C 4 olefins (propene, isobutene and 1-butene). Although the catalytic performance depends on the nature of the olefin fed the TTB-type catalysts prepared in the presence of elements of the V and VI groups such as Te, Sb and Bi have shown a high selectivity to partial oxidation products, especially that with Te. However, in the absence of these elements the TTB-catalysts present a high catalytic activity to deep oxidation. The selectivity to partial oxidation products decreases in the order: MoVNbPTe- > MoVNbPSb- > MoVNbPBi- > MoVNbP-TTB catalysts. The reaction products obtained in the oxidation of each olefin will be discussed according to their corresponding reaction mechanism and the characteristics of catalysts.

Crystal structures of KBa 2Nb 5O 15 and LaK 2Nb 5O 15 with the TTB-type structure

Single crystals of two niobates, KBa 2Nb 5O 15 and LaK 2Nb 5O 15, were synthesized by high-temperature reaction and the crystal structures were determined by single crystal X-ray diffraction data. Although the space groups for these compounds were different (the non-centrosymmetrical space group P4 bm (#100) for KBa 2Nb 5O 15 and the centrosymmetrical one P4/ mbm (#127) for LaK 2Nb 5O 15), both compounds had the same tetragonal tungsten bronze-type (hereafter TTB-type) structure. The lattice parameters and R-factors of KBa 2Nb 5O 15 (LaK 2Nb 5O 15) were a = 12.533(2) (12.563(2)) and c = 4.0074(9) (3.9179(9)) Å, and R 1 = 0.040 (0.047) and w R 2 = 0.131 (0.120), respectively. From the crystal structural analysis, it was clarified that distribution of two large cations was different from each other in the way that K and Ba atoms in KBa 2Nb 5O 15 were distributed statistically at two crystallographic sites and K and La atoms in LaK 2Nb 5O 15 were ordered.

Grain Growth of Microtubes During Sintering in Semiconducting Ba3Ce3−xBixTi5Nb5O30 (x=0.5, 1.0, 2.0, and 3.0) Ceramics

Well grown microtubes of Ba3Ce3−xBixTi5Nb5O30 (x=0.5, 1.0, 2.0, and 3.0) have been observed in the sintered ceramics. The microtubes have octagonal shapes at the cylindrical ends, hollow cores with diameters of 2–3 μm, lengths over a few tens of micrometers and wall thickness of 0.5–1.0 μm. In morphology, the tubes are very straight and are apparently welded to each other on the sides. The tubular growth as well as increase of wall thickness is observed to be more with greater Bi content, but when Ce is fully substituted by Bi, the tubes have grown into solid rods without hollowness. The growth spirals at the ends of tubes as seen by scanning electron microscope indicate that growth takes place in the longitudinal direction of the tubes. Observation of growth spirals on grains of a sintered ceramic is unusual. In fact, such features are usually seen in liquid phase growth and, therefore, we propose here grain growth in partially fluid conditions. The phase identification by X‐ray diffraction shows that the compound has a tetragonal tungsten bronze type structure. The resistivity measurements above room temperature indicate that these compounds are semiconductors and both resistivity and activation energy increase with Bi content.