Tungsten Bronze-type Structure Research Articles

Low Temperature Niobium Tungsten Oxides as Anodes for Fast Charging Li-Ion Batteries

The development of fast-charging Li-ion batteries is one of the important steps in building electric vehicles that need high power and fast charging. The reduction in the particle size to the nanometre length scale drastically improves the charging rate by increasing the surface area but lowers the volumetric energy density and can potentially result in more rapid degradation of the electrolyte via electrode-catalysed reactions. Recent reports show that niobium tungsten oxides (NbWOs) with µm sized particles serve as good anode materials for Li-ion batteries having high volumetric capacity with good cyclability at high rates while avoiding solid electrolyte interphase formation (SEI). The high rate performance of NbWOs is attributed to the topochemical charge storage mechanism that exhibits (i) minimal, highly reversible structural changes during the charge-discharge cycles, (ii) high Li-ion diffusion coefficients due to the presence of fast conducting tunnels, and (iii) high electronic conductivity upon lithiation.1,2 The NbWOs are known to crystallize into two families of different crystal structures depending on the composition and synthesis conditions. The NbWOs phases can either have tungsten bronze-type structures or block-type structures (commonly referred to as Wadsley-Roth phases). Different NbWO phases are reported in the literature3 which could serve as potential anode materials with better performance.In this work, a series of new NbWO phases have been synthesized at reduced annealing temperatures and the structure of these phases has been established using X-ray and neutron diffraction data. The electrochemical properties of NbWO/Li half cells were examined through a series of galvanostatic (dis)charge tests at varying current rates ranging from 0.2 C to 20 C. The NbWO phases delivers a high reversible capacity of 200 and 130 mAh/g at 0.2C and 10 C respectively vs. Li metal. Here we use operando powder X-ray diffraction to investigate the structural changes occurring at the anode during the charge-discharge process in the newly synthesised NbWO phases. Reference s : 1 K. J. Griffith, K. M. Wiaderek, G. Cibin, L. E. Marbella and C. P. Grey, Nature, 2018, 559, 556–563.2 C. P. Koçer, K. J. Griffith, C. P. Grey and A. J. Morris, Chem. Mater., 2020, 32, 3980–3989.3 N. C. Stephenson, Acta Crystallogr. Sect. B Struct. Crystallogr. Cryst. Chem., 1968, 24, 637–653. Figure 1

Designing rare earth-free high entropy oxides with a tungsten bronze structure for thermoelectric applications.

In recent years, forming high entropy oxides has emerged as one of the promising approaches to designing oxide thermoelectrics. Entropy engineering is an excellent strategy to improve thermoelectric performance by minimizing the thermal conductivity arising from enhanced multi-phonon scattering. In the present work, we have successfully synthesized a rare-earth-free single phase solid solution of novel high entropy niobate (Sr0.2Ba0.2Li0.2K0.2Na0.2)Nb2O6, with a tungsten bronze structure. This is the first report on the thermoelectric properties of high entropy tungsten bronze-type structures. We have obtained a maximum Seebeck coefficient of -370 μV K-1 at 1150 K, which is the highest among tungsten bronze-type oxide thermoelectrics. The minimum thermal conductivity of 0.8 W m-1 K-1 is obtained at 330 K, which is so far the lowest reported value among rare-earth-free high entropy oxide thermoelectrics. This synergistic combination of large Seebeck and record low thermal conductivity gives rise to a maximum ZT of 0.23 which is so far the highest among rare-earth free high entropy oxide-based thermoelectrics.

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.

Tungsten-titanium mixed oxide bronzes: Synthesis, characterization and catalytic behavior in methanol transformation

Tungsten oxide bronze-based materials show extremely adaptive structural and compositional features that make them suitable for functional properties modulation. Herein we report the preparation of a series of Ti-containing tungsten oxide catalysts presenting a hexagonal tungsten bronze-type structure. The insertion of Ti4+ within the structure (likely in the octahedral framework of the hexagonal tungsten bronze) leads to an increase in the number of strong acid sites, and the disappearance of W5+ surface species found in the undoped tungsten oxide. With the aim of studying the acid-redox properties of the titled catalysts, the catalytic transformation of methanol has been carried out in the presence and the absence of O2 in the feed. Both catalytic activity and the acid-redox properties of these catalysts are highly dependent on catalyst composition and reaction conditions applied (i.e. in the presence or in the absence O2 in the feed). Aerobic experiments show the depletion of the redox functionality (i.e. no formaldehyde detected in the products) when Ti4+ is incorporated in the framework (i.e. 100% selectivity to dimethyl ether). On the other hand, all the catalysts show the loss of the redox function and a decrease in the catalytic activity when anaerobic conditions are used. In the absence of oxygen, the catalysts are still active in the dehydration of methanol to dimethyl ether, i.e. they maintain their acid functionality even when oxygen is not present in the feed. The results are discussed in terms of the available surface active sites present in each case.

Tewite: A K–Te–W new mineral species with a modified tungsten-bronze type structure, from the Panzhihua-Xichang region, southwest China

Tewite: A K–Te–W new mineral species with a modified tungsten-bronze type structure, from the Panzhihua-Xichang region, southwest China

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.

Microstructures and Energy Storage Properties of Sr0.5Ba0.5Nb2O6 Ceramics with BaO–B2O3–SiO2 Glass Addition

Sr0.5Ba0.5Nb2O6 (SBN) ceramics with the BaO–B2O3–SiO2 (BBS) glasses have been successfully produced for high energy-storage density capacitor applications. The phase evolution, microstructures, dielectric properties and energy storage properties of the SBN-based glass ceramics were investigated. X-ray diffraction analysis revealed all ceramic samples were of tetragonal tungsten bronze-type structures. As the mass ratio of glass in the glass ceramics increased from 0.1wt.% to 10 wt.%, the dielectric constant of the glass ceramics increased initially and then decreased with the increasing glass content. Compared to pure Sr0.5Ba0.5Nb2O6 ceramics, the glass ceramics with a small amount of BBS glass addition illustrated better energy storage properties. The glass-ceramic with 0.5wt. % BBS glass addition showed the highest breakdown strength of 194 kV/cm and a maximum energy storage density of 0.286 J/cm3.

Preparation and characterization of Sr0.5Ba0.5Nb2O6 glass-ceramic on piezoelectric properties

We studied the influence of heat treatment time on the optical, thermal, electrical, and mechanical properties of strontium barium niobate (Sr1−xBaxNb2O6; hereafter SBN) piezoelectric glass-ceramics with tungsten bronze-type structure, which have good piezoelectric properties and are important lead-free piezoelectric materials. We found that the best heat treatment time is 4 h. The properties of the prepared materials are better than that of SBN ceramics and the glass-ceramic growth is faster than the SBN crystal when the heat treatment time of the SBN piezoelectric glass-ceramic is controlled, reducing the preparation costs greatly.

Phase transitions in relaxor ferroelectric materials with tungsten bronze type structures

Phase transitions in relaxor ferroelectric materials with tungsten bronze type structures

Microwave dielectric properties and microstructure of Ba6−3xNd8+2xTi18−y(Cr1/2Nb1/2)yO54 ceramics

The microwave dielectric properties and the morphology of Ba6−3xNd8+2xTi18−y(Cr1/2Nb1/2)yO54 (x = 0.75, 0 ≤ y ≤ 3.0) ceramics prepared under different sintering conditions were investigated in this work. The effects of substitutions on the microstructure and microwave dielectric properties were discussed. The X-ray diffraction (XRD) patterns of the sintered samples revealed a single-phase formation BaNd2Ti4O12 with a tungsten bronze type structure in the system. The results of energy dispersive spectrometer (EDS) and lattice parameters calculated on XRD data could confirm the substitution at B-site. A small amount of substitutions improved quality factor value (Q × f) and the temperature coefficient of resonant frequency (τf) but led to a decrease of the permittivity. The temperature coefficient of resonant frequency (τf) was found to decrease with increasing substitutions because of the declination of tolerance factor (t). And the τf could be adjusted from +62.4 ppm/°C to −7.3 ppm/°C with increment of substitutions. Finally, excellent dielectric properties were obtained as y was 0.5 sintered at 1400 °C for 2 h in air: εr = 88.6, Q × f = 11486 GHz, τf = +37.1 ppm/°C.

Fabrication of hexagonal/cubic tungsten oxide homojunction with improved photocatalytic activity

A homojunction with different phases of tungsten oxide photocatalyst was prepared via a hydrothermal method by adjusting the pH value. The structural analysis showed the homojunction consisted of tungsten bronze-type (hexagonal) and pyrochlore-type (cubic) structures. The rod-like hexagonal phase inserted into block cubic phase to form a well-defined homojunction structure under the proper pH condition. The activity of photocatalyst was tested through degradating RhB and water oxidation. The results indicated the homojunction photocatalyst displayed a higher photocatalytic activity compared with individual tungsten oxide under visible light and full sunlight spectra irradiation. The further investigations revealed the inner electric filed at the interface of the homojunction facilitated the separation of electron-hole pairs, resulting in the improved photoelectrochemical performance. The design of favorable tungsten oxide homojunction structure in the present work provided a promising method for the development of other efficient photocatalysts.

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.

Phase transition, microstructure, and dielectric properties of Li/Ta/Sb co-doped (K, Na)NbO3 lead-free ceramics

Li/Ta/Sb co-doped lead-free (K0.4425Na0.52Li0.0375)(Nb0.93−xTaxSb0.07)O3 (abbreviated KNLNSTx) piezoelectric ceramics, with Ta-doping ratio of x ranging from 0.0275 to 0.0675, were synthesized using the conventional solid-state reaction method at the sintering temperature of 1130°C. The effects of Ta content on the microstructure, dielectric properties, and phase transition behavior of the prepared ceramics were systematically investigated. The X-ray diffraction results show that all KNLNSTx ceramics formed a secondary phase, which is assigned to the tetragonal tungsten-bronze type (TTB) structure phase, and showed a phase transition from an orthorhombic symmetry to a tetragonal symmetry across a composition region of 0.0375<x<0.0475. The grain shape and size that correspond to the phase structure transformations can be clearly observed in the scanning electron microscopy images. As x increased to 0.0475, the KNLNST0.0475 ceramics changed from orthorhombic to tetragonal structure and showed excellent piezoelectric properties of d33=313pC/N, kp=47%, and εr=1825. By contrast, samples of x=0.0375 with orthorhombic symmetry exhibited poor piezoelectric properties, with d33=200pC/N and εr=1015. These results indicate that phase structure is vital in the piezoelectric properties of KNN lead-free ceramics.