Tungsten Bronze Type Research Articles

Thermal oxidation of oxynitride films as a strategy to achieve (Sr2Ta2O7)100-x(La2Ti2O7)x based oxide perovskite films with x = 1.65

•This study concerns STLTO compounds of the ferroelectric (Sr2Ta2O7)100-x(La2Ti2O7)x solid solution. The purpose is to produce the STLTO composition x = 1.65 as thin films by thermal oxidation of the corresponding oxynitride composition. Indeed, the combination of an STLTO oxide target with a dioxygen-rich reactive atmosphere during the sputtering deposition leads to Sr-deficient oxide thin films, shifting composition and structure from the perovskite to the tetragonal tungsten bronze type. An alternative synthesis pathway is to first deposit, under nitrogen-rich atmosphere, stoichiometric oxynitride films and produce, by thermal annealing under air, the stoichiometric oxide. For low oxidation temperatures ([550–600 °C]), samples remain intact and display an oxide character but still contain a significant amount of nitrogen, they could be described as intermediate phases containing nitrogen-nitrogen pairs as demonstrated by Raman. Dielectric characteristics of these original film materials are of interest with a tunability value of 26 % at 30 kV/cm (10 kHz).

Novel thermally stable, high quality factor Ba4(Pr0.4Sm0.6)28/3Ti18−yGa4y/3O54 microwave dielectric ceramics

AbstractHigh dielectric constant Ba4(Pr0.4Sm0.6)28/3Ti18−yGa4y/3O54 (BPSTG) (0 ≤ y ≤ 1) microwave ceramics with enhanced dielectric properties were synthesized via the traditional solid‐state processing for the first time. The crystal structure, microstructure, and microwave dielectric characteristics were investigated in details. X‐ray diffraction results showed that all well‐sintered samples belonged to a single tungsten bronze type phase. Results showed that partial substitution of Ga3+ for Ti4+ is an effective way in adjusting the temperature coefficient of resonant frequency (TCF) value of the Ba4(Pr0.4Sm0.6)28/3Ti18O54 ceramics and increased the quality factor (Qf) values. The variation tendency of dielectric constant was in accordance with the ionic polarizability. The Qf values highly depend on the grain size from the scanning electron microscopy results. A near zero TCF value can be obtained by carefully adjusting the substitution of Ga3+ content. Infrared reflectivity (FIR) spectra were employed to reveal the relationship between the phonons and microwave dielectric properties. At last, excellent combinations dielectric properties of εr = 78.5, Qf = 12,400 GHz, TCF = +2.1 ppm/°C were achieved in BPSTG ceramics when y = 0.75.

Spontaneous Non-stoichiometry and Ordering in Degenerate but Gapped Transparent Conductors

Spontaneous Non-stoichiometry and Ordering in Degenerate but Gapped Transparent Conductors

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

Features of the Crystal Structure and Vibrational Spectra of Ba1.65Sr3.35Nb10O30–Ba4Na2Nb10O30 Barium-Strontium Niobates with the Structure of Tetragonal Tungsten Bronze

The crystal structure and vibrational spectra of Ba1.65Sr3.35Nb10O30–Ba4Na2Nb10O30 barium-strontium niobates are investigated by neutron diffraction and Raman spectroscopy in the concentration range x = 0–1 with steps of 0.1 and 0.05. All compounds under study have a structure of the tetragonal tungsten-bronze type with the space group of P4mm symmetry. The concentration dependences of the crystal-cell parameters, interatomic bonds, and vibrational modes for a number of solid solutions of the (1–х)Ba1.65Sr3.35Nb10O30–хBa4Na2Nb10O30 system are obtained. Anomalies in the behavior of the crystal parameters and vibrational modes are observed for the concentration dependences at x ~ 0.5. This may be due to a change in the nature of the sodium occupation of vacancies in various crystallographic positions of the tetragonal structure.

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.

Suppression of Ti3+ generation in Ba3.75Nd9.5Ti17.5M0.5O54 (M = Cu, Cr, Al, Mn) ceramics

The suppression of Ti3+ generation in Ba3.75Nd9.5Ti18O54 (BNT) ceramic sintered at high temperature is investigated by substituting four metal ions M (M = Cu2+, Cr3+, Al3+, Mn4+) for Ti4+ at B-site. The XRD results show BNT ceramics substituted by M are formed as single tungsten-bronze type structured phase. All M substitutions have significant impact on the suppression of oxygen vacancies (VO••) and Ti3+ generation in BNT ceramic, which is confirmed by XPS results. BNTM ceramics have noticeable increase of Q×f values and decrease of positive τf values, but tiny decline in εr values. The SEM images of BNTM ceramics show a promotion of grain size by substituting M for Ti, which may be conducive to improvement on Q×f values.

Correlation between structures and microwave dielectric properties of Ba3.75Nd9.5-xSmxTi17.5(Cr1/2Nb1/2)0.5O54 ceramics

In this study, the influences of substitution of Sm3+ for Nd3+ on the crystal structure, Raman spectroscopy and microwave dielectric properties in Ba3.75Nd9.5Ti17.5(Cr1/2Nb1/2)0.5O54 (BNTCN) ceramics were investigated. The XRD results showed that all Ba3.75Nd9.5-xSmxTi17.5(Cr1/2Nb1/2)0.5O54 (BNTCN-Sx) samples were crystallized as the orthorhombic tungsten-bronze type like structure. The unit cell volume of samples decreased as x increased, and the blue shift of Raman spectral peaks (Ag(1) and Bg) also confirmed this result. Moreover, the increase of tilt angles indicated that more and more oxygen octahedra became more tilted, consequently leading to the decrease of temperature coefficient of resonant frequency (τf) with increase of x value. The packing fraction and Q×f value of BNTCN-Sx ceramics reached the maximum value, whereas the FWHM of Ag(1) and internal strain η reached the minimum value at x = 3. The low internal strain resulted from stability of the crystal structure, which was conducive to improvement of Q×f value in the BNTCN-Sx ceramics. At last the BNTCN-Sx ceramics sintered at 1380 °C for 4 h exhibited excellent microwave dielectric properties: εr = 83.6, Q×f = 11,597 GHz, and τf = +1.3 ppm/°C when x = 4.

Crystal structure, Raman spectroscopy and microwave dielectric properties of Ba3.75Nd9.5Ti18-z(Al1/2Nb1/2)zO54 ceramics

The influences of substitution of (Al1/2Nb1/2)4+ on the crystal structure, Raman spectroscopy and microwave dielectric properties in Ba3.75Nd9.5Ti18O54 (BNT) ceramics were investigated in this work. The sintering samples were mainly measured by SEM, XRD and Raman spectrometer. The results showed that all Ba3.75Nd9.5Ti18-z(Al1/2Nb1/2)zO54 (BNT-(AN)z) samples formed the orthorhombic tungsten-bronze type like structure. Lattice parameters of BNT-(AN)z samples were refined by Rietveld method, and unit cell volume of samples decreased as z increased. The blue shift of Raman spectral peaks was confirmed to the result of Rietveld refinement. Moreover, Raman spectra revealed that flexible oxygen octahedra networks became stressed-rigid and oxygen octahedra became more tilted, which was ascribed to Al3+ and Nb5+ occupational disorder when (Al1/2Nb1/2)4+ substituted for Ti4+. With increase of z value, the dielectric constant (εr) and temperature coefficient of resonant frequency (τf) decreased. The BNT-(AN)2 ceramic sintered at 1375 °C for 4 h exhibited excellent microwave dielectric properties: εr = 73.9, Q×f = 13,177 GHz, and τf = +0.3 ppm/°C. The relationship of the bond valence of the B-site to Raman shift of the Ag mode, dielectric constant (εr) and τf were analyzed.

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.

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.

Role of structural modulation in electrical properties of tungsten bronze (Ca0.28Ba0.72)2.5−0.5xNaxNb5O15 ceramics

(Ca0.28Ba0.72)2.5−0.5xNaxNb5O15 ceramics (CBNN, x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) were prepared by the conventional solid-state reaction method. Pure tungsten bronze structure could be obtained in all compositions according to X-ray diffraction (XRD) patterns. Raman spectroscopy results showed that introducing Na+ in A sites could induce stronger interaction between A-sites ions and NbO6 octahedron, and also lead to the more distortion degree of NbO6 octahedron at higher Na+ contents. It was found that accompanying with the structural evolution from ‘unfilled’ to ‘filled’ tungsten bronze type by modulating Na+ concentration, some unique dielectric and ferroelectric behavior emerged for CBNN ceramics, especially when x>0.4. Natural ferroelectric–paraelectric phase transition at the Curie Temperature (Tc) was determined for all compositions, while only the samples with x>0.4 showed slight dielectric anomaly around 125–200K, which was attributed to the local structural fluctuation of stronger NbO6 octahedron distortion in the ab plane. In addition, the more intense dependence of ε on the frequency measured around Tc could be attributed to the ions disorder in mainly A2 sites caused by simultaneous occupation of Na+ and Ba2+ ions when x>0.4. Normal ferroelectric hysteresis loops were observed in all compositions, and the related mechanisms for ferroelectric variations were discussed in detail. The better comprehensive dielectric and ferroelectric properties was obtained at x=0.8 due to the bigger distortion degree of NbO6 octahedron polar unit and the highest densification.

Structural, Microstructural and Electrical Properties of Strontium Barium Niobate (SBN60) Ceramics

Tungsten bronze type Sr0.6B0.4Nb2O6 ceramics were synthesized by solid state process. Cerium was used as dopant to improve its electrical properties. Single phase nature of the SBN60 composition were obtained successfully by this method in were confirmed through powder X-ray diffraction anlaysis. Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy with energy dispersive X–ray (SEM/EDX) and photoluminescence (PL) spectrometry were carried out to investigate its vibrational and surface phenomena which occurs during sintering. Dielectric measurements were carried out for pure and Ce doped SBN60. The obtained results were discussed in detail.

Microstructure and low–temperature relaxor behavior of dense K2Nb4O11 ceramics derived from sol–gel route

High purity tetragonal tungsten-bronze type K2Nb4O11 fine powders were synthesized via an economical sol–gel route and the crystal structure was solved by Rietveld refinement of X–ray powder diffraction. Structural evolution during calcination was found to occur via translation of the edge shared octahedra into a corner sharing coordination. Using nano-structured powder as precursor, dense K2Nb4O11 ceramics were successfully fabricated by pressure-less sintering. This maiden achievement allowed for primary dielectric characterization of this ceramic compound. The quantitative analysis based on empirical parameters, ΔTm and γ, reveals a relaxor nature in the temperature region below 200 K. More remarkably, the dielectric constant and loss are almost frequency independent in the wide range of 1 Hz–1 MHz at room temperature and a high dielectric constant and low loss (ɛr = 200 and tan δ = 0.007) makes them good materials for dielectric components for embedded capacitors and multilayer ceramic capacitors.

Diffraction Studies of Tungsten Bronze Type Relaxor Ferroelectrics

Ferroelectric materials are essential for modern electronic applications, from consumer electronics to sophisticated technical instruments. Relaxor ferroelectric materials provide the advantage of high dielectric constants over broad temperature ranges not seen in traditional ferroelectrics. Tungsten bronze type compounds have been shown to display a variety of industrially relevant optical and electronic properties amongst others. There is a fundamental relationship between the physical properties displayed by ferroelectrics and the crystal structures in which they form. Of particular interest are compositions and temperatures near phase transition. These are import because near phase transitions, particularly morphotropic phase transitions, electromechanical properties are often dramatically enhanced. [1,2] This work focuses on the structural investigation of the tungsten bronze type relaxor ferroelectric materials in the BaxSr3-xTi1-yZryNb4O15 (0 ≤ x ≤ 3; 0 ≤ y ≤ 1) system. A combination of X-ray, neutron (ToF and constant wavelength) and electron diffraction were employed to map the entire room temperature phase space. In addition, morphotropic phase boundary compositions were determined accurately. Variable temperature synchrotron X-ray diffraction studies were utilised to further explore the phase diagram for non-ambient conditions. Temperature dependent phase transitions were determined and the relationship between composition and transition temperature analysed. Structural models used in this work resulted from Rietveld refinements against powder diffraction data. [3] This work will shed light on new lead free relaxor ferroelectric materials.

Incommensurate modulations of relaxor ferroelectric Ca0.24Ba0.76Nb2O6 (CBN24) and Ca0.31Ba0.69Nb2O6 (CBN31).

CBN crystals show a one- and a two-dimensionally modulated modification. The former is isotypic with orthorhombic Ba4Na2Nb10O30 and the latter with the tetragonal tungsten bronze type of crystal structure. The orthorhombic form irreversibly transforms to the tetragonal polymorph at the ferroelectric phase transition near 603 K. Orthorhombic and tetragonal CBN24 slightly differ in the distribution of the Ba and Ca atoms over the incompletely filled Me1 and Me2 sites. The tetragonal symmetry is further broken in orthorhombic CBN24 by different amplitudes of the positional modulations of O atoms which are symmetrically equivalent in the TTB structure. A similar orthorhombic phase of CBN31 could be obtained by quenching from 1473 K.

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.