Bronze Structure Research Articles

Effect of Intrinsic Nb5+ Vacancy on Dielectric and Polarization Behaviors of KSr2Nb5O15: First‐Principles Investigation

The Nb in the structure of ferroelectric KSr2Nb5O15 (KSN) is designated as Nb1 or Nb2 as it is surrounded by differing amounts of oxygen atoms. The dielectric properties and spontaneous polarization intensity of KSN with different Nb5+ vacancies are studied using first‐principles calculations and the influence mechanism of intrinsic cation vacancies on the dielectric properties and spontaneous polarization intensity of KSN is clarified. The dielectric constants and polarization responses to the materials are different when vacancy is formed at these two types of Nb sites, respectively; vacancy at the Nb1 site increases the dielectric constant, whereas vacancy at the Nb2 site decreases it. Furthermore, the experimental results support the effects of Nb5+ vacancy on the polarization behavior of the material, including polarizability and polarization intensity. This study provides a theoretical foundation for better understanding and improving the dielectric properties and polarization behavior of tungsten bronze structure ferroelectrics.

Phase Composition, Structure, and Properties of Beryllium Bronze after Thermal and Thermoacoustic Treatment

Phase Composition, Structure, and Properties of Beryllium Bronze after Thermal and Thermoacoustic Treatment

Unraveling the structure and electrochemical supercapacitive performance of novel tungsten bronze synthesized by facile template-free hydrothermal method

In this study, we report a facile hydrothermal method for preparing novel tungsten oxide bronze (HxWO3) and nanostructured tungsten oxide (WO3) using different structure-directing agents. The effects of phase and morphology of the hydrothermally fabricated nanostructured WO3 on the charging/discharging performance of electrochemical pseudocapacitors are thoroughly investigated. Most of the as-prepared samples exhibit WO3•0•33H2O orthorhombic structure, whereas post annealing at 350 °C promotes phase transformation to a phase-pure hexagonal structure (h-WO3). Scanning electron microscope (SEM) images show a variation of the prepared samples’ morphology between one-, two-, and three dimensions. The difference in structure between the developed bronze and the known WO3 phases is confirmed by Raman spectroscopic investigations. The optical bandgap estimated from diffuse reflectance is the highest for novel HxWO3 (2.84 eV) and h-WO3 (2.73– 2.80 eV), whereas it is lowest for the monoclinic phase (2.66 eV). X-ray photoelectron spectroscopy (XPS) analysis confirms the formation of higher amount of oxygen vacancies in h-WO3 than HxWO3. An exceptional capacitance of 1280 F/g at 1 A/g is obtained for h-WO3 when tested in 0.5-M H2SO4 aqueous electrolyte, which is superior to HxWO3. The symmetrical supercapacitor device of HxWO3 exhibits an energy density of 8 W h/kg with high stability and good capacitive retention. The number and size of the various crystallographic tunnels in the fabricated h-WO3 are found to be the primary driving forces for improving the capacitive performance and stability.

Metal-insulator transition in Ba3−xSrxNb5O15

We investigated single crystals of ${\mathrm{Ba}}_{3\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ with a tetragonal tungsten bronze (TTB) structure, which exhibits a metal-insulator transition with $x$. We found that there is a one-dimensional anisotropy with a higher dc conductivity along the $c$ axis in the metallic phase, but it disappears in the insulating phase. The optical conductivity spectra of the metallic samples show similar anisotropy to the dc conductivity, but the anisotropy persists even in the insulating phase. These experimental results, together with the result of magnetic susceptibility, indicate that the disorder caused by the Sr ions loosely bound to the surrounding oxygen ions plays an important role in the metal-insulator transition.

Twin-boundary engineering and thickness effect on strontium-barium niobate thin films on MgO substrate

The Sr0.5Ba0.5Nb2O6 (SBN50) films with different thicknesses synthesized by a high-current discharge rf-sputtering in an oxygen atmosphere. All synthesized films have the structure of tetragonal tungsten bronze. The dependence of the unit cell parameters of the film material demonstrates a size effect, the magnitude of which is comparable to the distortions resulting from the ferroelectric relaxor phase transformation of SBN crystals of the same composition. The twinning of the same type is observed in all films when the twinned film components are rotated relative to each other by 36.8 degrees around the fourth-order rotational axis. The orientation of the films relative to the substrate can be written as [001]SBN∥[001]MgO; [100]SBN+∥[310]MgO and [100]SBN−∥[31¯0]MgO, where the sign "+" and "–" marks the twin components. It is noteworthy that such an orientation of the film relative to the substrate corresponds to the maximum possible tensile stress of heteroepitaxial films acting from the MgO single crystal substrate. This type of twinning is not typical for SBN50 films on MgO grown by other synthesis methods. The appearance of this unusual type of twinning is associated with the peculiarities of synthesis. At the initial moment of sputtering, the substrate is additionally heated by the sputtered substance’s plasma. The substrate temperature can also be used in other synthesis methods to control twinning and strain in barium strontium niobate thin films.

The effect of co-substitution on the microwave dielectric properties of Ba6-3xNd8+2xTi18O54(x=2/3) ceramics

In this work, the microwave dielectric properties of Ba4(Nd1-yBiy)28/3Ti18-x(Al1/2Ta1/2)xO54(0≤x≤2, 0.05≤y≤0.2) ceramics co-substituted by A/B-site were studied. Firstly, (Al1/2Ta1/2)4+ was used for substitution at B-site. At 0≤x≤1.5, the above mentioned ceramic was found to exist in single-phase tungsten bronze structure, but at x = 2.0, the secondary phase appeared. Although the dielectric constant decreased by doping the (Al1/2Ta1/2)4+, but the quality factor was observed to improve by 40% and the temperature coefficient of resonant frequency decreased by 75%. Based on the above results, Bi3+ was introduced to Ba4Nd28/3Ti17(Al1/2Ta1/2)O54. The introduction of Bi3+ reduced the sintering temperature, greatly improved the dielectric constant, and ultimately decreased the temperature coefficient of resonant frequency, but it led to deterioration of quality factor. At last, with appropriate site-substitution content control (x = 1.0,y = 0.15), excellent comprehensive properties (εr = 89.0, Q × f = 5844 GHz @ 5.89 GHz，TCF = +8.7 ppm/°C) were obtained for the samples sintered at 1325 °C for 4 h.

Ordering of A-cations in lead-free oxides with a filled tetragonal tungsten bronze structure

By X-ray powder diffraction, the structure of polycrystalline lead-free oxides K4Bi2Nbl0O30 and Na2Sr4Nb10O30 was refined with an emphasis on ordering in the A-sites of the tetragonal tungsten bronze crystal lattice. It was shown that the studied oxides have different types of cationic ordering, which does not change at ferroelectric phase transition. The results are consistent with the piezoresponse force microscopy data.

Evaluation of the Behavior and Resistance to Cavitation Erosion of the Structure of the CuSn12-C Bronze Obtained by Thermal Treatment of Quenching and Tempering

The paper presents the results of the research on the vibration cavitation erosion of the CuSn12-C bronze, subjected to the volumetric heat treatment of quenching to 700 °C, with water cooling, followed by tempering to 250 °C, with slow cooling in the air. The applied heat treatment shows the improvement of the behavior and the resistance to the cavitation erosion, compared to the initial state, of molded semi-finished product. The evaluation of the behavior and the resistance to the cavitation erosion is made by comparing the value of the parameters MDEmax, Rcav and Rz with those of the delivery state, as well as on the comparison of the characteristic curves MDE(t) and MDER(t) with those of the CuNiAl I-RNR bronze (used at propeller casting). The tests were conducted on the vibrating apparatus with piezoceramic crystals from the Cavitation Laboratory of the Polytechnic University of Timisoara.

Structural, dielectric and energy storage properties of Neodymium niobate with tetragonal tungsten bronze structure

The present work reports the effect of Nd3+ substitution on the structural, dielectric and energy storage properties of the polycrystalline Ba2NaNb5O15(BNN) ceramics. The ferroelectric lead-free Ba2Na1-xNdx/3Nb5O15 ceramics were prepared by conventional solid-state reaction technique. The room temperature X-ray study and Raman spectroscopy investigations have allowed evidencing the stabilization of a tetragonal tungsten bronze structure with P4bm space group for all the studied compounds. Dielectric study carried out on the prepared ceramics has permitted to determine the temperature of phase transitions (Tc). It was found that the dielectric constant increased with increasing the amount of Nd in BNN matrix. Room temperature energy storage property was determined using P-E hysteresis loops. The optimum discharge density was found for the composition x = 0.7 with Wrec = 18.1 mJ cm−3) at 1 Hz. Finally, the dynamic of phase transition was approached using thermal Raman spectroscopy investigations.

Sr2Nb6O13F8·4H2O and Sr3Nb2O2F12·2H2O: A Variant of Three-Dimensional Tungsten Bronze and a Polar Molecular Oxide Fluoride.

Crystals of two strontium niobium oxyfluorides, Sr2Nb6O13F8·4H2O and Sr3Nb2O2F12·2H2O, have been grown in phase pure forms via hydrothermal reactions using SrCO3, Nb2O5, and an aqueous HF solution. Single-crystal X-ray diffraction suggests that Sr2Nb6O13F8·4H2O, crystallizing in the orthorhombic centrosymmetric space group, Pbam (No. 55), reveals a new variant of the three-dimensional tungsten bronze structure with three-, four-, and five-membered rings that are composed of corner-sharing NbO2(O/F)2F2, NbO4(O/F)F, NbO3(O/F)3, and SrO3F6 groups. Sr3Nb2O2F12·2H2O with the noncentrosymmetric polar space group, Cmc21 (No. 36), however, reveals a molecular structure consisting of Nb(O/F)2F5 pentagonal bipyramids and two unique Sr2+ cations interacting with F, O/F, and water molecules. Band gaps calculated by the Kubelka-Munk function based on the ultraviolet-visible diffuse-reflectance spectra of Sr2Nb6O13F8·4H2O and Sr3Nb2O2F12·2H2O are estimated to be ca. 3.22 and 4.11 eV, respectively, in which the values are related to the contents of electronegative F atoms and the Nb-O(F)-Nb bond angles influenced by structural distortion. An interesting phase transition reaction from Sr3Nb2O2F12·2H2O to thermodynamically more stable Sr2Nb6O13F8·4H2O occurs under a hydrothermal condition.

Enhanced dielectric, ferroelectric and magnetic properties of Ba4Sm2Fe2Nb8O30 RT multiferroics prepared by microwave sintering

High density Ba4Sm2Fe2Nb8O30 (BSFN) multiferroics ceramics with tetragonal tungsten bronze structure had been prepared by microwave sintering (MS) for 30min and conventional sintering (CS) methods for 4 h at 1275 °C. Single tungsten bronze phase and equiaxial grains are obtained for the MS BSFN ceramics, while a small amount secondary phase of SmNbO4 is observed in the CS BSFN ceramics with columnar grains. Compared to Ba4Sm2Fe2Nb8O30 ceramics prepared by CS method, enhanced dielectric, ferroelectric and magnetic properties are achieved for the MS BSFN ceramics. The values of electric polarization Pr and coercive electric field Ec are 2.11 μC/cm2 and 7.14 kV/cm for the MS BSFN ceramics, respectively. Meantime, the magnetic polarization Mr of 0.410emu/g and coercive magnetic field Hc of 2930Oe are also obtained for the MS BSFN ceramics. Based on the density, crystal structure, point defect and grain, the reasons of enhanced dielectric, ferroelectric and magnetic properties are discussed for the MS BSFN ceramics It is indicated that Ba4Sm2Fe2Nb8O30 is an intrinsic room temperature multiferroic materials.

Pinched P–E hysteresis loops of KSr2Nb5O15 ferroelectric ceramics

Pinched polarization–electric field (P–E) loop was reported in many perovskites, while it is rarely observed in tungsten bronze structures. Here, the KSr2Nb5O15 (KSN) ceramics were prepared by using the KSN calcined and microcrystalline powder as raw materials. Among them, the KSN microcrystalline were fabricated by the molten salt synthesis, which had many K+/Sr2+ vacancies. The pinched P–E hysteresis loops was found in the KSN ceramics with the larger grain size. In order to investigate the effect of K+/Sr2+ vacancies on remnant polarization, the grain-oriented KSN ceramics were prepared by using microcrystalline powder. The ferroelectric hysteresis loops were tested under different conditions. The results suggested that the electric field, temperature and frequency had a significant influence on the shape of P–E loop. The characteristic could be attributed to the pinning effect of point defects on domain wall motion.

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.

High‐temperature dielectric and energy storage properties of Bi0.5Na0.5TiO3‐based ceramics modified by Sr0.8Na0.4Nb2O6

AbstractSr0.8Na0.4Nb2O6with a tungsten bronze structure is introduced into perovskite‐structured 0.94(Bi0.5Na0.5)TiO3‐0.06BaTiO3composition (abbreviated as BNT‐BT‐xSNN, x = 0‐0.04). The temperature stability of dielectric properties and energy storage performance is found to be effectively enhanced by Sr0.8Na0.4Nb2O6dopant. When x is 0.03, the temperature ranges covering |ε'‐ε'150°C|/ε'150°C≤15% andtanδ≤ 0.02 are 43°C‐404°C and 90°C‐422°C, respectively. More importantly,ε′can be retained as high as 3304 at 150°C. Besides, the variances of energy storage density and its efficiency are 6.4% and 5.3%, respectively, in the temperature range from room temperature (RT) to 180°C. Therefore, this work provides a new method of compositional modification in BNT‐based materials to improve their temperature stability of dielectric and energy storage properties.

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.

The behavior to cavitation erosion of CuSn12-C bronze structures, as obtained by in-depth heat treatments

CuSn bronze alloys are some of the most used in the casting of freshwater ship propellers. Through in-depth heat treatments, the microstructure and hardness are substantially altered when compared with the initial state of the material after casting. The work highlights the change in the behavior and resistance to the vibrating cavitation erosion of the CuSn12-C bronze microstructures, after being subject to two in-depth heat treatments (hardening at 700ºC, followed by cooling in water, and two hardening treatments at 700ºC, with cooling in water, followed by tempering at 500ºC and 250ºC, with cooling in air). The experimental tests are performed using the vibrating standard piezoelectric crystal assembly, available in the Cavitation Research Laboratory at the Polytechnic University from Timisoara.

ИССЛЕДОВАНИЕ ПРОДУКТОВ ТЕРМОЛИЗА ВОЛЬФРАМОФОСФАТОВ ЦЕЗИЯ

The article deals with the synthesis, study of thermal decomposition and identification of the thermolysis products of cesium tungstophosphates that are promising compounds in the field of materials science, catalysis and other fields of science and technology.
 Compounds with the Keggin anion structure are synthesized from aqueous solutions: Cs3[PW12O40] ∙ 9H2O; Cs5Na2[PW11O39(H2O)] ∙ 5H2O and Cs5[PW11O39Ni0,5Cu0,5(H2O)] ∙ 4H2O. The processes of their thermal decomposition are investigated and some regularities of their thermolysis are established. Thermolysis products are identified: Cs3PW12O40, phases with the structure of pyrochlore and hexagonal tungsten bronze of the composition Cs10/13Na4/13P2/13W22/13O6 and Cs10/13P2/13Ni1/13Cu1/13W22/13O6. The unit cell parameters of the phase with the pyrochlore structure are determined. Research results confirm that phosphorus, nickel and copper ions are included in the structure of pyrochlore and hexagonal tungsten bronze. Phases similar to this chemical composition are not previously known in the literature. The studied tungstophosphates and their thermolysis products are promising compounds for obtaining heterogeneous catalysts for the oxidation of organic compounds and selective sorbents. The research results can be useful for predicting the thermal properties and phase composition of thermolysis products of similar polyoxometallates in order to obtain new compounds with the structure of pyrochlore and hexagonal tungsten bronze, as well as composite materials based on them.

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.

The Alloying Element Effect on the Structure and Mechanical Properties of Copper-based Aluminum Bronze

The Alloying Element Effect on the Structure and Mechanical Properties of Copper-based Aluminum Bronze