Tetragonal Tungsten Bronze Research Articles

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.

Structural and mechanical properties of Sr-doped barium niobate thick films

Polycrystalline strontium barium niobates Sr x Ba1 − x Nb2O6 (SBN, 0.40 ≤ x ≤ 0.75) were prepared by standard solid state ceramic method at relatively at low temperatures. Thick SBN films prepared by simple, low-cost, screen printing route were characterized by XRD. Preliminary structural analysis exhibits the formation of tetragonal tungsten bronze crystal structure at room temperature. Texture coefficient [TC(hkl)], dislocation density (ρD), density of crystallites per unit surface area (Ψ), mechanical properties, and microwave behavior of synthesized materials are reported.

Correlation between upconversion photoluminescence and dielectric response in Ba-substituted (Sr1−xBax)4(La0.85Ho0.025Yb0.125)2Ti4Nb6O30

The filled tetragonal tungsten bronze (Sr1−xBax)4(La0.85Ho0.025Yb0.125)2Ti4Nb6O30 (SBLTNx: Ho-Yb) ceramics with different Ba substitution levels (x) are prepared. The upconversion photoluminescence (UC-PL) and dielectric permittivity are investigated. The substitution of Sr2+ ions at the A2-sites by larger Ba2+ ions results in substantial variation of the UC-PL intensity as a function of substitution level x. Furthermore, the dielectric response to the substitution of Sr2+ by Ba2+ suggests a close correlation between the UC-PL intensity and dielectric permittivity. The origin for this correlation is discussed based on the random stress field (RSF) model.

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.

Ferroelectric and optical properties of Ba5Li2Ti2Nb8O30 ceramics potential for memory applications

Giant grained (42 μm) translucent Ba5Li2Ti2Nb8O30 ceramic was fabricated by conventional sintering technique using the powders obtained via solid state reaction route. These samples were confirmed to possess tetragonal tungsten bronze structure (P4bm) at room temperature. The scanning electron microscopy established the average grain size to be close to 20 μm. The photoluminescence studies carried out on these ceramics indicated sharp emission bands around 433 and 578 nm at an excitation wavelength of 350 nm which were attributed to band-edge emission as the band gap was 2.76 eV determined by Kubelka–Munk function. The dielectric properties of these ceramics were studied over wide frequency range (100–1 MHz) at room temperature. The decrease in dielectric constant with frequency could be explained on the basis of Koops theory. The dielectric constant and the loss were found to decrease with increasing frequency. The Curie temperature was confirmed to be ~370 °C based on the dielectric anomaly observed when these measurements were carried out over a temperature range of 30–500 °C. This shows a deviation from Curie–Weiss behaviour and hence an indicator of the occurrence of disordering in the system, the γ = 1.23 which confirms the diffuse ferroelectric transition. These ceramics at room temperature exhibited P–E hysteresis loops, though not well saturated akin to that of their single crystalline counterparts. These are the suitable properties for ferroelectric random access memory applications.

Upconversion photoluminescence and dielectric properties in Er3+ and Yb3+ co-doped Sr4La2Ti4Nb6O30

Filled tetragonal tungsten bronze (TTB) structure Sr4La2Ti4Nb6O30 co-doped by Er3+ and Yb3+ (SLTN: xEr3+–yYb3+) ceramics with different x and y were synthesized, and their crystal structure, upconversion photoluminescence (UC-PL), and dielectric permittivity were investigated for the first time. Under 980nm infrared (IR) excitation, bright single-band UC green (515–570nm) emission was observed for SLTN: xEr3+–yYb3+ (x=0.05, y=0), however, featured two-bands UC emission (515–570nm and 640–690nm) was confirmed for all SLTN: xEr3+–yYb3+ (x=0.05, y≠0) samples. Two-photon energy transfer process was proved through pumping laser power dependence of emission intensity measurement. It is believed that energy back transfer (EBT) process should be mainly responsible for the two-bands UC emission. In addition, the dielectric properties were also discussed for SLTN: xEr3+–yYb3+ system.

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.

Unified approach for determining tetragonal tungsten bronze crystal structures.

Tetragonal tungsten bronze (TTB) oxides are one of the most important classes of ferroelectrics. Many of these framework structures undergo ferroelastic transformations related to octahedron tilting deformations. Such tilting deformations are closely related to the rigid unit modes (RUMs). This paper discusses the whole set of RUMs in an ideal TTB lattice and possible crystal structures which can emerge owing to the condensation of some of them. Analysis of available experimental data for the TTB-like niobates lends credence to the obtained theoretical predictions.

Superconducting properties of theKxWO3tetragonal tungsten bronze and the superconducting phase diagram of the tungsten bronze family

We report the superconducting properties of the K$_{x}$WO$_{3}$ tetragonal tungsten bronze. The highest superconducting transition temperature ($T_{c}=2.1$K) was obtained for K$_{0.38}$WO$_{3}$. $T_{c}$ decreases linearly with increasing K content. Using the measured values for the upper critical field $H_{c2}$, and the specific heat $C$, we estimate the orbital critical field $H_{c2}$(0), coherence length $\xi$(0), Debye temperature $\Theta _{D}$ and coupling constant $\lambda _{ep}$. The magnitude of the specific heat jump at $T_{c}$ suggests that the K$_{x}$WO$_{3}$ tetragonal tungsten bronze is a weakly-coupled superconductor. The superconducting phase diagram of the doped tungsten bronze family is presented.

Structural peculiarities and point defects of undoped and Cr- and Ni-doped Sr0.61Ba0.39Nb2O6 crystals

Crystals of strontium barium niobate solid solutions, both nominally pure Sr0.61Ba0.39Nb2O6 and doped with transition metal (Cr3+ and Ni3+) ions, have been investigated by X-ray diffraction. The chemical composition of the undoped crystal was determined taking into consideration vacancies in niobium Nb(2) and oxygen sites and the statistical distribution of strontium in two (Sr(1) and Sr(2)) sites in the tetragonal tungsten bronze type crystal structure. It was established that the introduction of chromium or nickel oxides into the melt initial composition greatly influences the strontium and barium content in the crystals obtained and the distribution of these species in the crystallographic structure. The probability of finding the transition metal ions in regular and interstitial positions depending on their concentration and valence state is discussed.

Towards novel multiferroic and magnetoelectric materials: dipole stability in tetragonal tungsten bronzes

We discuss the strategy for development of novel functional materials with the tetragonal tungsten bronze structure. From the starting composition Ba6GaNb9O30, the effect of A- and B-site substitutions on the dielectric properties is used to develop an understanding of the origin and stability of the dipolar response in these compounds. Both tetragonal strain induced by large B-site cations and local strain variations created by isovalent A-site substitutions enhance dipole stability but result in a dilute, weakly correlated dipolar response and canonical relaxor behaviour. Decreasing cation size at the perovskite A2-site increases the dipolar displacements in the surrounding octahedra, but insufficiently to result in dipole ordering. Mechanisms introducing small A-site lanthanide cations and incorporation of A-site vacancies to induce ferroelectricity and magnetism are presented.

On the nature of phase transitions in the tetragonal tungsten bronze GdK2Nb5O15 ceramics

Phase transitions in gadolinium potassium niobate GdK2Nb5O15 (GKN) ceramics have been investigated by x-ray diffraction, dielectric susceptibility, differential scanning calorimetry, and Raman scattering. The results of our complementary studies show that GKN exhibits two phase transitions at Tc1 = 238 °C and Tc2 = 375 °C attributed to the ferroelectric-antiferroelectric-paraelectric (FE-AFE-PE) phase transitions. According to the x-ray diffraction analysis, the FE and PE phases were refined in the polar P4bm and centrosymmetric P4/mbm space groups. For the intermediate phase, the structure is refined in the space group P4nc with doubling of the c unit cell parameter, which is compatible with an AFE phase. This result was confirmed by Raman spectroscopy since new low-frequency lines are activated in the AFE phase due to the Brillouin zone-folding effect, typical for the modulated phases. The presence of the AFE phase between the FE and PE phases and the complex nature of tetragonal tungsten bronze crystal structure can explain the large thermal hysteresis observed at the FE-AFE transition between heating and cooling cycle and the strong depression of the Curie-Weiss temperature T0. The semi-phenomenological Ising-like model based on the obtained experimental data is proposed to account for the observed FE-AFE-PE transition sequence.

Growth and Characterization of Ba2LnFeNb4O15 (Ln = Pr, Nd, Sm, Eu) Relaxor Single Crystals

Ba2LnFeNb4O15 (Ln = Pr, Nd, Sm, and Eu) single crystals were grown from high-temperature solution under controlled atmosphere. Chemical, structural, and dielectric characterizations were performed on four crystals with tetragonal tungsten bronze (TTB) structure. The main difference between the four crystals’ compositions is the proportion of rare-earth, as well as vacancies, located in the square site. All crystals displayed an incommensurately modulated structure at room temperature in the paraelectric state. The corresponding basic structures were refined using the P4/mbm space group (R = 0.0274; 0.0416; 0.0328; 0.0357 for Ln = Pr, Nd, Sm, and Eu, respectively). Examination of the structural parameters confirmed the central role played by square-shaped channels in the crystal chemistry of these materials. All crystals display a relaxor behavior with Tm (1 MHz) ranging from 110 to 270 K. The chemical, structural, and physical properties of the crystals are discussed and compared, for the latter, with tho...

A-site size effect in a family of unfilled ferroelectric tetragonal tungsten bronzes: Ba₄R(0.67)Nb₁₀O₃₀ (R = La, Nd, Sm, Gd, Dy and Y).

The effect of A-cation size on the structural and electrical properties in a family of ferroelectric tetragonal tungsten bronzes (TTBs) Ba4R(0.67)□(1.33)Nb10O30 (R = La, Nd, Sm, Gd, Dy and Y; □ = vacancy) was investigated. In each case, the crystal structure, as determined from lab-based ambient powder X-ray diffraction (PXRD), is metrically tetragonal and can be refined in the P4bm space group. XRD data show an increased splitting of hk0 00l reflections with decreasing R cation size indicating an increasing tetragonal distortion (measured by tetragonality c/a). Dielectric data and ferroelectric measurements indicate that the ferroelectric Curie temperature, T(C), increases with decreasing R size and so a direct correlation between T(C) and tetragonality/ionic radius of R is demonstrated. Rietveld refinements show that the large A2-site is fully occupied by Ba(2+) and, in addition to the R cation size, the presence of vacancies at the A1-site (perovskite-like site) is also shown to strongly affect the stability of ferroelectricity in this structure type.

Structure and thermal expansion of the tungsten bronze Pb2KNb5O15

The structure and thermal expansion behavior of the tetragonal tungsten bronze oxide Pb2KNb5O15 were investigated by neutron powder diffraction and high-temperature X-ray diffraction. Below the Curie temperature, T(C) (orthorhombic phase, T(C) ≈ 460 °C), the cell parameters a and c increase with temperature, while b decreases. The thermal expansion coefficients are α(a) = 1.29 × 10(-5) °C(-1), α(b) = -1.56 × 10(-5) °C(-1), and α(c) = 1.62 × 10(-5) °C(-1). Temperature-dependent second harmonic generation (SHG), dielectric, and polarization-electrical field (P-E) hysteresis loop measurements were performed to study the symmetry and electric properties. We show that the distortion and cooperative rotation of NbO6 octahedrons are directly responsible for the negative thermal expansion coefficient along the polar b axis. It is suggested that Pb-O covalency, especially in the large and asymmetric pentagonal prisms, may be related to orthorhombic distortion and abnormal spontaneous polarization along the b axis. This study shows that tungsten bronze families are possible candidates for exploring negative thermal expansion materials.

Defect Structure, Phase Separation, and Electrical Properties of Nonstoichiometric Tetragonal Tungsten Bronze Ba0.5–xTaO3–x

New insight into the defect chemistry of the tetragonal tungsten bronze (TTB) Ba(0.5-x)TaO(3-x) is established here, which is shown to adapt to a continuous and extensive range of both cationic and anionic defect stoichiometries. The highly nonstoichiometric TTB Ba(0.5-x)TaO(3-x) (x = 0.25-0.325) compositions are stabilized via the interpolation of Ba(2+) cations and (TaO)(3+) groups into pentagonal tunnels, forming distinct Ba chains and alternate Ta-O rows in the pentagonal tunnels along the c axis. The slightly nonstoichiometric Ba(0.5-x)TaO(3-x) (x = 0-0.1) compositions incorporate framework oxygen and tunnel cation deficiencies in the TTB structure. These two mechanisms result in phase separation within the 0.1< x < 0.25 nonstoichiometric range, resulting in two closely related (TaO)(3+)-containing and (TaO)(3+)-free TTB phases. The highly nonstoichiometric (TaO)(3+)-containing phase exhibits Ba(2+) cationic migration. The incorporation of (TaO)(3+) units into the pentagonal tunnel and the local relaxation of the octahedral framework around the (TaO)(3+) units are revealed by diffraction data analysis and are shown to affect the transport and polarization properties of these compositions.

Triangular Exchange Interaction Patterns in K3Fe6F19: An Iron Potassium Fluoride with a Complex Tungsten Bronze Related Structure

The synthesis and structural and magnetic characterizations of K3Fe6F19, a new iron potassium fluoride with a complex tungsten bronze related structure, are presented. This phase was found during the investigation of relatively low-temperature (600 °C) synthesis conditions of classical tetragonal tungsten bronze (TTB) fluorides and can be considered an intermediate that forms at this temperature owing to faster crystallization kinetics. The K3Fe6F19 compound has an orthorhombic structure (space group Cmcm (63), a = 7.6975(3) Å, b = 18.2843(7) Å, c = 22.0603(9) Å) related to the TTB one, where the perovskite cage is substituted by a large S-shaped channel simultaneously occupied by two potassium atoms. The magnetic structure, characterized by magnetization measurements on an oriented single crystal and powder neutron diffraction, is dominated by the presence of interconnected double stripes of antiferromagnetic triangular exchange interaction patterns alternately rotated in clock- and anticlockwise fashion. The magnetic order takes place in a wide temperature range, by increasing progressively the interaction dimensionality.

Relaxor nature in lead-free Sr5LaTi3Nb7O30 tetragonal tungsten bronze ceramics

Lead-free tetragonal tungsten bronze Sr5LaTi3Nb7O30 ceramics were prepared and the correlation of the relaxor nature and crystal structure was studied using dielectric spectroscopy and powder X-ray diffraction. Three dielectric relaxations were observed below the deviation temperature TD ∼ 330 K. Relaxation I and II followed the Vogel-Fulcher law with the freezing temperatures of 189 K and ∼90 K. Low temperature relaxation III, which was first observed in filled tungsten bronze, followed well the Arrhenius law. Dielectric response becomes static below 50 K. Polarization-field (P-E) hysteresis loops were evaluated from 183 K to 298 K. Pr value of 0.41μC/cm2 was observed at 183 K. Deviation of lattice parameter c from the linear contraction and increasing of tetragonality (c/a ratio) were observed below TD, reflecting the structure change during the formation of polar nanoregions and the following freezing process. Opposite tendency was observed below 100 K for all the lattice parameters, corresponding to relaxation III. Generally, the main dielectric relaxation I and II were attributed to flipping and breathing of polar nanoregions along c axis, while the concerted rotations of the oxygen octahedra in the ab plane were suggested as the origin of relaxation III.

Low-temperature synthesis of K0.5FeF3 with tunable exchange bias

Fluorides K0.5FeF3 with tetragonal tungsten bronze structure have been fabricated by solid state reaction at low sintering temperature in the range between 150 °C and 400 °C with the assistance of crystal water during the grinding and sintering processes. Unusual magnetic properties have been observed, including positive exchange bias field (HE) with negative vertical magnetization shift (Mshift), and smaller field cooling (FC) magnetization than the zero field cooling one below 53 K. The results are explained by a core-shell structure consisting of antiferromagnetic core and spin glass (SG) shell with antiferromagnetic interfacial coupling between the pinned interface spins and the SG shell spins. The sign of HE and Mshift can be changed by increasing the cooling field in the FC process, which is attributed to the competition between the antiferromagnetic interfacial coupling and the Zeeman energy of magnetization of the SG shell.

Transmission Electron Microscopy of Mn-doped KNN Ceramics

Smart materials like piezoelectrics and ferroelectrics play a crucial role in applications such assensors and actuators,radio-frequency switching, drug delivery, chemicals detection, and power generation and storage. K0.5Na0.5NbO3 (KNN) is one of the leading lead free piezoelectric materials being considered as an alternativeto Pb(Zrx,Ti1-x)O3 (PZT), which is currently the most widely used material for electromechanical applications. Although pure KNN has inferior electromechanical properties compared to PZT,efforts are on going to tailor and improve its piezoelectric coefficients by doping and texturing.Although the piezoelectric constant (d33) of undoped KNN is unsuitable for practical electromechanical applications, properties comparable to PZT at room temperature (d33&gt;400 pC/N) have been reported for modified KNN ceramics. Electromechanical properties are however, very much dependent on the crystalline phase content, crystallographic orientation, microstructure, interfaces and domain configuration.Mn is an indispensable dopant for both PbO-based as well as PbO-free ceramics like BaTiO3, SrTiO3, KNbO3 and KTaO3. It has been reported to improve the density, mechanical quality factor, electromechanical properties and to reduce dielectric loss. Mn has been successfully used to reduce the leakage current and lower the orthorhombic to tetragonal phase transition temperature (TO-T) in KNN single crystals. It has also been shown to improve the density and properties of KNN–LiTaO3–LiSbO3. However, the effect of Mn on the KNN domain structure and phase assemblage has not yet been reported. In this work, KNN ceramics doped with Mn on the B-site (Mn content was 0.5, 1.0. 1.5 and 2 mole%) were synthesized by a conventional mixed oxide method. Transmission electron microscopy (TEM)(Hitachi 9000) studies were carried out to analyse the effect of B-site Mn doping on the ferroelectric domain structure and phase assemblage.Undoped KNN ceramics had large grains (&gt;30 &gt;m) which contained large (&gt;1 &gt;m wide) wedge shaped ferroelectric domains. KNN doped with 0.5 mole % Mn exhibited a smaller grain size (~2 mm) in which a well defined domain structure was observed with widths approximately an order of magnitude smaller than those in undoped KNN. For KNN doped with 2 mole % Mn, the presence of a second phase, Figure 1c, was often observed. Electron diffraction patterns from the second phase were consistent with a tetragonal tungsten bronze (TTB) structured compound although more work is required to definitively determine the phase assemblage. The domain structure became increasing complex as Mn concentration increased, suggesting that the presence of Mn on the B-site disrupts polar order.In conclusion, TEM analysis demonstrated that Mn doping changes the domain structure of KNN ceramics: for low Mn content, well defined ferroelectric domains and for high Mn content, tangled domains and second phase were the main features.These microstructure details elucidate reasons that may account for the inferior piezoelectric properties of KNN at higher Mn concentration.