Relaxor ferroelectric and photocatalytic properties of BaBi4Ti4O15

ABSTRACT

Aurivillius phase BaBi₄Ti₄O₁₅ micro-sized powders were produced by solid-state reaction and their photocatalytic properties were reported for the first time. X-ray diffraction revealed the polar orthorhombic structure. BaBi₄Ti₄O₁₅ ceramics exhibited diffuse phase transition at ～ 410°C. The freezing temperature of 274°C was obtained by fitting the Vogel–Fulcher law. The distinct ferroelectric domain switching current peaks in current – electric field (I-E) loop and piezoelectric coefficient d₃₃ value of 7.0?±?0.1 pC/N at room temperature further demonstrated relaxor ferroelectric behaviour of BaBi₄Ti₄O₁₅. UV-vis absorption spectra indicated that BaBi₄Ti₄O₁₅ had a direct band gap of 3.2?eV. The photocatalytic study showed 15% degradation of Rhodamine B (RhB) solution by BaBi₄Ti₄O₁₅ powders after 3.5?h UV-vis irradiation. The RhB degradation rate was further enhanced by depositing Ag nanoparticles on the BaBi₄Ti₄O₁₅ powders surface. This work suggested that the relaxor ferroelectric BaBi₄Ti₄O₁₅ is promising for photocatalytic applications.     

Structural and Ferroelectric Properties of Aurivillius Phase Materials

Aurivillius phase materials are important for ferroelectric, piezoelectric, and microelectromechanical (MEM) devices. These materials were synthesized by solution chemistry route. Thin films of Sr 1 m x Ba x Bi 2 TaNbO 9 and [1-xBaTiO 3 - xSrZrO 3 ] were deposited by spin coating and pulsed laser deposition. Both powder and thin films were characterized using x-ray diffraction, impedance spectroscopy, and Raman spectroscopy. Improved remnant polarization is observed in Sr 1 m x Ba x Bi 2 TaNbO 9 thin films.     

Room-temperature multiferroic behaviour in Co/Fe co-substituted layer-structured Aurivillius phase ceramics

Room-temperature single-phase multiferroic materials have attracted the considerable attention of may scientists due to their technological applications in the next-generation electronic devices. Here, we report the structural evolution and its relationship with the ferroelectric and magnetic properties of Aurivillius Bi_3.25Sm_0.75Ti_{(3-x) (}Fe,Co)_xO₁₂ (BSmT:FCx); for (0≤x ≤ 0.4) ceramics to elucidate the room-temperature multiferroics induced by Fe/Co co-substitution. The XRD analysis and structural refinements reveal that incorporating of Co, and Fe ions into Ti sites of BSmT host lattices gives rise to a single orthorhombic phase with no evidence of secondary phases. The X-ray photoelectron spectroscopy (XPS) spectra indicate the existence of oxygen vacancies in the investigated samples. Pure BiT samples exhibit diamagnetic behavior, while BSmT:FCx; for (0≤x ≤ 0.4) exhibits ferromagnetic behavior at room temperature. As a result, the sample with x = 0.4 displayed the highest remnant and saturation magnetization values. It is believed that the occurrence of magnetism causes by the magnetic double exchange interaction between the non-equivalent magnetic ions and oxygen. The reversible polarization induced by an electric field indicates the ferroelectric character of the ceramics at room temperature. The co-substituting samples show unsaturated P-E hysteresis loops, which could be caused by the domain pinning induced by the accumulation of oxygen vacancies near the domain boundaries. We expect these compounds are promising for developing electronic devices for future applications.     

Optimisation of SrBi2(Nb,Ta)2O9 Aurivillius phase for lead-free electrocaloric cooling

The influence of different substitutional mechanisms on the electrocaloric effect of a lead-free SrBi₂(Nb_0.2Ta_0.8)₂O₉ Aurivillius phase was studied for application in electrocaloric cooling systems. The A-site substitution with barium efficiently reduced the temperature of maximum permittivity from about 300?°C to 100?°C. The A-site substitution induced phenomena that are typical of strong relaxor ferroelectrics such as significant broadening of the permittivity peak and an increase in its frequency dispersion and with a depolarization temperature below room temperature. These features directly influenced the electrocaloric effect. A direct measurement system, based on a modified-differential scanning calorimeter, was used to analyze the EC effect of the dense (Sr_0.5Ba_0.5)Bi₂(Nb_0.2Ta_0.8)₂O₉ ceramics. In accordance with the relaxor characteristics, the EC effect was found to increase continuously over a broad temperature range above the room temperature. This was attributed to the alignment of field induced polar nanodomains. Directions for optimization towards a high-performing EC ceramic were identified.     

Breakdown field enhancement and energy storage performance in four-layered Aurivillius films

In dielectric capacitors, ferroelectric thin films with slim polarisation electric (P-E) hysteresis loops, which are mainly characterised by small residual polarisation (P_r) and large saturation polarisation (P_s) are expected to obtain high recoverable energy density (U_r) and efficiency (η). However, a lower breakdown in ferroelectric thin films usually impedes this result. Here, through the co-doping of La³⁺ and Pr³⁺ ions, a larger U_r of 54.27 J/cm³ and high η of 85.6% were obtained in four-layered Aurivillius phase ferroelectric thin films capacitors due to the enhanced breakdown electric field. The doped films annealed at relatively low temperatures showed similar energy storage properties compared with those of the prototype and higher energy storage efficiency compared with that of higher annealing films. In addition, the obtained thin film shows excellent energy storage properties in a wide frequency range, fatigue durability and good thermal stability. These results indicated that four-layered Aurivillius films are promising candidate materials for dielectric energy storage capacitors. The co-doping of double ions was an effective way to improve energy storage performance.     

Synthesis,structural analysis and dielectric properties of the double-layer Aurivillius compound Pb1-2xBi1.5+2xLa0.5Nb2-xMnxO9

The double-layer Aurivillius phases Pb_1-2xBi_1.5+2xLa_0.5Nb_2-xMn_xO₉ (x = 0, 0.1, 0.3, and 0.5) were synthesized by a molten salt method using a mixture of K₂SO₄/Na₂SO₄. The effect of composition on the structure, morphology and dielectric properties was investigated. X-ray diffraction showed that single-phase samples with a non-polar, orthorhombic A2₁am structure were obtained for x = 0, 0.1 and 0.3. The unit cell becomes more orthorhombic with increasing x as the degree of distortion of the BO₆ octahedra in the perovskite layer increases. Raman spectroscopy showed the typical modes of the orthorhombic double-layer Aurivillius structure and indicated that the La³⁺ ions prefer to occupy the perovskite A-site, Mn³⁺ occupies the B-site, and the Pb²⁺ ions are found in the Bi₂O₂ layer. The morphology of the samples was probed by scanning electron microscopy, which showed anisotropic, plate-like crystallites that increased in size with increasing x. The dielectric constant significantly increased with x, and the ferroelectric properties became more relaxor-like.     

Influence of synthesis method on the properties of mixed Aurivillius phase Bi5TiNbWO15

The ceramic material Bi₅TiNbWO₁₅ (BTNW) was obtained and is characterized by a layered, Aurivillus type structure (MBLPO—mixed bismuth oxide layered perovskites) with m = 1.5. In order to optimize the production, BTNW was synthesized by two methods: synthesis of a mixture of simple oxides Bi₂O₃, Nb₂O₅, WO₃, TiO₂ and synthesis of a mixture of appropriate, layered Aurivillus type structures Bi₂WO₆ (m = 1) and Bi₃TiNbO₉ (m = 2). Synthesized solid solutions were consolidated with a conventional method. The crystal structure of Bi₂WO₆ and Bi₃TiNbO₉ Bi₅TiNbWO₁₅ was examined at room temperature with an X-ray diffraction method. Microstructure and chemical composition were analyzed. The authors analyzed the temperature—permittivity ɛ relationship for the ceramic with m = 1,5, which was obtained with two methods, as well as for m = 1 and m = 2.     

Relaxor behavior and photocatalytic properties of BaBi2Nb2O9

Lead-free Aurivillius phase BaBi₂Nb₂O₉ powders were prepared by solid-state reaction. Ferroelectric measurements on BaBi₂Nb₂O₉ (BBNO) ceramics at room temperature provided supporting evidence for the existence of polar nanoregions (PNRs) and their reversible response to an external electric field, indicating relaxor behavior. The photocatalytic degradation of Rhodamine B reached 12% after 3 hours irradiation of BBNO powders under simulated solar light. Silver (Ag) nanoparticles were photochemically deposited onto the surface of the BBNO powders and found to act as electron traps, facilitating the separation of photoexcited charge carriers; thus, the photocatalytic performance was significantly improved. The present study is the first examination of the photochemical reactivity of a relaxor ferroelectric within the Aurivillius family with PNRs.     

Enhanced ferroelectricity of CaBi2Nb2O9-based high-temperature piezoceramics by pseudo-tetragonal distortion

(LiBi)-doped CaBi₂Nb₂O₉ (CBN) ceramics were synthesized by the conventional solid-state sintering method. The effects of (LiBi) additives on the crystalline structure were explored by Rietveld refinements. The results showed that pseudo-tetragonal distortion was induced by (LiBi) additives when x ≥ 0.35. This structural distortion increased the spontaneous polarization along a-axis and decreased the quits spontaneous polarization along b-axis, which was beneficial to the polarization switching, thus promoting the ferroelectricity and decreasing the coercive field (E_C) of CBN based ceramics. The Ca_0.60(Li_0.5Bi_0.5)_0.40Bi₂Nb₂O₉ (CBNLB-40) ceramics possess the optimum ferroelectric property with 2P_r of 10 μC/cm² and a coercive field (E_C) lower than 100 kV/cm. Moreover, it exhibits a good ferroelectric fatigue-free property within 10⁷ switched cycles. This work may provide a new method to promote the performance of Aurivillius ferroelectric-based non-volatile memory devices.     

Focus on the ferroelectric polarization behavior of four-layered Aurivillius multiferroic thin film

The well-saturated ferroelectric hysteresis loops with double remnant polarization up to 50?μC/cm² were obtained in four layered Aurivillius-type multiferroic Bi₅FeTi₃O₁₅ thin film. Pulsed positive-up negative-down polarization measurements demonstrate the intrinsic ferroelectric polarization, which present optimal rectangularity and polarization value. The hysteresis loops measurements with larger frequency range of 0.2–100?kHz indicate stable and ultra-fast switching speed of ferroelectric domains. Persistent retention properties were observed, and they are also independent of the applied electric field. In fatigue test an increased dielectric constant is observed along with the suppression of switchable polarization. Both of them can be restored partly to their original values via the stimulating of high electric field. The block domain switching due to the oxygen vacancies aggregated on domain walls are discussed for those characteristics. It is providing important contributions of domain wall pinning in the polarization degradation of Aurivillius-type ferroelectric films with four layers.