X. W. Tang

Multiferroic properties of Aurivillius phase Bi6Fe2−xCoxTi3O18 thin films prepared by a chemical solution deposition route

The magnetic and ferroelectric properties of Co-doped Bi6Fe2Ti3O18 thin films Bi6Fe2−xCoxTi3O18 (BFCTO) (0 ≤x≤ 1) are investigated. The coexistence of room-temperature ferromagnetism and ferroelectricity is observed in BFCTO thin films. The x = 0.6 sample exhibits a higher remnant magnetization Mr of 8.41 emu/cm3 and a remnant polarization Pr of 17.6 μC/cm2 compared with other BFCTO thin films. The ferromagnetism can be ascribed to the spin canting of Fe- and Co-based sublattices via the Dzyaloshinskii-Moriya interaction [I. Dzyaloshinsky, J. Phys. Chem. Solids 4, 241 (1958); T. Moriya, Phys. Rev. 120, 91 (1960)]. The change in the remnant polarization is discussed in terms of the variation of grain size and oxygen vacancies caused by Co-doping.

Kinetically Enhanced Electrochemical Redox of Polysulfides on Polymeric Carbon Nitrides for Improved Lithium–Sulfur Batteries

The kinetics and stability of the redox of lithium polysulfides (LiPSs) fundamentally determine the overall performance of lithium-sulfur (Li-S) batteries. Inspired by theoretical predictions, we herein validated the existence of a strong electrostatic affinity between polymeric carbon nitride (p-C3N4) and LiPSs, that can not only stabilize the redox cycling of LiPSs, but also enhance their redox kinetics. As a result, utilization of p-C3N4 in a Li-S battery has brought much improved performance in the aspects of high capacity and low capacity fading over prolonged cycling. Especially upon the application of p-C3N4, the kinetic barrier of the LiPS redox reactions has been significantly reduced, which has thus resulted in a better rate performance. Further density functional theory simulations have revealed that the origin of such kinetic enhancement was from the distortion of molecular configurations of the LiPSs anchored on p-C3N4. Therefore, this proof-of-concept study opens up a promising avenue to improve the performance of Li-S batteries by accelerating their fundamental electrochemical redox processes, which also has the potential to be applied in other electrochemical energy storage/conversion systems.

Magnetic and dielectric properties of Aurivillius phase Bi6Fe2Ti3−2xNbxCoxO18 (0 ≤ x ≤ 0.4)

We investigate the structural, magnetic, and dielectric properties of Bi6Fe2Ti3−2xNbxCoxO18 (0 ≤ x ≤ 0.4). The room-temperature ferromagnetism is observed in the Nb and Co co-doped samples compared with the paramagnetic behavior in Bi6Fe2Ti3O18. The ferromagnetism in Bi6Fe2Ti3−2xNbxCoxO18 can be understood in terms of spin canting of the antiferromagnetic coupling of the Fe-based and Co-based sublattices via Dzyaloshinsky-Moriya interaction. Moreover, doping Co at Ti sites can significantly enhance the ferromagnetic Curie temperature compared with the substitution of Co for Fe in the Aurivillius compounds. The dielectric loss of Bi6Fe2Ti3−2xNbxCoxO18 (0.1 ≤ x ≤ 0.4) exhibits a relaxation process. The rather large activation energy in the 0.1 ≤ x ≤ 0.3 samples implies that the relaxation process is not due to the thermal motion of oxygen vacancies inside ceramics.

Large remnant polarization and magnetic field induced destruction of cycloidal spin structure in Bi1−xLaxFeO3 (0 ≤ x ≤ 0.2)

We prepared a series of Bi1−xLaxFeO3 (0 ≤ x ≤ 0.2) ceramics with a sol-gel method and find that both the magnetization and dielectric constant show an abrupt anomaly near a critical field Hc, which is attributed to the destruction of the cycloidal antiferromagnetic spin structure. The critical field Hc decreases substantially from ∼20 T for the x = 0 sample [Y. F. Popov et al., JETP Lett. 57, 69 (1993)] to ∼2.8 T for the x = 0.17 sample and finally to 0 T for the x = 0.2 sample at room temperature (RT). It is also found that Hc increases with decreasing temperature. The variation of Hc with La substitution and temperature can be ascribed to the change in the magnetic anisotropy and isotropic superexchange interaction, respectively. We have also discussed the magnetodielectric effects in these samples in terms of the Ginzburg-Landau theory and the spin-phonon model. Moreover, increasing the doping level of La to 0.15 greatly improves the RT leakage-current and ferroelectric (FE) properties. A RT square-sha...

Improved ferroelectric polarization of V-doped Bi6Fe2Ti3O18 thin films prepared by a chemical solution deposition

We prepared V-doped Bi6Fe2Ti3O18 thin films on Pt/Ti/SiO2/Si (100) substrates by using a chemical solution deposition route and investigated the doping effect on the microstructure, dielectric, leakage, and ferroelectric properties of Bi6Fe2Ti3O18 thin films. The Bi5.97Fe2Ti2.91V0.09O18 thin film exhibits improved dielectric properties, leakage current, and ferroelectric properties. The incorporation of vanadium resulted in a substantially enhanced remnant polarization (2Pr) over 30 μC/cm2 in Bi5.97Fe2Ti2.91V0.09O18 thin film compared with 10 μC/cm2 in Bi6Fe2Ti3O18 thin film. It is demonstrated that the improved properties may stem from the improvement of crystallinity of the films with the contribution of suppressed oxygen vacancies and decreased mobility of oxygen vacancies caused by the V-doping. The results will provide a guidance to optimize the ferroelectric properties in Bi6Fe2Ti3O18 thin films by chemical solution deposition, which is important to further explore single-phase multiferroics in the ...

Enhanced multiferroic properties of Aurivillius Bi6Fe1.4Co0.6Ti3O18 thin films by magnetic field annealing

We investigate the effect of high magnetic-field annealing on the microstructural, ferroelectric, and magnetic properties of Bi6Fe1.4Co0.6Ti3O18 thin films. The magnetic field can lower the energy barrier for nucleation and improve the grain connectivity. The application of magnetic field of 6T parallel to the film plane can substantially enhance the remnant polarization Pr from 18.1 to 29 μC/cm2 as a result of the variation in grain size and growth orientation caused by magnetic field annealing. Moreover, the remnant magnetization Mr is substantially improved from 2.48 to 4.56 emu/cm3 arising from the enhanced exchange coupling due to the better grain connectivity. These results demonstrate that high magnetic-field annealing is an effective way to optimize multiferroic properties of the Aurivillius compounds.

Bi3.25La0.75Ti3O12 thin film capacitors for energy storage applications

Environmentally benign Bi3.25La0.75Ti3O12 (BLTO) thin film capacitors were prepared by a cost effective chemical solution deposition method for high energy density storage device applications. Low annealing temperature annealed BLTO thin films showed very slim hysteresis loops with high maximum and small remnant polarization values. Increasing the applied electric field to 2040 kV/cm, the optimized BLTO thin films show a high recoverable energy density of 44.7 J/cm3 and an energy efficiency of 78.4% at room temperature. Additionally, the BLTO thin film capacitors exhibited excellent fatigue endurance after 4 × 108 cycles and a good thermal stability up to 140 °C, proving their strong potential for high energy density storage and conversion applications.

Room temperature multiferrocity and magnetodielectric properties of ternary (1-x) (0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xBiFeO3 (0 ≤x≤ 0.9) solid solutions

(1–x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xBiFeO3 (0 ≤x≤ 0.9) ternary ceramics were prepared by the modified Pechini method. X ray diffraction patterns manifest that the samples undergo a gradual structural transformation from the rhombohedral-tetragonal morphotropic phase boundary to the rhombohedral phase with the increasing content of BiFeO3. Room temperature ferromagnetism was observed in BiFeO3-rich samples with x ≥ 0.6, and this ferromagnetism can be ascribed to the suppression of the spiral spin structure with the canting of antiferromagnetically ordered spins. The BiFeO3-rich samples (x = 0.6–0.8) exhibit superior ferroelectric properties with the maximum remanent polarization of 44.7 μC/cm2, as confirmed by the positive-up negative-down measurement to exclude the contribution of leakage current. The coexistence of room-temperature ferromagnetism and ferroelectricity manifests that the lead-free BiFeO3-based solid solutions are quite promising multiferroic materials and may be important for potential a...

Ultrahigh energy storage in lead-free BiFeO3/Bi3.25La0.75Ti3O12 thin film capacitors by solution processing

Thin film ferroelectric capacitors (TFFCs) with excellent energy storage have attracted increasing attention due to the electronic devices toward miniaturization and integration. BiFeO3 (BF)/Bi3.25La0.75Ti3O12 (BL) based thin films are prepared by chemical solution deposition for energy storage. Ultrahigh energy storage with a recoverable energy density Ure of 54.9 J/cm3 and an efficiency η of 74.4% is observed in the bilayered BF/BL thin films. Further improvement of energy storage is realized in trilayered BL/BF/BL thin films with a Ure of 65.5 J/cm3 and an efficiency η of 74.2% at an electric field of 2753 kV/cm as well as excellent fatigue endurance up to 109 cycles. The results suggest that BF/BL based thin films can be used as lead-free TFFCs in energy storage applications.

Magnetic, dielectric properties, and scaling behaviors of Aurivillius compounds Bi6−x∕3Fe2Ti3−2x(WCo)xO18 (0 ≤ x ≤ 0.15)

We investigate the structural, magnetic, dielectric properties, and scaling behaviors of Aurivillius compounds Bi6−x∕3Fe2Ti3−2x(WCo)xO18 (0 ≤ x ≤ 0.15). The room-temperature weak ferromagnetism is observed for the W/Co co-doped samples. The results of the dielectric constant er, complex impedance Z″, the dc conductivity σdc, and hopping frequency fH manifest that the dielectric relaxation of the x = 0 sample and the doped samples in the dielectric anomaly region (450–750 K) can be ascribed to the trap-controlled ac conduction around the doubly ionized oxygen vacancies and the localized hopping process of oxygen vacancies, respectively. The scaling behaviors reveal that the dynamic process of both electrons in the x = 0 sample and oxygen vacancies in the doped samples is temperature independent. The ferroelectric Curie-temperature Tc decreases slightly from 973 K to 947 K with increasing the doping level of W/Co. In addition, the dielectric loss exhibits a dielectric relaxation above 800 K with the rather ...