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Effects of B-site Ta5+ substitution on the ferroelectric transition and crystal structure modulation were investigated for Sr2Na(Nb1−xTax)5O15 (x = 0–0.5) tungsten bronzes. The weakened ferroelectricity and enhanced relaxor behaviors were observed with increasing Ta, while the room temperature crystal structure changed from non-centrosymmetric to centrosymmetric. Raman spectra showed a decreasing intensity of the internal vibrations with increasing Ta, revealing different bonding mechanisms between Nb/Ta cations and O anions with reduced covalence by Ta-substitution, which resulted in weakened polarization. Two sets of superlattice reflections were detected in the selected area electron diffraction patterns: commensurate modulation for all compositions (type I), described by the wave vectors q 1 = 1 4 (a 0 ∗ + b 0 ∗) + 1 2 c 0 ∗ and q 2 = 1 2 (a 0 ∗ + b 0 ∗) ; extra reflections of 1/2[0 0 l] at some local areas for Ta-containing compositions (type II), indicating the possible ordering of B-site cations. The distinct bonding state between the B-site cation and the O anion dominates the crossover from ferroelectric to relaxor behaviors in the present compounds. [ABSTRACT FROM AUTHOR]

In the present study, enhanced antiferromagnetic characteristics and room temperature magnetoelectric coupling are achieved by Fe-substitution for Mn in h-ScMnO3. The antiferromagnetic Néel temperature (TN) increases from 131 K for ScMnO3 up to 140 K for ScMn0.8Fe0.2O3. Electric field controlled magnetism is confirmed for ScMn0.8Fe0.2O3 at room temperature, combining with the short-range magnetic spin ordering. The Rietveld structural refinement and selected area electron diffraction confirm the noncentrosymmetric P63cm structure for the present ceramics. The ferroelectric domain walls at the atomic scale are determined by the aberration-corrected high-angle annular dark-field scanning transmission electron microscopy; meanwhile, the spontaneous polarization of 5.6 μC/cm2 is determined. The noticeable magnetoelectric coefficient αME ∼ 0.78 mV/cm Oe (measured under a magnetic field up to 3000 Oe) is obtained in ScMn0.8Fe0.2O3, which is quite stable with the applied magnetic field, reflecting coupling of the short-range spin ordering with the ferroelectric ordering. The enhanced antiferromagnetic behaviors are discussed with the effect of Fe-substitution, which reduce the magnetic frustration and enhance the exchange interaction. [ABSTRACT FROM AUTHOR]

SmFeO3 has received the increasing scientific attention as a promising room‐temperature single‐phase multiferroic material with great application potential in the next‐generation spintronic devices. However, the ferroelectricity of SmFeO3 has remained as a controversial issue. In the present work, high resistivity incipient ferroelectric CaTiO3 was introduced as a charge blocking layer on an SmFeO3 thin film to suppress the leakage current. CaTiO3/SmFeO3/Nb–SrTiO3 (1 0 0) thin films were prepared by a PLD (pulsed laser deposition) process, and the epitaxial growth of thin films was confirmed by XRD (X‐ray diffraction) and HRTEM (high resolution transmission electron microscope) images. The interface strain resulted in the polar non‐centrosymmetry in space group P42/mc for SmFeO3, and subsequently the room‐temperature ferroelectricity was achieved with a robust remanent polarization of about 4.9 μC/cm2. The 180° switchable domain structure was confirmed by PFM (piezoelectric force microscope). Meanwhile, strong magnetoelectric coupling with magnetoelectric coefficient (αME) of about 2.5 V/(cm Oe) was observed. The present work has revealed the structural origin of ferroelectricity in an SmFeO3 thin film and provided the great reference for its room temperature multiferroic applications. [ABSTRACT FROM AUTHOR]

Ion substitution was the most efficient way for BiFeO3 modification. In this work, polar Na0.5Bi0.5TiO3 and non-polar CaTiO3 were chosen to modify BiFeO3, and the effects of Na0.5Bi0.5TiO3 and CaTiO3 co-substitution on the crystal structure, dielectric, ferroelectric and magnetic properties of BiFeO3 ceramics were investigated. All ceramics exhibited rhombohedral R3c phase with dense microstructure. Two dielectric anomalies were observed, one related to the valence change of Fe ions and the other to the ferroelectric phase transition. The DSC and dielectric analysis confirmed that the Curie temperature TC linearly increased with increasing Na0.5Bi0.5TiO3 content, while the magnetic transition temperature TN nearly unchanged. Meanwhile, the leakage current of the ceramic samples with Na+ was three orders of magnitude lower than that of a sample without Na+ ion. Due to the decrease in leakage current density, the breakdown field strength and piezoelectric properties were improved. In addition, the magnetic properties of all co-substituted ceramics were improved to a certain extent, which indicated that the co-doping of A-site and B-site affected the magnetic properties of the present ceramics. [ABSTRACT FROM AUTHOR]

BiFeO3 is the most typical multiferroic materials. In this paper, the effects of different types of ion replacement on the structure and properties of Bi(Mg,M)O3 (M=Hf, Ta) modified 0.75BiFeO3–0.25BaTiO3-based ceramics were investigated. XRD and TEM analysis showed that all samples were characterized by the coexistence of rhombohedral R3c and tetragonal P4mm phases. Two dielectric anomalies were observed for all samples. Among them, the dielectric relaxation at 300 ~ 500 °C showed obvious frequency dispersion, which was related to the diffusion of oxygen ion vacancies. A frequency independent dielectric anomaly was detected accompanied by thermal hysteresis at around 625 ~ 650 °C, which indicated the first-order ferroelectric transition of all samples. Meanwhile, the ferroelectric transition temperatures of all samples hardly change, which was attributed to the fixed A-site substitution contents, indicating that the ionic species has little effect on the phase transition temperature. The leakage current of the ceramics was significantly reduced after composite ion replacement. The magnetic properties of the ceramics were significantly improved after the addition of (Mg, M) composite ions, and the magnetic properties of (Mg1/2Hf1/2)3+-substituted samples is better than that of (Mg2/3Ta1/3)3+-substituted samples. The increased magnetic properties is attributed to the suppression of the helical spin structure of the ceramic antiferromagnetic sequence by the B-site ion replacement. [ABSTRACT FROM AUTHOR]

Hexagonal rare‐earth ferrites (h‐RFeO3) have attracted much scientific attention due to their room‐temperature multiferroicity. However, it is still a hard job to obtain h‐RFeO3 bulk materials because of the meta‐stability of such hexagonal phase, and the evaluation of room‐temperature ferroelectric and magnetoelectric characteristics in such materials is also a challengeable issue. In the present work, Yb1−xInxFeO3 ceramics with the stable hexagonal structure were obtained by introducing chemical pressure, where the unique ferroelectric domain structures of sixfold vortex combined with tenfold vortex with a typical size of ~400 nm were determined. Symmetry of the present system evolved from centrosymmetric orthorhombic Pbnm (x = 0–0.4) to non‐centrosymmetric hexagonal P63cm (x = 0.5 and 0.6) with a ferroelectric polarization up to 3.2 μC/cm2, and finally to centrosymmetric hexagonal P63/mmc (x = 0.7 and 0.8). The Curie point decreased monotonically from 723 K to a temperature below room temperature with increasing x, and the antiferromagnetic phase transition above room temperature was determined for all compositions. Meanwhile, a large linear magnetoelectric coefficient (αME) up to 0.96 mV/cm Oe was obtained at room temperature, and this indicated the great application potential for magnetoelectric devices. [ABSTRACT FROM AUTHOR]

The prototypical Ruddlesden‐Popper compound Ca3Mn2O7 has been predicted to possess hybrid improper ferroelectricity, where the polarization is induced by the condensation of two oxygen octahedral distortion modes. Nevertheless, it is a big challenge to switch the polarization at room temperature in Ca3Mn2O7 since the presence of intermediate nonpolar Acaa phase generally leads to the complex domain morphology. Here, the effects of Fe/Nb cosubstitution on hybrid improper ferroelectricity in Ca3Mn2O7 are reported, and easy polarization switching at room temperature is achieved in Ca3[Mn0.5(Fe0.5Nb0.5)0.5]2O7. The ferroelectric phase transition occurs directly from I4/mmm to A21am at a temperature far above room temperature without intermediate nonpolar Acaa phase. The distinct transition pathway forms the alternating 180°‐type ferroelectric domains rather than the irregular 90°‐type ferroelastic domains stacked along [001], resulting in easy polarization switching at room temperature. Moreover the enhanced ferroelectric polarization (Pr ~2.0 μC/cm2) is obtained due to the increased anti‐ferrodistortive displacements of Ca cations at A‐site, arising from the larger amplitudes of oxygen octahedral distortions. Chemical pressure is emphasized here for the tunability of phase transition, domain morphology, and ferroelectric characteristics, and it provides a useful approach for designing and creating high‐performance improper ferroelectrics. [ABSTRACT FROM AUTHOR]

The copresence of ferroelectricity and ferromagnetism inside the same compound is a rather scarce phenomenon, and therefore, the development of room-temperature multiferroic materials is a great challenge. In the present work, La0.5Y0.5FeO3 ceramics with La and Y ions partially ordered as a rocksalt type structure have been prepared. Such ordering leads to a polar P21nm phase, which is consistent with a particular type of hybrid improper ferroelectricity that has rarely been observed. Saturated polarization-electric field loops are determined at room-temperature with a remnant polarization of 0.11 μC/cm2, together with a room-temperature predominantly antiferromagnetic order accompanied by a weak magnetization, which renders our La0.5Y0.5FeO3 ceramics an example of a room-temperature multiferroic compound. [ABSTRACT FROM AUTHOR]

Abstract: In the present work, h‐RFeO3 multiferroic ceramics are designed and created by introducing chemical pressure (In‐substitution for Lu) in LuFeO3. Lu1−xInxFeO3 (x = 0‐0.75) ceramics are prepared by the standard solid‐state reaction process. The crystal structure of the present ceramics is tuned from centrosymmetric Pbnm (x = 0) to non‐centrosymmetric P63cm (x = 0.4–0.6), and subsequently to centrosymmetric P63/mmc (x = 0.75), while the Pbnm and P63cm biphase structure is detected for x = 0.25. The Curie temperature for the polar P63cm (x = 0.4–0.6) phase decreases from >1000 to ≈550 K with increasing x. Cloverleaf ferroelectric domain structures are determined in polar Lu0.5In0.5FeO3 samples, and the ferroelectric domain walls at atomic scale are evaluated by the aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (HAADF STEM), where the spontaneous polarization of 1.73 µC cm−2 is determined for x = 0.5. The spontaneous polarization is also confirmed by calculating the site displacement from the centrosymmetric phase based on the X‐ray diffraction (XRD) data. Meanwhile, two magnetic transitions are determined for all compositions, that is, paramagnetic to antiferromagnetic transition at Néel temperature TN (≈350 K for x = 0.4–0.6), and antiferromagnetic to weak‐ferromagnetic transition at spin‐reorientation temperature TSR. The co‐presence of ferroelectric and antiferromagnetic orders confirms the present ceramics as promising room‐temperature multiferroic materials. [ABSTRACT FROM AUTHOR]

Abstract: In the present work, h‐RFeO3 multiferroic ceramics are designed and created by introducing chemical pressure (In‐substitution for Lu) in LuFeO3. Lu1−xInxFeO3 (x = 0‐0.75) ceramics are prepared by the standard solid‐state reaction process. The crystal structure of the present ceramics is tuned from centrosymmetric Pbnm (x = 0) to non‐centrosymmetric P63cm (x = 0.4–0.6), and subsequently to centrosymmetric P63/mmc (x = 0.75), while the Pbnm and P63cm biphase structure is detected for x = 0.25. The Curie temperature for the polar P63cm (x = 0.4–0.6) phase decreases from >1000 to ≈550 K with increasing x. Cloverleaf ferroelectric domain structures are determined in polar Lu0.5In0.5FeO3 samples, and the ferroelectric domain walls at atomic scale are evaluated by the aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (HAADF STEM), where the spontaneous polarization of 1.73 µC cm−2 is determined for x = 0.5. The spontaneous polarization is also confirmed by calculating the site displacement from the centrosymmetric phase based on the X‐ray diffraction (XRD) data. Meanwhile, two magnetic transitions are determined for all compositions, that is, paramagnetic to antiferromagnetic transition at Néel temperature TN (≈350 K for x = 0.4–0.6), and antiferromagnetic to weak‐ferromagnetic transition at spin‐reorientation temperature TSR. The co‐presence of ferroelectric and antiferromagnetic orders confirms the present ceramics as promising room‐temperature multiferroic materials. [ABSTRACT FROM AUTHOR]

Effects of postdensification annealing on microwave dielectric properties of Ba([ Mg1− x Co x]1/3Nb2/3)O3 ceramics have been investigated. The 1:2 ordered complex perovskite structure is obtained in the entire composition range of the as-sintered samples, and the ordering degree decreases with increasing x. With increasing x, the dielectric constant (εr) increases, the temperature coefficient of resonant frequency (τ f) shifts toward negative and can be tuned to near-zero for x = 0.6, while the Q × f value decreases. With postdensification annealing at 1400°C in air for 24 h, the ordering degree increases, and Q × f value can be significantly improved without obvious change in εr, while the composition for obtaining near-zero τ f shifts to x = 0.8. Annealing at 1375°C and 1425°C in air for 24 h does not improve microwave dielectric properties more than annealing at 1400°C. A good combination of microwave dielectric properties of Ba([Mg1− xCo x]1/3Nb2/3)O3 ceramics after postdensification annealing at 1400°C in air for 24 h has been obtained for x = 0.8: εr = 31.7, Q × f = 76,900 GHz, τ f = 3.3 ppm/°C. [ABSTRACT FROM AUTHOR]