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The substitution of rare-earth ions has been recognized as an effective way to modify the multiferroic properties, especially to realize the electric field-controlled magnetism in BiFeO3 ceramics, in which the symmetry modulation of R3 c/ Pna21 and the unlocked ferromagnetism through breaking the cycloidal spin structure are the key issues. Here, the enhancement of ferroelectric polarization and weak ferromagnetism are investigated in Bi1− xLa xFeO3 ceramics together with the magnetoelectric coupling coefficient αME with the development of the Pna21 phase. The expected electric field-controlled magnetism is achieved at the phase boundary through the electric field-induced transition of Pna21/ R3 c, where the variation of magnetization reaches 34.18 emu/mol (58.95%) at x = 0.18 after poling.

Rare earth ion (RE)-substitution has been recognized as an effective way for improving multiferroic properties especially for realizing the electric field-controlled magnetism in BiFeO3 ceramics, where the symmetry modulation of R3c-Pna21-Pbnm and unlocking ferromagnetism by destroying the cycloidal spin structure are the key points. In Lu-substituted BiFeO3 ceramics, however, the Pna21 phase is not involved. In the present work, La/Lu co-substitution was investigated to involve the Pna21 phase in BiFeO3 ceramics. The phase transition from R3c to Pna21 occurred with an increase in x, and the mixed structure of R3c, Pna21, and Pbnm phases was determined in Bi0.82−xLaxLu0.18FeO3 ceramics. The greatly enhanced ferroelectric polarization and the switching from antiferromagnetic to the weak ferromagnetic state through the destruction of the cycloidal spin structure were achieved. Subsequently, the direct magnetoelectric coupling coefficient αME up to 2.02 mV/cm Oe was obtained. The increased effective ionic radius of RE in the present ceramics played a key role in involving Pna21 phase in RE-substituted BiFeO3 ceramics, and the critical value was about 0.104 23(2) nm.

In this study, the structure evidence of the Pna21 phase and the field-induced transition of Pna21/R3c have been quantitatively investigated in Bi1-xSmxFe0.99Ti0.01O3 (x = 0.12, 0.14, and 0.16) ceramics together with the subsequent electric field-controlled magnetism. A sequential phase transition of R3c-Pna21-Pbnm is detected by Rietveld refinement of the x-ray diffraction data. Further, the electric field-induced phase transition is investigated near the morphotropic phase boundary at x = 0.14, where the structure is tuned from Pna21 to R3c with poling treatment, and this transition could become reversible with as-annealing after poling. For x = 0.14, the structure is tuned from Pna21 to R3c with poling treatment, and this transition could become reversible with as-annealing after poling. The phase transition is confirmed by Rietveld refinements together with VSM measurements, providing an opportunity for electric field-controlled magnetism. The magnetoelectric coupling is enhanced, and the linear magnetoelectric coefficient αME up to 0.91 mV/Oe·cm is obtained in Bi1-xSmxFe0.99Ti0.01O3 ceramics. More importantly, the R3c and Pna21 structures are confirmed by selected area electron diffraction and dark-field analysis at x = 0.14, and the high-resolution TEM image demonstrates the coexistence of the R3c and Pna21 structures.

The crossover from normal to relaxor ferroelectrics in Sr0.25Ba0.75(Nb1−xTax)2O6 (x = 0, 0.1, 0.25, and 0.5) ceramics was determined together with the structure evolution. A single-phase tungsten bronze structure in space group P4bm (x = 0 and 0.1) or P4/mbm (x = 0.25 and 0.5) was obtained in the entire composition range. The lattice parameter a increased, while c, tetragonality c/a, and cell volume decreased with increasing x. Meanwhile, the dielectric maximum temperature Tm went down and the dielectric peak exhibited an enhanced diffused nature with a stronger frequency dispersion, where the saturated P–E loop (x = 0 and 0.1) transformed to a slim relaxor loop (x = 0.25) and finally to the linear one (x = 0.5). Raman spectra results revealed the reduced covalence between the B-site cation and the oxygen ion as a result of Ta-substitution. The coexistence of Nb5+ and Ta5+, which possessed the distinct covalence with the oxygen ion, played the key role in the structural variation and crossover from normal to relaxor ferroelectrics in the present system.

Hybrid improper ferroelectricity (HIF) has received increasing scientific attention since it provides great potential for realizing single phase room temperature multiferroicity with strong magnetoelectric coupling, and Ruddlesden–Popper (R–P) compounds AO(ABO3)n with even n are the most important candidates for HIFs. However, the modulation of ferroelectric polarization is recognized as a challenging issue. Here, Sr3−xBaxSn2O7 ceramics with a large remanent polarization with respect to other R–P HIFs (∼1.0 μC/cm2) combined with a reduced coercive field are reported. The polarization increases obviously with the increasing tolerance factor, inconsistent with the previous understanding. This is because the amplitude of tilt increases and rotation decreases, which results in the increased distortion of SnO6 octahedra. That is, the polarization of R–P compounds should be determined by the combined effects of tilting and rotation and not be simply evaluated by the tolerance factor.

The aging effect and the metastable ferroelectric state are investigated in the Ba4Eu2(Ti0.9Zr0.1)4Ta6O30 ceramic, where the tetragonal tungsten bronze structure with a P4bm space group is confirmed. When the compound is cooled from temperatures far above its transition temperature (TC), a very slim polarization-electric field curve is observed at 300 K. After 3 days of aging at 300 K, an obvious increase in the remanent polarization and the coercive electric field can be obtained. Moreover, a gradual recovery of the endothermic peak is detected in the differential scanning calorimeter curves when the compound is cooled from temperatures above TC and aged at 300 K for several days. These phenomena are interpreted as the result of a metastable ferroelectric state and the related aging effect, which may be deeply concerned with the compositional inhomogeneity and the hindered superstructure transformation. Through manipulating A and B site cation substitutions, the range of the metastable ferroelectric state can be tuned, and compounds with a different ferroelectric transition process can be obtained.

Lead-free relaxors Ba5RSn3Nb7O30 (R = La, Nd, Sm) with a tetragonal tungsten bronze structure were prepared by a standard solid-state reaction process, and the dielectric and ferroelectric properties were systematically determined together with the structures and microstructures. Rietveld refinement results suggested that a single-phase tungsten bronze structure in space group P4/mbm was obtained for all the compositions. Both the dielectric constant (e′) and the dielectric loss (tan δ) exhibited a broad maximum with evident frequency dispersion far below room temperature, which indicated the typical relaxor nature. The selected area electron diffraction patterns revealed that the relaxor behavior in the present system originated from the incommensurate structure modulation. The dark-field transmission electron microscopy images confirmed the presence of polar nanoregions in the areas generating the satellite reflections of incommensurate structure modulation, which implied a possible correlation between the incommensurate structure modulation and polar nanoregions in tungsten bronze relaxors.

Effects of B site ions on the relaxor to normal ferroelectric crossover in Ba4Sm2Zr4(NbxTa1-x)6O30 (x = 0, 0.3, 0.5, 0.7, 0.9, and 1) tungsten bronze ceramics were investigated. The tetragonal tungsten bronze structure in space group P4/mbm was confirmed for compositions of x = 0, 0.3, and 0.5, while space group P4bm was confirmed for compositions of x = 0.7, 0.9, and 1. With the increasing Nb content in the B site, the dielectric properties showed a crossover from the relaxor to normal ferroelectric behavior, accompanied with the disappearance of frequency dispersion, increased ferroelectric transition temperature, and larger dielectric constant value. The room temperature Raman spectra for compositions of x = 0 and x = 1 showed different vibration modes of the oxygen octahedral, and the temperature dependence of the integrated intensity of Raman bands indicated the different structural transition process. Unlike the effects of A site ions, the onset of the relaxor/normal ferroelectric behavior of Ba4Sm2Zr4(NbxTa1-x)6O30 was mainly caused by the different electron configuration of Nb and Ta.

Ferroelectricity and polarization reversal dynamics in Ba4Sm2Ti4Ta6O30 tungsten bronze ceramics were investigated by measuring dielectric spectra and the evolution of hysteresis loops over a wide temperature range. With decreasing temperature, the dielectric properties and differential scanning calorimetry results showed diffuse peaks at ∼280 K with large thermal hysteresis, suggesting a first order ferroelectric transition. A dielectric relaxation was observed at low temperature that followed the Vogel–Fulcher relationship. The saturation and remanent polarizations of the Ba4Sm2Ti4Ta6O30 ceramics showed remarkable dependence on the applied field and temperature. The temperature dependence of the coercive field was divided into three linear regions and fitted to the Vopsaroiu model. Activation energies for polarization reversal of 0.73, 0.79, and 0.65 eV were determined for the following three regions: (I) the diffuse ferroelectric transition region (323.15–293.15 K), (II) the region just below the ferroe...

Ba5RTi3Nb7O30 (R = La, Nd, Sm) tungsten bronze ceramics were prepared, and the dielectric and ferroelectric properties were investigated over a broad temperature range. The relaxor nature was determined for all compositions in their permittivity curves, and a second anomaly of the dielectric loss (tan δ) was observed around 250 K in Ba5NdTi3Nb7O30 and around 275 K in Ba5SmTi3Nb7O30. Both the maximum and remanent polarization tended to decrease and vanish at low temperatures in the ferroelectric phase for all compositions, which was referred to as the low temperature re-entrant relaxor behavior. The remanent polarization increased with decreasing temperature first and then reached the maximum value at the re-entrant temperature (Tr). For Ba5RTi3Nb7O30 (R = La, Nd, Sm), Tr decreased with the radius of R3+ cations and the applied field amplitude.

Effects of Ca-substitution on the crystal structure and dielectric properties were investigated for Ba5SmTi3Nb7O30 tungsten bronze ceramics. The tetragonal tungsten bronze structure in space group P4bm was determined in Ba4CaSmTi3Nb7O30 ceramics, which was the same for Ba5SmTi3Nb7O30. The c-axis off center displacement in both B1 and B2-sites for Ba5SmTi3Nb7O30 was slightly larger than that in BaCaSmTi3Nb7O30, while the distortion of Ti/Nb(2)O6 octahedra in Ba4CaSmTi3Nb7O30 was obviously suppressed compared with that in Ba5SmTi3Nb7O30. Though there were also two dielectric anomalies observed in Ba4CaSmTi3Nb7O30, the relaxor ferroelectric nature was changed to the normal ferroelectric one, and both the endothermal peak in differential scanning calorimetry curve and the significant thermal hysteresis of the phase transition confirmed the first order ferroelectric transition. These results indicated that the ferroelectric nature of Ba4CaSmTi3Nb7O30 was more close to that in Ba4Sm2Ti4Nb6O30, and it could be c...

Ba5LaTi3Nb7O30 tungsten-bronze ferroelectric ceramics were synthesized and characterized. The tetragonal tungsten bronze structure in space group P4/mbm was confirmed, and a broad permittivity peak with strong frequency dispersion was observed around 250 K where the peak points well fitted the Vogel-Fulcher relationship [H. Vogel, Phys. Zeit. 22, 645 (1921); G. S. Fulcher, J. Am. Ceram. Soc. 8, 339 (1925)]. The temperature dependence of the ferroelectric hysteresis indicated the paraelectric to ferroelectric phase transition in the temperature range between 153 and 273 K. The high temperature permittivity curve deviated from the Curie-Weiss law in quite a narrow temperature region above Tmax, which reflected the weak correlations between the polar nanoregions. The Curie-Weiss constant (C) was 1.218 × 105 K, which was consistent with that for the displacive type ferroelectric. No DSC peak was detected over the temperature region investigated here. Moreover, the Curie-Weiss constant below Tmax (C′) was just...

Dielectric response of tetragonal tungsten bronze dielectrics (BaxSr1−x)4Nd2Ti4Nb6O30 was investigated over a broad temperature and frequency range, and the obvious composition-dependent dielectric anomalies with respect to x value were discussed in detail in association with the radius differences between A1- and A2-site ions. With decreasing the magnitude of radius difference between A1- and A2-site ions, the normal ferroelectric peak above 400K became weaker, and two relaxor peaks at lower temperatures became obvious. The low- and high-temperature relaxor behaviors, which followed well the Vogel-Fulcher relationship, were associated with the polar clusters caused by the off-center Nb∕Ti displacements and the incommensurate tilting modulation, respectively.

Dielectric relaxation and ultrasonic attenuation spectra of Sr4La2Ti4Nb6O30 tungsten bronze ceramics were investigated over a broad temperature range. A broad permittivity peak with strong frequency dispersion was observed around 200K, which followed well the Vogel-Fulcher relationship, and is attributed to relaxor ferroelectric behavior with spin glass nature. The ultrasonic investigation provided further evidence on this thermally activated process, and both the dielectric relaxation and the freezing of polar clusters were clearly observed in the ultrasonic velocity shift and ultrasonic attenuation spectra.