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.
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.
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 q1=14(a0∗+b0∗)+12c0∗ and q2=12(a0∗+b0∗); 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.
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