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Single crystals of 2-methylbenzimidazolium perchlorate were prepared for the first time with a slow evaporation method from an aqueous solution of a mixture of 2-methylbenzimidazole (MBI) crystals and perchloric acid HClO4. The crystal structure was determined by single crystal X-ray diffraction (XRD) and confirmed by XRD of powder. Angle-resolved polarized Raman and Fourier-transform infrared (FTIR) absorption spectra of crystals consist of lines caused by molecular vibrations in MBI molecule and ClO4− tetrahedron in the region ν = 200–3500 cm−1 and lattice vibrations in the region of 0–200 cm−1. Both XRD and Raman spectroscopy show a protonation of MBI molecule in the crystal. An analysis of ultraviolet-visible (UV-Vis) absorption spectra gives an estimation of an optical gap Eg~3.9 eV in the crystals studied. Photoluminescence spectra of MBI-perchlorate crystals consist of a number of overlapping bands with the main maximum at Ephoton ≅ 2.0 eV. Thermogravimetry-differential scanning calorimetry (TG-DSC) revealed the presence of two first-order phase transitions with different temperature hysteresis at temperatures above room temperature. The higher temperature transition corresponds to the melting temperature. Both phase transitions are accompanied by a strong increase in the permittivity and conductivity, especially during melting, which is similar to the effect of an ionic liquid.

In this work, we presented investigations on the microstructural, magnetic, dielectric and conductive properties of (1-x)Na0.5Bi0.5TiO3-xCoFe2O4 (NBT-CFO) composite ceramics (x = 0.1, 0.2, 0.3 and 0.5). The composites were synthesized using the in-situ sol-gel method, and the resulting CFO/NBT embedded structure was observed in the 0.9NBT–0.1CFO sample. Rietveld refinement confirmed that the embedded structure had remarkable effects on the c/a ratios of NBT-CFO composites. Increasing normalization saturation magnetizations were observed with increasing of CFO contents, thus revealing the influence of phase interfaces on magnetism. Dielectric relaxation anomalies were observed on both temperature dependent permittivity and loss spectra. Two sets of anomalies in these spectra were caused by space charges on the phase interfaces of embedded CFO grains and the NBT matrix. The conduction behavior of ceramics with and without the embedded structure obeyed the double power law and Jonscher's fraction power law, respectively, which inferred a transformation from the AC to the DC conduction mechanism with the disappearance of the embedded structure.

The 0.9SrTiO3–0.1NiFe2O4 (STO/NFO) composite ceramics sintered at O2/N2 atmospheres were investigated. The N2-sintered STO/NFO (STO/NFON) had a larger microstrain (0.092(47)%) than the O2-sintered STO/NFO (STO/NFOO) revealed by the Rietveld refinement and the Williamson-Hall analysis. The maximum polarization of STO/NFON increased to ~7.1 μC/cm2, but the larger leakage current in STO/NFOO resulted in a 20% larger false remanent polarization value. Moreover, STO/NFON presented a lower activation energy (Ea, 1.22 eV), which implied more oxygen vacancies ( V O ⋅ ⋅ ). The amount of Ti3+ ions (60%) in STO/NFON increased as well, which formed defect dipoles [Ti4+·e− V O ⋅ ⋅ −Ti4+·e] and fixed V O ⋅ ⋅ . The composite of STO/NFON also showed decreased V O ⋅ ⋅ content (~53%) owing to the fixing effect and presented low leakage current, increased impedances, and the improved ferroelectric hysteresis loop. The anomaly dielectric constants peaks indicated different conductivity mechanisms related to V O ⋅ ⋅ , and the STO/NFON ceramic possessed a low rate of dielectric loss.

Lead-free ferroelectric Na0.5Bi0.5TiO3 (NBT), 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 (NBT-6BT) and (Na0.5Bi0.5)0.94Ba0.06(Nb0.05Ti0.95)O3 (Nb-NBBT6) ceramics were synthesized via a multi-ionic sol-gel method process. The effects of Nb on the structural, dielectric and ferroelectric properties were studied. Doped Nb ions distributed uniformly in the Nb-NBBT6 ceramics and shrank the grain sizes of Nb-NBBT6. All NBT, NBT-6BT and Nb-NBBT6 ceramics consisted of the rhombohedral phase and the tetragonal phase according to XRD Rietveld refinements. The dielectric constants of the Nb-NBBT6 ceramics were improved compared to NBT and NBT-6BT below 200 °C. The introduction of Nb ions enhanced the dielectric stability, and the temperature difference between two dielectric peaks increased from 153 °C for NBT to 197 °C for Nb-NBBT6. The remnant polarization (Pr) of Nb-NBBT6 ceramics decreased, but the reduced Ec of 7.01 kV/cm and the maximum polarization of 40.79 μC/cm2 (90 kV/cm) revealed the enhanced ferroelectric property. The temperature-dependent ferroelectric hysteresis loops of Nb-NBBT6 ceramics (65–150 °C) exhibited the largest energy-storage density (W) of 0.399 J/cm3 at 90 °C.

The existence of Bi-fluctuation dispersing in Na0.5Bi0.5TiO3 (NBT) relaxor ferroelectric is hinted in other recent studies. However, this fluctuation has not been directly observed yet. We introduc...

Morphotropic phase boundaries (MPB) consisting of rhombohedral, nonpolar orthorhombic and orthorhombic bridging phases were formed within the range of 0.1 ≤ x ≤ 0.2 in Bi1-xNdxFe0.97Ti0.03O3 ceramics. The intricate phase coexistence was verified by X-ray diffraction refinements, new modes at ~305 cm−1 in Raman spectra and microstructure analyses. Impedance tests confirmed the MPB ranges via resistance fluctuations and revealed additional arcs from phase boundaries. Ferroelectric and magnetic properties were synchronously improved at MPB resulting in the apparent magnetodielectric effect with a permittivity change rate of ~1.5% at 10000 Oe. Additionally, magnetic domains were reversed at low voltages.

Morphotropic phase boundaries (MPB) consisting of rhombohedral and orthorhombic phases formed within the range of 0.1 ≤ x ≤ 0.15 in Bi1-xErxFe0.97Ti0.03O3 ceramics. Orthorhombic phase transition was verified by the appearance of a new mode at 310 cm−1 in Raman spectra. Bridging phases as intermedia phases of MPB were rare in Bi1-xErxFe0.97Ti0.03O3. Polarization and permittivity improved at MPB. Moreover, impedance spectroscopy revealed additional arcs with a distinct small polaron conduction mechanism. Er3+ ions enhanced magnetic properties and induced a permittivity change at Neel temperature. The magnetodielectric effect resulted in a permittivity change rate of ~1% at 10000 Oe.

Lead-free 0.9Na0.5Bi0.5TiO3 − 0.1CoFe2O4 (0.9NBT-0.1CFO) composite ceramics with novel embedded microstructures were synthesized by an in-situ sol-gel method. The structural evolution, ferroelectricities and dielectricities were studied. The embedded structures were formed around 1000 °C − 1100 °C with contributions of the strong grain boundary mobility and small size of CFO grains, and then were destroyed at 1150 °C. The 0.9NBT-0.1CFO ceramic sintered at 1100 °C showed the optimal ferroelectric hysteresis loop with a remanent polarization of ~ 52 μC/cm2. Anomaly peaks in the temperature dependent permittivity curves were observed in all ceramics with embedded structures. These Debye-like type peaks generated by the interface effects of NBT and CFO appeared around ~620 °C, and could reach relatively large values of ~15000 at 100 kHz. The change rate of permittivity between zero magnetic field and 500 Oe reached 2.4%, indicating a noteworthy magnetoelectric effect.

The 0.8Bi3.15Nd0.85Ti3O12 (BNdT)-0.2CoFe2O4 (CFO) composite multiferroic ceramics have been fabricated by spark plasma sintering (SPS) at 850 °C. The relative density of as-sintered SPS ceramic reaches 97.4 (±0.3)%. The composites are composed of pure BNdT and CFO phases without any preferred c-orientation. The a-orientation preference is more obvious perpendicular to the pressure direction. The average grain-sizes of BNdT and CFO are 163 and 146 nm, respectively. The BNdT phase has more grains below 100 nm (∼20%). The super energy-dispersive X-ray analyses suggest no serious reaction between BNdT and CFO. The Raman spectrum verifies the nano-structure of the SPS ceramic via the broadening bands and peak shifts. The Curie temperature of the SPS ceramic declines to 560 °C with stabilized dielectric loss. The grain boundary resistance plays a dominant role on impedance above 700 °C. The remanent polarization approaches to 15.2 μC/cm2 (300 kV/cm) with lower coercive fields (−89/+95 kV/cm).

A series of experimental and theoretical studies have demonstrated that Bi1-xNdxFeO3 ceramics with morphotropic phase boundaries (MPBs) exhibit a very complex phase-transition process including the displacive ferroelectric phase transition and reorientation of antiferromagnetic alignments [Chem. Mater. 23, 2166 (2011); Adv. Funct. Mater. 25, 552 (2015)]. In this work, such complex phase-transition process in the Bi0.85Nd0.15FeO3 ceramic is detected by electric and magnetic responses as the function of frequency. First, the phase transition from orthorhombic Pbam phase (O2) to orthorhombic Pbnm phase (O1) is found to be accompanied by the change in the local strain induced by MPBs. Second, the electric response at O2→O1 transition with the disappearance of antiferroelectric order showed the relaxation and reversible rotation of the ferroelectric order coupled with the local strain. The moderate active energy of such coupling (∼2.7 eV) facilitates the possibility of recovering the deformation of Bi1-xNdxFeO3 ceramic by an external electric field. Third, the weak magnetic response is frequency-independent, and the reorientation of the antiferromagnetic order is also reversible. Finally, the rotation of ferroelectric/magnetic order connected to MPBs in the ceramic is discussed.

Phase-pure Aurivillius Bi5Ti3FeO15 nanoparticles were successfully prepared using a sol–gel method. Behaviours of crystallisation were characterised by XRD, DTA, FT-IR, Raman spectra and HRTEM. Mass spectrum was used to monitor the combustion products of all organics in the xerogel during calcination. An intermediate product, Bi2O2CO3, which blocks the generation of the pyrochlore phases, was confirmed by XRD patterns and IR spectra. Calculated by XRD data using the Debye–Scherrer method, the average grain size in powders calcinated at 500 °C was 17.7 nm and grew to 80.5 nm at 850 °C. The local symmetry connected to FeO6 octahedral was broken. The XRD refinement and XPS analyses inferred the valence increase of ferric ions. The maximum magnetisation of nanoparticles prepared at 850 °C was 4.2 emu/g. The field cooling (FC) and zero field cooling (ZFC) tests showed a wide range of the blocking temperature (T B) around 8–45 K answering for the lack of hump corresponding to the transition from superparamagnetism to blocking state.

The ferroelectric domain structures were analyzed in strained multiferroic BiFeO3 ceramics via X-ray diffraction (XRD), scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). XRD refinements indicate that the SPS method introduced the large strain into the ceramic samples even though ~50% strain reduction could be conducted through annealing process. The large strain increased the contents of unusual 71° ± 1° grain boundaries and deformed 71° domain walls (~89° twin boundaries) both in the as-sintered sample and the annealed sample, which could accommodate the grain boundary energy and the large thermal strain. The large strain was more conducive to form typical curved and bridge-like 180° domain morphologies. And the domain topographies heavily relied on the grain orientations.