In recent years, much attention has been paid to layer-structured Bi4Bim−3Fem−3−xMxTi3O3m+3 (BFMTO, M = Co, Mn) compounds due to their potential as high temperature single phase multiferroic materials. However, BFMTO single crystals have been rarely reported in the past, though they are better candidates for studying the corresponding intrinsic multiferroics as well as the platform for making future devices, due to their structural complexity and difficulties in fabrication. In this article, Bi5Fe0.9Co0.1Ti3O15 single-crystalline nanoplates were synthesized by the hydrothermal method. The ferromagnetic domain structure of the nanoplate was investigated by electron holography. Denser phase contours were observed and the closed magnetic flux lines indicated a significant magnetic interaction between the neighboring nanoplates, which proved the ferromagnetic nature of the sample. Furthermore, M–H loops of the sample were also measured, in which the ferromagnetic Curie temperature reached ∼730.2 K. Besides, ferroelectric domains were also detected by using a piezoresponse force microscope. All the above-mentioned results indicate the first verification of the room temperature (RT) multiferroic behaviour in such single crystals, which will be useful for both future devices and understanding the underlining physics.

In this article, morphology control of Aurivillius Bi11Fe3Ti6O33 (4.5-BFCTO) microcrystals was investigated in detail in the hydrothermal process, where NaOH concentration and citric acid play a critical role. When NaOH concentrations are 0.3–0.5 M, 0.6 M, and 0.7–1.5 M4.5-BFCTO microflowers, microsheets and truncated tetragonal bipyramid particles formed, respectively. For the 0.3–0.5 M-sample, (Bi2O2)2CO3(OH)2 intermediate product, rather than (Na/Bi)7FexCoyTi5−x−yO21−δ of 0.6 M and 0.7–1.5 M samples, acts as a template and is responsible for the morphology change due to the competition between OH− and CO32− ions originating from the decomposition of citric acid. For the 0.6 M and 0.7–1.5 M samples, the morphology was controlled by the Gibbs free energy along the [001] direction. Besides, the dependence of magnetic properties on the morphologies was also investigated. This research on the morphology control could serve as guidance to potentially synthesize larger BFCTO single crystals and shed light on further designing nanodevices.

By SiO2 shell coating and Nd3+ doping, layer-structured hexagonal phase Gd2O2CO3:Yb3+, Er3+ nanoparticles were synthesized successfully through a homogeneous precipitation method. The detailed mechanism was investigated and the results indicate that the SiO2 shell and Nd3+ doping ions can effectively prevent hexagonal phase Gd2O2CO3 from decomposing into cubic phase Gd2O3 during heat treatment, because the SiO2 shell limits the diffusion of CO2 gas and the Nd3+ doping increases the decomposition temperature of Gd2O2CO3. Compared with non-layer-structured GdVO4:20%Yb3+, 2%Er3+, 4%Nd3+ particles with similar diameters and morphologies and lower phonon energies, layer-structured Gd2O2CO3:20%Yb3+, 2%Er3+, 4%Nd3+/SiO2 hexagonal phase particles show much a stronger upconversion emission, suggesting that layer-structured materials are more appropriate as upconversion hosts. Furthermore, the paramagnetic properties of Gd2O2CO3:Yb3+, Er3+, Nd3+/SiO2 nanoparticles were also demonstrated.

Size and morphology are critical to monitoring the properties of nanocrystals. In this work, Bi6Fe1.9Co0.1Ti3O18 (BFCTO) nanocrystals, which are a visible-light active photocatalyst with room temperature ferromagnetism, were successfully synthesized through a hydrothermal process. Furthermore, the morphology dependent magnetism and band gap of the BFCTO-1.00/BFCTO-1.50/BFCTO-2.00 nanocrystals are studied by adjusting the NaOH concentration to 1.00 M/1.50 M/2.00 M in the hydrothermal process, respectively. As the {117} facet ratio increases, the absorption edge has an obvious red-shift by 32 nm and the corresponding bandgap is reduced from 2.58 to 2.42 eV. Remarkably, the specific surface area normalized degradation rate of BFCTO-1.50 was considerably higher than those of BFCTO-1.00 and BFCTO-2.00 due to the optimal ratio of {001} and {117} facets.

•The physical properties of the layer-structured BFTO could be greatly affected by the layer number n.•Partial substituted of Co for Fe could break the AFM or PFM state in pure BFTO at room temperature, thus exhibits ferromagnetization.•A systematic investigation of the influence of layer number n on the room temperature ferromagnetic properties of the Co-substituted BFTO has been done.Aurivillius Bi4Bin-3Fen-3.2Co0.2Ti3O3n+3 (BFCT, n=4–7) ceramics were prepared by the hot-press method. X-ray diffraction and high-resolution transmission electron microscopy analysis indicate a successful synthesis of pure phase BFCT samples. All samples show significant magnetic moments at the room temperature. Interestingly, the remanent magnetization (2Mr) increases with the increase of n, which was found to be 21.53 emu/mol, 413.49 emu/mol, 472.78 emu/mol and 727.03 emu/mol for n=4, 5, 6 and 7 ceramics, respectively. This phenomenon can be ascribed to the increased coupling probability of Fe-O-Co with increasing n. Besides, the odd-layered compounds are likely to be beneficial for enhancing the 2Mr, due to that their crystalline structures may endure larger distortions when compared with the even-layered compounds.