•La1 − xCaxMnO3 (x = 0.3 and 0.5) nanoparticles were synthesized by molten salt method.•Microstructural characterizations are performed by XRD, TEM and HRTEM.•The MR value of 77% was achieved under 5 T and 50 K in the La0.5Ca0.5MnO3 samples.•The origins of large MR values in the present LCMO samples are discussed.Single-crystalline La1 − xCaxMnO3 (LCMO, x = 0.3 and 0.5) nanoparticles synthesized by molten salt method, have been studied based on their microstructural characterization, magnetic and electrical transport property measurements. X-ray diffraction patterns and electron diffraction patterns reveal that all the LCMO nanoparticles crystallize in the orthorhombic perovskite structure (Pnma space group). TEM images show that the LCMO nanoparticles exhibit irregular shapes and their particle sizes are increased with increasing the annealed temperatures. The resolved (200) lattice fringes in high-resolution TEM images confirm the single crystalline nature of the LCMO nanoparticles. Magnetization measurements versus temperature show a paramagnetic – ferromagnetic transition for all the nanoparticles. Room temperature and low temperature M – H loops demonstrate that the LCMO nanoparticles exhibit ferromagnetic behavior at 5 K, whereas a paramagnetic behavior at 300 K. Resistivity behavior of the LCMO nanoparticles demonstrate that they exhibit a broad metal — insulator (M − I) transition, and the transition temperature shifts towards a high temperature under an external magnetic field. The magnetoresistance (MR) values of the LCMO nanoparticles were increased monotonically with decreasing the temperature below 300 K, and finally reached constant values at temperatures below 100 K. The MR value of the La0.5Ca0.5MnO3 nanoparticles annealed at 650 °C for 3 h, was found to be 77% at 50 K in 5 T, and 57% for the La0.7Ca0.3MnO3 nanoparticles annealed at 650 °C for 3 h. Such large MR values of the present LCMO nanoparticles make them ideal candidates for the spintronic devices.

La-modified Bi1−xLaxFeO3 (x = 0.15–0.40) and BaTiO3-modified (Bi1−yBay)(Fe1−yTiy)O3 (y = 0.10−0.33) multiferroic ceramics were synthesized via solid-state reactions, and their structural and vibrational properties were investigated by X-ray diffraction (XRD) and Raman spectroscopy. The XRD patterns reveal that the La-modified Bi1−xLaxFeO3 system exhibits an rhombohedral – orthorhombic structural transition at x = 0.40. The degree of rhombohedral distortion in the Bi1−xLaxFeO3 ceramics was decreased with increasing the La content, resulting in the disappearance of some Raman active modes (e.g. A1-4, E(TO1), E(TO2), E(TO6)). Similarly, in the BaTiO3-modified (Bi1−yBay)(Fe1−yTiy)O3 system, a composition-driven structural transition from rhombohedral (R3c) phase to cubic phase appeared at y = 0.30, indicating a morphotropic phase boundary between the rhombohedral and cubic phases. It is found that the Raman mode frequencies of the A1-1, A1-2, A1-3 and E(TO3) modes are slightly dependence of either the La content in Bi1−xLaxFeO3 system or the BaTiO3 content in (Bi1−yBay)(Fe1−yTiy)O3 system, whereas the frequencies of the E(TO8) and E(TO9) Raman active modes are significantly dependent upon the modified compositions. Such compositional dependence of the Raman active mode frequencies are discussed and interpreted based on a spring oscillator model in combination with the structural phase transition, the degree of rhombohedral distortion, and the B–O bond length in the BO6 octahedron. The present results are useful for understanding the correlation between the structural and physical properties of the La-modified Bi1−xLaxFeO3 and the BaTiO3-modified (Bi1−yBay)(Fe1−yTiy)O3 ceramics.

In this work, we report on the synthesis of BiFeO3 microcrystals by a facile molten-salt technique with the mixed medium of NaCl–KCl as the molten salt. The microstructures of BiFeO3 microcrystals were characterized by X-ray diffraction, Raman spectra, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results show that almost pure phase BiFeO3 powders with the average particle size of 1.0 μm were synthesized at temperatures of 700–800 °C. However, the prolonged holding time and the change of molten salt ratios with respect to the starting oxides have no remarkable effects on the size and morphology of the BiFeO3 powders. Raman spectra of the BiFeO3 multiferroic ceramics prepared by the BiFeO3 microcrystals derived from the molten-salt method, reveal five Raman peaks at around 125, 145, 212, 268, and 504 cm−1. With increasing sintering temperature, the Raman peak at 145 cm−1 disappeared, a slight blue frequency shift was observed in the Raman peak at 212 cm−1, but no frequency shift was observed in the Raman peaks at around 125, 268, and 504 cm−1. The dielectric constants of the BiFeO3 multiferroic ceramics prepared from the BiFeO3 microcrystals synthesized by the molten-salt method, are in the range of 60–250 (measured @103 Hz and room temperature), and the corresponding dielectric losses are in the range of 0.05–0.2, which are dependent upon the molten-salt processing parameters.

Single-phase cubic pyrochlore-typed Bi1.5MgNb1.5O7 (BMN) dielectric ceramics were synthesized at temperatures of 1050–1200 °C by solid-state reaction method. Their atomic-scale microstructures and dielectric properties were investigated. X-ray diffraction patterns revealed that the BMN ceramics had an average cubic pyrochlore structure, whereas the Raman spectra indicated that they had an essentially cubic symmetry with small local deviations at the A and O' sites of the cubic pyrochlore structure. This was confirmed by selected electron area diffraction (SAED) patterns, where the reflections of {442} (not allowed in the cubic pyrochlore with Fd  3¯m symmetry) were clearly observed. SEM and TEM images revealed that the average grain size was increased with the sintering temperature, and an un-homogeneous grain growth was observed at high temperatures. HRTEM images and SAED patterns revealed the single-crystalline nature of the BMN ceramic grains. Energy dispersive spectroscopy (EDS) elemental mapping studies indicated that the compositional distributions of Bi, Mg, Nb and O elements in the ceramic grains were homogenous, and no elemental precipitation was observed at the grain boundary. Quantitative EDS data on ceramic grains revealed the expected cationic stoichiometry based on the initial composition of Bi1.5MgNb1.5O7. Dielectric constants of all the BMN samples exhibited almost frequency independent characteristic in the frequency range of 102–106 Hz, and the highest value was 195 for the BMN ceramics sintered at sintered at 1150 °C with the highest bulk density. The dielectric losses were stable and less than 0.002 in the frequency range of 102–105 Hz. The high dielectric constants of the present BMN samples can be ascribed to the local atomic deviations at the A and O' sites from the ideal atomic positions of the pyrochlore structure, which affect the different polarization mechanisms in the BMN ceramics, and which in turn enhance the dielectric constants of the BMN ceramic samples.

Here, we report the atomic-scale microstructural characterization and dielectric properties of crystalline cubic pyrochlore Bi1.5MgNb1.5O7 (BMN) nanoparticles with mean size of 70 nm, which were synthesized by sol–gel method. The crystallinity, phase formation, morphology, and surface microstructure of the BMN nanoparticles were characterized by X-ray diffraction (XRD), Raman spectra, transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM), respectively. The phase evolution of the BMN nanoparticles investigated by XRD patterns showed that uniform cubic pyrochlore BMN nanoparticles were obtained after calcination at temperature of 800 °C, and their structural information was revealed by Raman spectrum. TEM images demonstrated that the BMN nanoparticles had a spherical morphology with an average particle size of 70 nm, and their crystalline nature was revealed by HRTEM images. In addition, HRTEM images also demonstrate a terrace–ledge–kink (TLK) surface structure at the edges of rough BMN nanoparticles, where the terrace was on the (100) plane, and the ledge on the (001) plane. The formation of such a TLK surface structure can be well explained by a theory of periodic bond chains. Due to the surface structural reconstruction in the BMN nanoparticles, the formation of a tetragonal structure in a rough BMN nanoparticle was also revealed by HRTEM image. The BMN nanoparticles exhibited dielectric constants of 50 at 100 kHz and 30 at 1 MHz, and the dielectric loss of 0.19 at 1 MHz.

Application of microwave energy for materials processing is emerging as an innovative technology with many advantages over the conventional processing, and the rapid progress in this field suggests that microwave material processing (e.g., microwave and microwave-hydrothermal process) will play an outstanding role in the broad field of nanoscience and nanotechnology. This review article gives an up-to-date overview of the current microscopical and physical characterization of the products synthesized by microwave and microwave-hydrothermal process, particularly for oxide nanomaterials because they are indispensable for nanotechnological innovations due to their combinations of infinite variety of structural motifs and properties with manifold morphological features. Basic principles, advantages, and limitations of microwave and microwave-hydrothermal processes are first introduced, and then their recent applications in the synthesis of different classes of functional materials especially for oxide nanomaterials are critically reviewed. Next, the recent progress on the structural and physical characterizations is summarized and discussed. Finally, prospects for future researches within this field are elaborated.Highlights► Microwave processing of materials is emerging as an innovative technology. ► It has many advantages over the conventional processing. ► Many products are synthesized by microwave and microwave-hydrothermal process. ► Microscopical and physical characterization of the products are reviewed.

Dielectric properties and ferroelectric domain configurations of multiferroic xBaTiO3–(1 − x)BiFeO3 (x = 0.10–0.33) solid solutions synthesized by conventional solid-state reaction, were reported. A structural transition from rhombohedral to pseudo-cubic structures appeared around x = 0.33, and the formation of impurity phase of Bi2Fe4O9 was effectively depressed by doping BaTiO3. Dielectric constants of xBaTiO3–(1 − x)BiFeO3 solid solutions decreased with increasing the frequency, and the degree of decrease was related to the doping content of BaTiO3. Transmission electron microscopy images revealed that the ferroelectric domain configurations in the multiferroic BiFeO3–BaTiO3 solid solutions with rhombohedral symmetry, exhibited a wavy character whereas a predominant intricate domain structure with fluctuating mottled contrast was observed in the multiferroic BiFeO3–BaTiO3 solid solution with pseudo-cubic phase structure. The presence of 1/2{1 1 1} superlattice spots in the selected area electron diffraction patterns taken from the multiferroic BiFeO3–BaTiO3 solid solutions with rhombohedral symmetry indicated that the ordered regions have a doubled perovskite unit cell.

Perovskite oxides exhibit a wide range of functional properties, such as ferroelectricity, piezoelectricity, pyroelectricity, non-linear dielectric behavior, as well as multiferroic property. These properties are indispensable for applications in microelectronic devices. Recent advances in science and technology of perovskite oxides have resulted in the feature sizes of microelectronic devices down-scaling into nanoscale dimensions. At the nanoscale perovskite oxides display novel physical properties that are different from their bulk and film counterparts. Understanding these size effects of perovskite oxides at the nanoscale is of importance for the developing a new generation of revolutionary electronic nanodevices. Due to these effects being dependent on the structure and finite size, considerable efforts have been made in the controllable synthesis of low-dimensional perovskite nanostructures such as perovskite oxide nanotubes (PONTs). PONTs can not only be used as building blocks for miniaturized microelectronic devices, but also offer fundamental scientific opportunities for investigating the intrinsic size effects of physical properties. This review article describes the recent progress made in the field of PONTs, which covers their synthesis, structural characterization, properties and applications. We begin this review with a comprehensive survey on the research activities on PONTs, and then focus on their synthesis strategies. Structural characterization and multifunctional properties of the PONTs prepared by the template synthesis are also summarized. Their potential applications in 3D memory devices, nano-scale fluidic control systems, nanoscale power generators and terahertz generators, are discussed. Finally, we conclude this review by providing our perspectives to the future directions of PONTs.

PX-phase PbTiO3 (PT) nanowires were synthesized by microwave-hydrothermal process, and their microstructures were characterized by electron microscopy. The PX-phase PT nanowires exhibit acicular morphology with diameter sizes of 20–80 nm and length over 1 µm. They tend to grow into a regular structure with parallel arrangement along their long axis in the [001] direction. Selected area electron diffraction patterns demonstrate the PX-phase PT nanowires with a 3-fold modulated periodicity along the [110] direction and a 4-fold modulated periodicity in the [001] direction. These results were also confirmed by the high-resolution transmission electron microscopy images.

In this work, we report on an efficient visible-light-responsive novel photocatalyst based on sillenite-type bismuth ferric nanocrystals with hexagonal-shaped morphology and particle size range of 18 - 33nm, which were synthesized by microwave hydrothermal method. Their microstructures were characterized by X-ray diffraction, Raman spectra, X-ray photoelectron spectroscopy and transmission electron microscopy. The high photocatalytic activity was evaluated by degrading the rhodamine B in aqueous solution under visible-light irradiation. The present results demonstrate that the sillenite-type bismuth ferritic nanocrystals are promising visible-light-responsive photocatalysts for degradation of organic compounds.

La0.7Ca0.3MnO3 (LCMO) manganite nanoparticles are synthesized via a sol-gel route at different annealed temperatures. Their structural, morphological, and magnetic properties are investigated. The X-ray diffraction patterns coupled with electron diffraction confirm that all the LCMO samples are single phase and crystallize in the orthorhombic perovskite structure (Pnma space group). The morphology of the samples observed by TEM, reveals a spherical shape with an average grain size lower than 50 nm. The resolved lattice fringes in high-resolution TEM images also reveal the single crystalline nature of the LCMO nanoparticles. Magnetization measurements versus temperature under low magnetic field (0.01 T) show a paramagnetic - ferromagnetic transition for all the samples. The Curie temperature (Tc) is found to be decreased with increasing the annealed temperature. A bifurcation is observed in the zero field-cooled and field-cooled magnetizations, indicating a competition between ferromagnetic and antiferromagnetic interactions in the nanoparticles at low temperatures. Field-cooled hysteresis measurements suggest a cluster glasslike behavior of the nanoparticles. Room temperature and low temperature M - H loops demonstrate that all the samples exhibit ferromagnetic behavior at 5 K, whereas a paramagnetic behavior at room temperature. Resistivity behavior of the LCMO samples shows that they exhibit a metal - insulator transition. Magnetoresistance of ~ 50% at the field up to 8 T was observed at 2 K in the LSCO samples annealed at 600 °C.

•Single-phase pyrochlore-typed BMN ceramics are synthesized via solid-state reactions.•Atomic-scale microstructural characterizations are performed.•Modulated pyrochlored structures along the [001] and 〈112〉 directions are observed.•Dielectric properties and leakage current characteristics are measured.Single-phase pyrochlore-type ceramic samples of Bi1.5MgNb1.5O7 (BMN) in the system of Bi2O3-MgO-Nb2O5 were synthesized via solid-state reactions, and their microstructures, dielectric properties, and leakage current characteristics were also investigated. X-ray diffraction patterns demonstrate the synthesized BMN samples have cubic phase pyrochlore-type structure, which is also confirmed by selected area electron diffraction (SAED) patterns. The highest density of the BMN samples sintered at different temperatures was 6.511 g/cm3, which was about 98.0% of the theoretical one. More than six Raman vibrational peaks observed in the BMN samples also confirmed the cubic phase pyrochlore-type structure of the synthesized BMN samples. TEM images reveal that the BMN samples have clean grain boundaries and no voids are observed at triple-grain boundary. Both SAED patterns and HRTEM images confirm the single crystalline nature of the BMN ceramic grains, and the superlattice structures (or modulated pyrochlore structures) along the [001] and 〈112〉 directions in the BMN samples. Such long-range ordered pyrochlore structures (superlattice structures) could enhance the polarizations of the electric dipoles formed along the superstructure direction, resulting in high dielectric constants of the BMN samples. The dielectric constants of the BMN samples were measured to be 180–195, which exhibit almost frequency independent characteristic in the frequency range of 102–106 Hz. The dielectric losses of the BMN samples are much stable and smaller than 0.002 below 105 Hz, which is ascribed to the existence of clean grain boundaries since they are benefit to reducing the extrinsic dielectric loss. The leakage current values of the BMN ceramics are in the range of 0.13–0.75 μA/cm2 (measured @ electric field E = 13.3 MV/m). A flat frequency independence of the relative dielectric constant, low dielectric loss, and small leakage current characters, make the BMN dielectrics attractive in the fields of high volume efficient multilayered ceramic capacitors with the capacitance weakly dependent on the frequency.

Perovskite oxides exhibit a wide range of functional properties, such as ferroelectricity, piezoelectricity, pyroelectricity, non-linear dielectric behavior, as well as multiferroic property. These properties are indispensable for applications in microelectronic devices. Recent advances in science and technology of perovskite oxides have resulted in the feature sizes of microelectronic devices down-scaling into nanoscale dimensions. At the nanoscale perovskite oxides display novel physical properties that are different from their bulk and film counterparts. Understanding these size effects of perovskite oxides at the nanoscale is of importance for the developing a new generation of revolutionary electronic nanodevices. Due to these effects being dependent on the structure and finite size, considerable efforts have been made in the controllable synthesis of low-dimensional perovskite nanostructures such as perovskite oxide nanotubes (PONTs). PONTs can not only be used as building blocks for miniaturized microelectronic devices, but also offer fundamental scientific opportunities for investigating the intrinsic size effects of physical properties. This review article describes the recent progress made in the field of PONTs, which covers their synthesis, structural characterization, properties and applications. We begin this review with a comprehensive survey on the research activities on PONTs, and then focus on their synthesis strategies. Structural characterization and multifunctional properties of the PONTs prepared by the template synthesis are also summarized. Their potential applications in 3D memory devices, nano-scale fluidic control systems, nanoscale power generators and terahertz generators, are discussed. Finally, we conclude this review by providing our perspectives to the future directions of PONTs.