As compared to bulk materials, magnetic nanoparticles possess distinct magnetic properties and attempts have been made to exploit their beneficial properties for technical and biomedical applications, e.g. for magnetic fluids, high-density magnetic recording, or biomedical diagnosis and therapy. Early magnetic fluids (MFs) were produced by grinding magnetite with heptane or long chain hydrocarbon and a grinding agent, e.g. oleic acid [152]. Later procedures for MFs precipitated Fe 3+/Fe 2+ of an aqueous solution with a base, coated the particles by oleic acid, and dispersed them in carrier liquid [161]. However, besides the elemental composition and crystal structure of the applied magnetic particles, particle size and particle size distribution determine the properties of the resulting MF. Many methods for nanoparticle synthesis including the preparation of metallic magnetic particles have been described in the literature. However, there still remain important questions, e.g. concerning control of particle size, shape, and monodispersity as well as their stability towards oxidation. Moreover, peptization by suitable surfactants or polymers into stable MFs is an important issue since each application in engineering or biomedicine needs special MFs with properties adjusted to the requirements of the system.

Synthesis and Characterization

Superionic conductors are compounds that exhibit exceptionally high values of ionic conductivity within the solid state. Indeed, their conductivities often reach values of the order of 1 Ω−1 cm−1, which are comparable to those observed in the molten state. Following Faraday's first observation of high ionic conductivity within the solids β-PbF2 and Ag2S in 1836, a fundamental understanding of the nature of the superionic state has provided one of the major challenges in the field of condensed matter science. However, experimental and theoretical approaches to their study are often made difficult by the extensive dynamic structural disorder which characterizes superionic conduction and the inapplicability of many of the commonly used approximations in solid state physics. Nevertheless, a clearer picture of the nature of the superionic state at the ionic level has emerged within the past few decades. Many different techniques have contributed to these advances, but the most significant insights have been provided by neutron scattering experiments and molecular dynamics simulations. This review will summarize the state of current knowledge concerning the crystal structures and conduction processes of superionic conductors, beginning with a comparison of the behaviour of two of the most widely studied binary compounds, AgI and β-PbF2. Each can be considered a parent of two larger families of highly conducting compounds which are related by either chemical or structural means. These include perovskite-structured oxides and Li+ containing spinel-structured compounds, which have important commercial applications in fuel cells and lightweight batteries, respectively. In parallel with these discussions, the relative importance of factors such as bonding character and the properties of the mobile and immobile ions (charge, size, polarizability, etc) in promoting the extensive lattice disorder which characterizes superionic behaviour will be assessed and the possibilities for predicting a priori which compounds will display high ionic conductivity discussed.

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Superionics: crystal structures and conduction processes

The solid electrolytes with fluorine-ion conductivity that were revealed during the analysis of the phase diagrams of the MF
 m
 -RF
 n
 systems within the program of search for new multicomponent fluoride crystalline materials carried out at the Shubnikov Institute of Crystallography, Russian Academy of Sciences, are described. The most widespread and promising materials are the nonstoichiometric phases with fluorite (CaF2) and tysonite (LaF3) structures, which are formed in the MF2-RF3 systems (M = Ca, Sr, Ba, Cd, or Pb; R = Sc, Y, or La-Lu). These phases have superionic fluorine conductivity due to the anion sublattice disorder. The ionic conductivity of crystals of both structure types has been studied and the limits of its change with composition and temperature are determined. Nonstoichiometric fluorides are used as solid electrolytes in chemical sensors, fluorine sources, and batteries. The prospects of the use of fluorine-ion conductors in solid-state electrochemical devices, principles of their operation, and the problems of optimization of their composition are discussed.

Nonstoichiometric fluorides—Solid electrolytes for electrochemical devices: A review

Metal-halide perovskite materials are crystalline semiconductors that can be processed at room temperature using solution-processible deposition techniques. In only a few years, perovskite-based solar cell efficiencies have seen a huge increase from 3 % to 22.1 %. These direct-bandgap materials are potential candidates for other optoelectronic device applications such as photodetectors, light-emitting transistors, and light-emitting diodes. In this Review, we present the current state-of-the-art in the research and development of perovskite light-emitting diodes (PeLEDs) based on metal halide perovskite semiconductors with an emphasis on size of crystallites and its effects on optical properties, device architectures, and PeLED performance parameters. A uniform pinhole-free morphology with small grain size is essential for high-performance PeLEDs. For this purpose, a p-i-n type device architecture with a p-type poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) layer was found to be more suitable than the n-i-p type. In PeLEDs based on bulk-phase perovskites, the average size of the crystallites is in the range 100–500 nm. The efficiency can be improved further by using perovskite nanoparticles with size <100 nm. Color of the emitted light from these materials can be tuned over a wide range, that is, from NIR (775 nm) to near-UV (410 nm), simply by changing the halide ion and the stoichiometric ratio between halide ions in the mixed-halide-type perovskites. Change of stoichiometry also affects the structural properties and the final film morphology, which in turn influences the radiative and non-radiative recombination rates of the charge carriers and hence the device performance. Furthermore, we also provide recent developments in PeLEDs based on inorganic perovskite nanocrystals that complement the efforts being made in hybrid PeLEDs.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/ente.201700356

K. Miyauchi

Papers

Inorganic Solid Fluorides: Chemistry and Physics