Intermetallic compounds are a class of materials composed of two or more metal or metalloid elements that combine in a specific ratio to form ordered crystal structures. Since intermetallics are typically not charge-balanced, their compositions are not constrained by the need for neutrality; a huge number of compounds are possible from the combination of 3 or 4 elements. These materials exhibit a wide range of properties that are distinct from those of their constituent elements, such as magnetism, superconductivity, and magnetocaloric or thermoelectric effects. Intermetallics combining rare earth elements and transition metal elements can exhibit complex magnetic behavior stemming from both metals. Carbon is also a component of great interest; its small size and electronegativity lead to formation of species such as monoatomic C4-, diatomic C22- (acetylide) and C24- (ethenide) that can either occupy interstitial sites between rare earth ions or bond to transition metals. This chapter describes the structures of ternary and quaternary intermetallics comprised of a rare earth, carbon, transition metal, and/or main group element. Focus is directed onto their common structural building blocks and on how these species and their connectivity change in going from ternary to quaternary compounds. Incorporation of hydride interstitials to form rare earth carbide hydride compounds is also discussed. Synthetic procedures are described at the beginning of the chapter, including high-temperature stoichiometric reactions, metal-flux synthesis, and post-synthesis introduction of carbon.
Upconversion luminescence represents a nonlinear optical process whereby high-energy photons are emitted through the conservation of energy from two or more low-energy photons. Owing to the unique electronic structure of rare earth ions, their rich emission levels are considered excellent upconversion luminescent carriers. Currently, upconversion luminescent materials based on trivalent lanthanide ions have been widely explored. With the advantages of large anti Stokes sites, narrow emission bands, long luminescence lifetime, good photostability, and low cytotoxicity, it has been applied in many fields such as fluorescence imaging, advanced information anti-counterfeiting, and biological therapy. This chapter approaches rare earth luminescent materials from the scale perspective, including molecular scale RE-complexes, nanoscale rare earth doped UCNPs, and micro scale Ln-MOFs. We elaborate and introduce the upconversion luminescence mechanisms at various scales, and provide ideas for subsequent research by combining material design, characterization, and application.
Photon upconversion describes the optical phenomenon where light is emitted at a shorter wavelength than the excitations source. Doped rare-earth materials, such as lanthanide (Ln3+) doped rare-earth fluoride nanoparticles (R-NPs) became most famous as upconverting nanoparticles. With highly tunable optical and magnetic properties, customizable particle design, good biocompatibility, and high resistance to photobleaching and degradation they emerged as promising specimens for technical and biomedical applications. Recent advances have focused on R-NPs capable of upconversion, aiming to also downshift the excitation wavelengths from the conventional 980 and 808 nm to the near-infrared transparency windows. This strategic shift seeks to generate more efficient probes for, for example, achieving high spatial resolution in bioimaging with enhanced tissue penetration capabilities. Many applications that benefit from these properties of rare-earth-based materials require access to high quality and well-defined nanoparticles. Consequently, there is a need for optimized synthesis routes towards R-NPs, including control of the material's crystalline phase, morphology, and size, while reproducibility and potential upscaling are relevant when seeking implementation to real-life. Moreover, minimized temperature requirements, avoidance of complex procedures, and reduced nanoparticle aggregation risks are key for improved synthesis efficiency. Manifold strategies have been demonstrated as suitable for the synthesis of R-NPs, which are explored in this chapter. Herein, the focus will be set on thermal decomposition, hydro(solvo)thermal, and microwave-assisted approaches, while also providing an outlook to strategies based on flow reactors and automated synthesis approaches.
