Preface

Jean-Claude G. Bünzli; Vitalij K. Pecharsky

Volume 45 of the Handbook on the Physics and Chemistry of Rare Earths features four chapters covering subjects ranging from gas-phase chemistry, to inorganic clusters and complexes, optical refrigeration, and organolanthanide chemistry.

The first chapter (Chapter 263) is devoted to an unusual aspect of f-element chemistry, gas-phase chemistry. Thanks to the development of sensitive mass spectrometry techniques, many new rare-earth and actinide molecular and cluster species have been identified; this in turn deepened knowledge of the basic chemistry of these elements and provided clues for understanding condensed-phase processes. Two strong features of this review are the discussion of experimental thermodynamic parameters obtained for several atomic and molecular ions and the validation of computational methods applied to the challenging f-elements. The following review (Chapter 264) deals with clusters formed by rare-earth elements with endohedral transition metal atoms. These heteroatomic species contain 6–10 rare-earth ions and are usually surrounded by halide ligands, building large {TRr}Xx entities. The chapter focuses on the synthesis and crystal growth of these stunning “anti-Werner” complexes by comproportionation of rare-earth trihalides in presence of the transition metal at elevated temperature and on the detailed description of their structure. Semiconductor optical detectors largely benefit from cryogenic technologies to improve their signal-to-noise ratio; present techniques have definite limitations and Chapter 265 presents an alternative, optical (laser) cooling. Indeed, some luminescent materials emit light of higher energy than the excitation source, anti-Stokes emission, and a few lanthanide ions doped into transparent matrices, such as trivalent erbium, thulium, and ytterbium, are ideal candidates for optical refrigeration. The final chapter (Chapter 266) describes arene-bridged rare-earth complexes with emphasis on compounds obtained by reduction reactions; this aspect of organolanthanide chemistry exemplifies the remarkable advances made by this field that started 60 years ago. Bimetallic lanthanide complexes bridged by ligands derived from arenes and complexes of ferrocene-based diamide ligands constitute a major subtheme in the chapter. Detailed discussions then follow on compounds derived from fused rings and their reactivity with white phosphorus.

We thank Professor William Evans, member of the Advisory Editorial board, for suggesting the authors of the last chapter and for his help in writing the preface.

Chapter 263. Gas-Phase Ion Chemistry of Rare Earths and Actinides

By Joaquim Marçalo and John K. Gibson

Universidade de Lisboa, Portugal, and Lawrence Berkeley National Laboratory, USA

Gas-phase chemistry studies of atomic and molecular rare-earth and actinide ions have a deep-rooted history of more than three decades. In gas phase, physical and chemical properties of elementary and molecular species can be studied in absence of external perturbations. Due to the relative simplicity of gas-phase systems compared to condensed-phase systems, solutions or solids, it is possible to probe in detail the relationships between electronic structure, reactivity, and energetics. Most of this research involves the use of a variety of mass spectrometry techniques, which allows one exerting precise control over reactants and products. Many new rare earth and actinide molecular and cluster species have been identified that have expanded knowledge of the basic chemistry of these elements and provided clues for understanding condensed-phase processes. Key thermodynamic parameters have been obtained for numerous atomic and molecular ions. Such fundamental physicochemical studies have provided opportunities for the refinement and validation of computational methods as applied to the particularly challenging lanthanide and actinide elements. Among other applications, the roles of ligands, solvent molecules, and counter ions have been examined at a molecular level. A deeper understanding of plasma chemistry, flame chemistry, radiolysis, and interstellar chemistry stems from these gas-phase studies. Important applications in analytical and biomedical mass spectrometry have also benefited from discoveries in this area.

The chapter starts with an introduction on gas-phase chemistry and associated experimental techniques as well as on electronic structures and energetics of lanthanide and actinide ions. It then focuses on reactions of these ions with oxidants, inorganic molecules, small hydrocarbons, and organic molecules. The review ends with considerations on hydrolysis, solvation, complexation, and interactions with biologically relevant molecules.

Chapter 264. Symbiosis of Intermetallic and Salt: Rare-Earth Metal Cluster Complexes with Endohedral Transition Metal Atoms

By Gerd Meyer

Universität zu Köln, Germany, and Iowa State University, USA

Rare-earth elements, R, form clusters with endohedral transition metal atoms, T, featuring six to eight R atoms. These heteroatomic species, {TRr} where r is the coordination number (CN) of T (6–8), are surrounded by halide ligands, X, building larger entities, {TRr}Xx. Examples are the prolific {TR6}X12R and {TR6}X10 compound types. Edge- and face-sharing of (mainly) octahedral clusters constitute a small number of oligomers, dimers to pentamers, with the tetrameric {T4R16} oligomers being the most abundant. Further connection of clusters via common edges leads to a variety of chains with numerous {TR3}X3-type compounds, exhibiting a surprising structural diversity. CNs of the endohedral atom of seven and eight are also observed in {TR7} and {TR8} clusters. Similar coordination environments are also seen in {TtRr} polar intermetallic compounds, for example, CN = 8–10 in {Ru11Lu20}. Thus, {TRr}Xx clusters—also coined as metal-rich halides or as anti-Werner complexes—may be understood as a symbiotic arrangement of (polar) intermetallics and salts. Consequently, bonding in both polar intermetallics and heterocluster complexes is mainly heteroatomic with minor homoatomic contributions. Synthesis and crystal growth is mostly accomplished by comproportionation of RX3 and R in the presence of T at high temperatures in refractory metal containers (mostly niobium or tantalum) followed by controlled cooling.

After presenting the components and synthesis of the cluster complexes described in this review, the chapter focuses on detailed descriptions of their crystal chemistry before ending with a discussion on their electronic structure. So far, not much is known about the physical properties, such as conductivity or magnetism, of these compounds, an issue that requires closer inspection in the immediate future.

Chapter 265. Solid-State Optical Refrigeration

By Markus P. Hehlen, Mansoor Sheik-Bahae, and Richard I. Epstein

Los Alamos National Laboratory, University of New Mexico, Albuquerque, and ThermoDynamic Films, Santa Fe, USA

Refrigeration is a technique almost as old as humanity. Until the middle of the nineteenth century, collecting of ice and snow, evaporating water, or dissolving salts into it were the major cooling technologies. The discovery of cyclic refrigeration was a major breakthrough, and vapor compression, vapor absorption, and gas cycle refrigerators meet most of everyday refrigeration needs with a few new technologies such as magnetocaloric, electrocaloric, and elastocaloric refrigeration emerging as promising tools. Semiconductor optical detectors largely benefit from cryogenic technologies to improve their signal-to-noise ratio. Their cooling is however challenging in view of constraints in size, weight, power, and especially vibrations. Present techniques, which include liquid helium and thermoelectric coolers, have definite limitations so that optical cooling may be a viable alternative. The latter phenomenon is based on the observation that some luminescent materials emit light of higher energy than the excitation source, the energy difference being supplied by vibrational levels. Lanthanide ions doped into inorganic matrices, in particular trivalent erbium, thulium, and ytterbium, have excited states with large quantum efficiencies, which make them ideal candidates for optical refrigeration. Although the efficiency remains small, laser-induced cooling allows one to reach lower temperatures (100 K) compared to thermoelectric cooling (170 K) and it is therefore an exciting technology with appreciable potential for solid-state optical devices.

The chapter starts with a brief history of refrigeration and solid-state cooling before concentrating on the description of experimental techniques, followed by a presentation of lanthanide-doped laser-cooling materials. It concludes on considerations aiming at improving the cooling efficiency and requirements for the design of practical lanthanide-based optical containing devices. It is anticipated that cooling temperatures down to liquid nitrogen temperature could be reached with Yb-doped...