Reduced rare-earth metal halides

Dorhout, P. K. Corbett, J. D. (1992). A Novel Structure Type in Reduced Rare-Earth Metal Halides. One-Dimensional Confacial Chains Based on Centered Square Antiprismatic Metal Units Y4Br40s and Er4Br40s, J. Am. Chem. Soc. 114, 1697-1701. 

Table 1 Binary reduced rare earth metal halides...

It gave birth to a new approach to reduced rare-earth metal halides that were previously synthesized only by the comproportionation route or by reduction with hydrogen, viz.,... 

But first the synthesis had to come John was interested in reduced metal halides, particularly for the post-transition metals cadmium, galHum, and bismuth (his Ph.D. dissertation was on anhydrous aluminum halides and mixed halide intermediates, a good start for what was to come ). However, he was not yet actively interested in rare-earth metals and their remarkable solubility in their halides. But these elements lured him one floor below where Adrian Daane headed the metallurgy section of Spedding s empire. He knew how to produce rare-earth metals with high purity and in sufficient quantity and also how to handle tantalum containers. What if one gave it a tr/ and reduced some rare-earth metal halides (John insists that this term is used correctly) from their respective metals at high temperatures under appropriate conditions. 

With the smaller, harder rare earth elements (Gd through Lu), such compounds have never been seen, except for scandium where scandium-defrcient hexagonal perovskites, ASc Xs (A = Rb, Cs X = Cl, Br, I), have been observed. These are discussed together with the perovskite-type halides of R = Sm, Eu, Dy, Tm, Yb below. All other complex hahdes with reduced rare earth metals contain clusters and are discussed in Rare Earth Metal Cluster Complexes. 

Ames Laboratory (Iowa State University, USA) investigating new solid state phases based on reduced rare earth halides. Since 1993, she has held a position at the University Jaume 1 of Castello (Spain) and became Associate Professor of Physical Chemistry in 1995. During the second semester of 2005, she held a visiting professor position at the Laboratory of Chemistry, Molecular Engineering and Materials of the CNRS-Universtity of Angers (France). Her research has been focussed on the chemistry of transition metal clusters with special interest in multifunctional molecular materials and the relationship between the molecular and electronic structures of these systems with their properties. She is currently coauthor of around 80 research papers on this and related topics. 

Neodymium, along with lanthanum, cerium and praseodymium, has low melting points and high boiling points. The fluorides of these and other rare earth metals are placed under highly purified helium or argon atmosphere in a platinum, tantalum or tungsten crucible in a furnace. They are heated under this inert atmosphere or under vacuum at 1000 to 1500°C with an alkali or alkaline earth metal. The halides are reduced to their metals ... 

Calcium metal is an excellent reducing agent for production of the less common metals because of the large free energy of formation of its oxides and halides. The following metals have been prepared by the reduction of their oxides or fluorides with calcium hafnium (22), plutonium (23), scandium (24), thorium (25), tungsten (26), uranium (27,28), vanadium (29), yttrium (30), zirconium (22,31), and most of the rare-earth metals (32). 

It is easy to reduce anhydrous rare-earth halides to the metal by reaction of more electropositive metals such as calcium, lithium, sodium, potassium, and aluminum. Electrolytic reduction is an alternative in the production of the light lanthanide metals, including didymium, a Nd—Pr mixture. The rare-earth metals have a great affinity for oxygen, sulfur, nitrogen, carbon, silicon, boron, phosphorus, and hydrogen at elevated temperature and remove these elements from most other metals. 

Ctudies of binary rare earth metal-metal halide systems have revealed not only a considerable number of new, reduced phases, but also a substantially new class of halides possessing a metallic rather than a truly reduced or localized character. The reduction properties of the chlorides, bromides, and iodides of the first four rare earth elements are summarized as follows in terms of the compositions of new lower phases or, where absent, the limiting solution compositions in mole % metal in liquid MX3 ... 

Many reduced (metal-rich) halides of group 4 (especially Zr) and the rare earth metals have been prepared. Most of these compounds are stabilized, by the metals forming Mg octahedral or other clusters having strong metal-metal bonds. The reactions to form these clusters are slow. Other nonmetals, especially oxygen, are undesirable impurities that may form more stable phases. Therefore the reactions are carried out with stoichiometric mixtures of pure halide and metal in degassed Ta or Nb tubes that have been loaded in an inert atmosphere and arc-welded shut. The welded ampule is then sealed in a protective quartz tube and heated to a temperature adequate to achieve a reaction in a week or more ( >600°C) . Yields may be small in some cases individual single crystals are produced as evidence of synthesis of a new material with metal-metal bonds. 

The most frequently encountered interstitial species are the second period main group elements C and N. From the main group atoms in the periodic table, B and Si have also been reported to occupy interstitial sites in the reduced rare earth halides. Their valence atomic orbitals, s, p,, p and p, transform as a g + ti in an octahedral field (Bursten et al. 1980). In addition to the ajg orbital, the metal-metal bonding levels of the clusters also contain a t u orbital which is well suited to overlap strongly with the orbitals on the interstitial. Figure 41 illustrates the molecular orbital diagram... 

Nagaki, D., Simon, A. Borrmann, H. (1989). Synthesis and Structure of Gd4l4Si and GdgIgSi The First Examples of Reduced Rare Earth Halides Containing Second-Row Non-Metal Interstitial Atoms, J. Less-Common Met. 156, 193-205. 

The comproportionation route (Corbett, 1983a, 1991) is widely used and is very efficient when pure phases are desired, especially when the phase relationships are known or can be anticipated. It led to a great variety of reduced rare-earth halides, binary, ternary, and higher, simple, and complex salts, and such that incorporate metal clusters interstitially stabilized by a non-metal atom or by a (transition) metal atom, for example,... 

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