Rare earth halide clusters

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. 

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... 

At this point, we note that except for H in Nb InH (Simon et al. 1967) and CsNbgli j H (Imoto and Corbett 1980), no other kind of interstitial species has been observed at the center of the metal octahedron in MgXg clusters. Possible explanations include the restrictive size of the interstitial site as well as X Z repulsions through the faces of the octahedron. In any case, since no examples exist in the rare earth halide family and no detailed theoretical treatment has been undertaken, we shall not discuss this specific example further. 

The previous structures, which contain distinct R octahedral clusters, necessarily have R/Z > 6. When R/Z > 4, we can find a finite number of these clusters condensed to give larger molecular units. For rare earth halide systems, only edge-sharing octahedra have been observed, as delineated in sect. 2, in which only two octahedra are involved and the interstitial moiety is a C2 dimer. Satpathy and Andersen (1985) applied the LMTO-ASA method to produce the energy DOS illustrated in fig. 43. Their model examined the interaction between an isolated GdioCljs cluster with two... 

Investigations of the electrical and magnetic properties of the metal-rich rare earth halides have focussed on the Gd halide hydrides (deuterides) and carbides, and the Tb halide hydrides (deuterides). Table 10 summarizes some significant electrical and magnetic data of Gd, Tb, Sc and Y compounds. The binary compounds, the carbide halides with cluster chains or planes, and the hydride halides are discussed in detail. 

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. 

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,... 

Of course, valence electron concentration is not only related to the metal atoms but also to the number and valence of the ligands. Ligand deficiency creates vacant coordination sites at metal atoms and results in cluster condensation, which is the fusion of clusters via short M-M contacts into larger units ranging from zero- to three-dimensional. The chemistry of metal-rich halides of rare earth metals comprises both principles, incorporation of interstitial atoms and cluster condensation, with a vast number of examples [22, 23]. 

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. 

Modelling of Anisotropic Exchange Coupling in Rare-Earth-Transition-Metal Pairs Applications to Yb3+-Mn2+ and Yb3+-Cr3+ Halide Clusters and Implications to the Light Up-Conversion (M. Atanasov, C. Daul H. U. Gtidel)... 

It has been mentioned in the introduction that the condensed cluster halides of the rare earth metals based on the MgXi2-type cluster with an interstitial atom (or molecular unit) generally exhibit a defect rocksalt structure. Figure 10 provides clear evidence for this remark. The NaCl subcell in the structure of GdijInCg, marked by strong streaks is only weakly distorted (a = 6.07, b = 6.10, c = 5.92 A, a = y = 90°, p = 91°) by the ordering of I atoms and Cj units and occupation of all voids around the C2 units by Gd atoms. 

The structure of Gd2Cl3 contains linear chains of trans-edge-sharing metal octahedra. However, there are distinct differences which require a separate discussion of this structure. First, the compounds R2X3 (together with SC7CI10) seem to be the only binary metal-rich (X/R < 2) halides of the rare earth metals. Second, the chains in the structure are formally derived from the Mg Xg-type cluster. Last, but not least, incorporation of interstitial atoms leads to a number of phases, whose structures are closely related to that of Gd2 CI3. The structural family is summarized in table 4. 

As illustrated in the previous section, the metal-rich rare earth metal halides and their interstitial derivatives provide a vast collection of compounds that transcends the structural chemistry of both molecules and extended solids. On the one hand, these substances can be considered as connected or condensed clusters of the MgXi2-or MgXg-type, which may contain interstitial species. On the other hand, many of them can be derived from the structures of simple salts, e.g. NaCl or La20jS. 

This concept, adding electrons via the incorporation of electropositive elements outside and inside the cluster, becomes more and more important when the number of valence electrons decreases. Zirconium, yttrium and the rare-earth elements associated therewith are well known to form octahedral clusters but virtually never without an interstitial. In box 3 elements that form halide clusters are printed in bold type, and the rare-earth elements (in the box) that seem to form clusters most easily are indicated by shading. 

Owing to the ample contributions of the groups of Corbett and Simon, one must have the impression that the so-called reduced halide chemistry of the rare-earth elements is that of interstitially stabilized [R Z] clusters. It is certainly dominated by these units when condensed metal clusters are considered. This chemistry has been reviewed several times (Corbett 1992, Simon 1995, Meyer 1988). 

Ishii T (2005) Eirst-principles calculations for the cooperative transitions of Yb dimer clusters in Y3AI5O12 and Y2O3 crystals. J Chem Phys 122 024705-1-024705-6 Rubio O (1991) Doubly-valent rare-earth ions in halide crystals. J Phys Chem Solids 52 101-174... 

The pattern as seen in Figure 5 may be transferred to a periodic table of the rare earth elements, see Figure 6. Only elements underlaid in red form clusters. The lower I3 is, the easier it is to produce cluster complexes. Elements underlaid in blue form stable divalent compounds, for example EUCI2 the divalent state with the electronic configuration 4f 5d° (with n =7, 14, 6, 13 for R = Eu, Yb, Sm, Tm) has the highest stability and, thus, is the easiest to achieve when the third ionization potential is the highest. The divalent chemistry of these elements is alkaline-earth and saltlike this is described in The Divalent State in Solid Rare Earth Metal Halides. 

The synthesis of compounds of the lanthanides containing cluster complexes follows in general the same routes as described in The Divalent State in Solid Rare Earth Metal Halides, the conproportionation route and the metallothermic reduction route, for example... 

The vast majority of metal-rich rare earth-metal halides form clusters R that incorporate an atom Z, or a small atom group, in the center of the cluster, hence heteroatomic clusters ZR need to be considered. These clusters are surrounded by ligands, usually haUdo ligands X of the triad chloride, bromide, and iodide. Figure 10 depicts such an isolated cluster complex [ ZR X i2X 6] ... 

Oxide-halides of the alkaline-earth-hke divalent lanthanides, OlGlXe (R = Eu, Yb, Sm) are discussed in The Divalent State in Solid Rare Earth Metal Halides. Although, these have the topology of cluster complexes with isolated... 

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