Halides metal-rich

Typically, Be-containing alloys and intermetallic phases have been prepared in beryllia or alumina crucibles Mg-containing products have been synthesized in graphite, magnesia or alumina crucibles. Alloys and compounds containing Ca, Sr and Ba have been synthesized in alumina , boron nitride, zircon, molybdenum, iron , or steel crucibles. Both zircon and molybdenum are satisfactory only for alloys with low group-IIA metal content and are replaced by boron nitride and iron, respectively, for group-IIA metal-rich systems . Crucibles are sealed in silica, quartz, iron or steel vessels, usually under either vacuum or purified inert cover gas in a few cases, the samples were melted under a halide flux . 

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]. 

A. Simon, Hj. Mattausch, G.J. Miller, W. Bauhofer and R.K. Kremer, Metal-rich halides 191... 

Stoichiometry. At present the observed limit of two halide ions per metal does not seem particularly important as a necessity for close approach of the cations and hence suitable band formation rather, it more probably results from other characteristics of this formal oxidation state for these elements. One possible fact to the contrary is that thorium (III) iodide is evidently not metallic (4), though it would probably meet the second criterion below. The general electronic conduction in sulfide vs. chloride melts in the metal-rich region (as well as in the solid state) may be attributed to the lower anion to cation ratio and therefore closer approach of the cations (5), although covalency as discussed below may be more significant. 

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 chemistry of reduced Nb and Ta halides is rich in clusters with various structures. The metal atoms assemble with metal-metal distances close to those in the metal into triangular and tetranuclear clusters but the dominant structural motif is that of the octahedral MgXi2 and Nbglg types. Binary, ternary, and quaternary compounds all crystallize in that type. The Me clusters are characteristic of the chemistry of the lower oxidation states of Nb and Ta, although not restricted to them. These electron-deficient clusters are based on metal ions with average oxidation numbers between III and I. 

In what follows, the existing metal-rich halides are discussed first along the line of their structural chemistry. As an ordering scheme, the increasing degree of cluster condensation is used whether the Z atoms are present or not, i.e. the kind of R/X framework alone is used as a guideline. The arguments used to understand the... 

In the second section this electron counting scheme is rationalized in terms of recent band structure calculations. The limitations of the ionic model as well as a variety of structural and electronic relationships for metal-rich halides are also discussed using these theoretical treatments. 

Crystallographic data of metal-rich rare earth halides with one-dimensional infinite chains. 

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. 

Crystallographic data of metal-rich rare earth halides with structures derived from GdjCla. Calculated with the atomic parameters of (a) Gd2Cl3, (b) a-GdjCljN, (c) P-YjCljN,. 

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