Ternary rare-earth halides

The crystal chemistry of the ternary rare-earth halides with monovalent cations A of the general composition A ,RyX3 +w has developed considerably in the past sesquidecade. Phase diagram determinations by Blachnik (Blachnik and Selle 1979, Blachnik and Jager-Kasper 1980) have contributed much to get an overview of the existence of the more stable compound classes, especially with A=K, Rb, Cs. The respective crystal chemistry has been the subject of a previous review (Meyer 1982). Since then many systems have been reinvestigated or the phase diagram was even determined for the first time, especially by... 

Examples for ternary rare-earth halides of the A2RX5 type are known for all of the halides and most A+ ions although there are only few examples with A=Na, Cs (table 5). 

One of the best known classes of ternary rare-earth halides is that of A3R2X9-type compounds. As the A cation takes part in closest packed layers of the composition AX3 and, therefore, requires with respect to its coordination number of 12 large A ions, namely Cs, Rb, and to a much lesser extent K". They also do not occur with X=F (table 6). 

A rather uncommon composition for ternary rare-earth halides is that of A4RX7-type compounds. Their structure and synthesis was reported only recently (Reuter et al. 1996) and, up to date, two examples are well established, Cs4YbCl7 and CS4YCI7 (Seifert and Biichel 1998). The preparation is remarkable because it was carried out in aqueous solution. 

Ternary rare-earth-carbon-halide compounds... 

Nomenclature and scope 54 4.2. Ternary rare-earth(II) halides 97... 

Reduced halides 58 4.3. Ternary rare-earth(II,lII) halides 102... 

Ternary rare-earth(lll) halides 65 5.3. Triangles, [R3], Di(delta)hedra 107... 

After the derivatives of simple binary halides, we will discuss the formation and structures of ternary halides in the systems AX/RXz (with z = 2, 3 and with A an alkali metal, lithium through cesium, or an pseudo-alkali cation such as In, Tr, NHJ) and BX2/RXz (with B a divalent cation) and some mixed systems. Complex rare-earth halides have been reviewed eighteen years ago (Meyer 1982), so that we will focus on generalities and recent developments. 

Box 2. Examples for reactions to ternary rare-earth(m) halides... 

G. Meyer and M.S. Wickleder have described the synthesis and structures of the many types of rare-earth halides. They have classified them as simple, complex, binary, ternary, quaternary, multinuclear complex, and other categories needed to deal with this most studied of the rare-earth compounds. The structure types are skillfiilly illustrated to show the elementary architecture of each type. 

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,... 

Numerous binary and ternary diene polymerization initiator systems with neodymium as the rare-earth metal component have been designed empirically and investigated since the early discoveries in the 1960s. Commercially used neodymium-based catalysts mostly comprise Nd(III) carboxylates, aluminum alkyl halides, and aluminum alkyls or aluminum alkyl hydrides [43, 48,50-52]. Typically, the carboxylic acids, which are provided as mixtures of isomers from petrochemical plants carry solubilizing aliphatic substituents R. They are treated with the alkylaluminum reagents to generate the active catalysts in situ (Scheme 11). 

A common and most easily to foUow way to prepare rare-earth hahdes, even in large quantities, is the so-called ammonium haUde route (Reed et al. 1939, Meyer and Ax 1982, Meyer 1989). Two steps are involved in this procedure. In the first step, the respective rare earth (most commonly the sesquioxide, R2O3) is converted into a ternary anunonium halide, (NIl4)3RX<, or (NH4)2RXs, which is, in the second step, decomposed to the respective rare-earth trihalide RX3 recovering ammonium halide (box 1). The first step may be carried out by dissolving the oxide and ammonium halide in hydochloric... 

For ternary and quaternary chlorides and bromides it is normally sufficient to dissolve the respective rare-earth and alkali halides or carbonates in l drohalic acid solution, evaporate to dryness and heat this intermediate product in a stream of the respective hythogen halide gas at temperatures between 300 and 500 C (Meyer 1983a,b). 

The XEOL of rare earths (usually Sm, Eu, Gd, Tb and Dy) present at the 100-1000 ppm level in a wide variety of simple, binary, ternary and quaternary oxide host systems have been reported by Jaworowski et al. (1968), Takashima et al. (1969), Kawaguchi et al. (1969), Saranathan et al. (1970), DeKalb et al. (1970) and D Silva et al. (1970). In addition a limited number of halides, oxyhalides, sellenides and tellurides have been shown by Low et al. (1974,J,... 

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