Rare earth-carbon-halide

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

The rare earth chlorides can be separated through sublimation but a very high temperature and good vacuum are required. Recently [46] Eu2+ has been obtained pure by the distillation of its halides using the fact that Eu2+-halides are less volatile than the halides of trivalent rare earths. Sm, Eu and Yb oxides can be reduced to the divalent state by carbon and volatilized selectively from a mixture with other rare earth oxides [47]. 

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. 

From the trend in acidities of the hydrogen halides in water, it follows that fluoride is the most basic or nucleophilic of the halides and iodide the least basic if the hydrogen ion is considered the reference acid. It should be recalled (p. 169) that this order of halide basicities is the same as that toward small, multicharged ions with rare-gas structures (for example, Be2+, A 3, and Si4+). A different, and sometimes reversed, order of basicities or nucleophilicities is observed toward certain ions of the post-transition metals (for example, Cu+, Hg +). For a number of ions (for example, Be+2, B+3 and Ta+6), fluoride complexes may exist in aqueous solution, whereas the other halo-complexes do not. Only a few of the elements having positive valence states form no halo-complexes the most important of these are carbon, the rare earths, the alkali metals, and the heavier alkaline-earth metals. 

Zone melting causes impurities to migrate to one end of a cylindrical metal sample by generating a narrow molten zone and moving it repeatedly in one direction along the cylinder axis. Impurities more soluble in the liquid phase (metals, some halides, and carbon) move in the direction of zone travel, while those more soluble in the solid (interstitials) move in the opposite direction. This technique produces pure rare earth metals . ... 

The rare earth elements are very electropositive, and, as a consequence, they generally form ionic compounds. Mineralogically, the REEs therefore form oxides, halides, carbonates, phosphates and silicates, borates or arsenates, but not sulphides. (Henderson 1996). Their oxidation states are given in Table 3.2. 

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). 

Shen, Y., Shen, Z., Zhang, Y., Hang, Q., 1997a. A comparison of polymerization characteristics and mechanisms of e-caprolactone and trimethylene-carbonate with rare earth halides. Journal of Polymer Science Part A Polymer Chemistry 35, 1339—1352. 

Equal four-layer slabs are also observed for carbide halides CR2 X2 and (C2)R2 X2 with single carbon atoms and dicarbon units, respectively, residing in all of the octahedral interstices between the metal double layers. There are also three-layer slab structures of the composition CR2)X in which one halide layer is missing such that the sequence is of the RRX type. In principle, they may be obtained with the same rare earth elements as the hydride halides. 

Rare earths being good electropositive metals combine readily with all the nonmetal elements forming compounds in the solid state. However, with the exception of halides, only oxygen, carbon and sulphur have been observed to form stable gaseous vapors with the majority of rare-earth metals. We shall not be considering halides in this paper, but will review the gaseous species of rare-earth elements with other nonmetal elements. 

The formation of rare earth silicates can be accelerated by the addition of flux material to the reaction mixture. The accelerators may be halides, carbonates, sulfates or oxides of alkali metals, earth alkaline metals, lead, zinc, bismuth, etc. (Leskela and Niskavaara, 1981). The amount of the accelerator represents only a few percent of the total weight of the reaction mixture. For example, using alkah fluorides it is possible to reach a complete conversion to silicate in the temperature range 1000-1300°C (Watanabe and Nishimura, 1978 Leskela and Niskavaara, 1981). 

Well known sintering aids in category (1) are alkali-earth oxides or rare-earth oxides such as Y2O3 (17,18) and CaO (19,20). These can be added not only as oxide but also as nonoxide compounds such as halide, nitride, carbide, nitrite, or carbonate. Some of the transition elements such as NiO and Ti02 can be classified as category (2) additives. Rare earth or alkali-earth oxide additive reacts with aluminum oxide of AIN powder (i.e. oxide layer of AIN powder) to form aluminate liquid at a high temperature Eq. (4) and promotes liquid-phase sintering of AIN powder. 

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