Rare earth carbide halides

Binary rare-earth compounds such as carbides, sulfides, nitrides, and hydrides have been used to prepare anhydrous trihalides, but they offer no special advantage. Treating these compounds at a high temperature with a halogen (98) or hydrogen halide (115) produces the trihalide, e.g.,... 

R2X2C and R2X2C2. Figure 29 shows the different structures of rare earth metal carbide halides based on planar four-layer slabs. 

At the end of the description of the rare earth metal carbide halides it seems worthwhile to summarize some facts. These compounds contain single C atoms, or Cj entities with C-C single and double bonds. The kind of species seems entirely related to the number of residual valence electrons at the metal site. As we are dealing with electropositive metals, these electrons will be transferred to MOs of the unit and it is the number of vacant antibonding MOs which determines the kind of carbido species. Thus, the ideas of Atoji (1961) concerning binary carbides can be extended to the rare earth metal carbide halides. A more detailed discussion of the bonding will be given in sect. 3. 

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. 

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. 

JY Chan, SM Kauzlarich. Rare-earth halides as fluxes for synthesis of tantalum and niobium carbide. Chem Mater 9 531, 1997. 

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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