Binary rare earth carbides

Summary of the formation of the binary rare earth carbides 77... 

Thermodynamic properties of binary rare earth carbides 105... 

According to the reported information on the binary rare-earth-carbon phase diagrams, a survey on formation of the binary rare earth carbides has been made and is given in table 2. 

Binary rare earth carbides and their lattice parameters (A) . 

The formation of the binary rare earth carbides has been summarized in table 2. The crystal structure and lattice parameter data listed in this table were quoted from the review Critical evaluation of binary rare earth phase diagrams (Gschneidner and Calderwood 1986). The listed lattice parameters were assessed by them and are the mean values when more than one acceptable set of data were presented for an individual compound. In this section, the crystal structures of each binary rare earth carbide will be evaluated in detail. 

Despite these theoretical investigations, a systematic study of the relationship of the chemical bonds with the formation and the structural stability of the binary rare earth carbides remains to be carried out. 

Drs. G.-y. Adachi, N. Iraanaka, and Z. Fuzhong review the rare earth carbides (chapter 99) emphasizing the thermodynamics, phase diagrams, crystal structures, and physical properties. The binary rare earth carbides present an exceptionally wide range of compositions and structures both as solids and gas-phase molecules. More complex carbides with additional metal and non-metal components also receive attention. 

While perhaps with not as wide variety as the borides, the rare earth elements also form interesting compounds with carbon to form the rare earth carbides. The phases are particularly rich for the relatively metal-rich carbide compounds. Adachi etal. have written a detailed, 129-page long review on the rare earth carbides, while Gschneidner and Calderwood have comprehensively reviewed phase diagrams and lattice constants of binary rare earth carbides. ... 

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.,... 

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. 

The author considered it best not to include in the reference book the properties of certain little-studied compounds rarely used in practice. Thus, in the presentation of the information on carbides, borides, nitrides, and other classes of metal-like compounds, no data are given on the refractory compounds of metals of the platinum group for the sulfides, data are given only for the class of sulfides of the rare-earth metals and actinides, in most of which the properties of refractory compounds in the wide sense are most clearly expressed, the proportion of ionic bond, in particular, being small. It was, however, found e qpedient to consider also the properties of oxysulfides of the rare-earth metals and actinides, which are very similar to the properties of sulfides and are obtained simply by replacement of two atoms of sulfur in a sesquisulfide by two atoms of oxygen. This is one of the few exceptions where the tables of the reference book give the properties of ternary and not binary compounds. 

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