Gaseous rare earth carbides

Thermodynamic stability of gaseous rare earth carbides 105... 

The high-temperature Knudsen effusion mass spectrometric technique has been used to study equilibria involving gaseous rare-earth-containing molecules. The existence of a large number of stable gaseous rare earth carbides has been established. The enthalpies AHZ of the reactions... 

The carbide systems are next to oxides in importance both in condensed state as well as in gaseous state. However, a few significant differences between the carbides and the oxides systems should be noted. Carbon, unlike the diatomic gaseous oxygen, not only is solid at ambient temperature, but even at high temperatures the carbon vapors in equilibrium with graphite contain many more species besides the diatomic carbon species. This difference between the carbide systems and the oxide systems is also found in the gaseous rare-earth carbides. 

In contrast to the rare-earth-oxygen systems, the rare-earth monocarbide vapors are exception rather than the rule. Only the CeC molecule has so far been observed in the gas phase (Gingerich 1969, Kingcade et al. 1983). On the other hand, dicarbides of the formula, RC2, R = rare-earth atom, have been observed for most of the rare-earth elements (Gingerich 1980a, 1985). The recent review by Gingerich (1985) describes the complexity of gaseous rare-earth carbides. For primary references one should refer to this review. 

The atomization enthalpies (4//,) and the enthalpies of formation (,AH°) of ternary gaseous rare-earth carbides (kJ/mol). 

According to De Maria and co-workers (1972), the dicarbide is always the most abundant gaseous molecular species in the equilibrium vapor over the corresponding rare-earth-carbide-graphite system, followed by the tetracarbide. Table 8 shows examples of equilibrium partial-pressure ratios, P(RC2)/P(R) and P(RC4)/P(R), at the specified temperatures. 

The selected values for the reaction enthalpies were combined with ancillary literature data to yield the atomization energies AH° q and the standard heats of formation AHf 298,15 of the gaseous di-rare-earth carbides (table 10). 

The gaseous carbide species containing rare-earth atoms are quite complex. The high temperature mass spectrometric data again are the primary source for... 

---