Sesquicarbide

The C-C distance in CaC2 is close to that in ethyne (120.5 pm) and it has been suggested that the observed increase in the lanthanoid and actin-oid carbides results from a partial localization of the supernumerary electron in the antibonding orbital of the ethynide ion [C=C] (see p. 932). The effect is noticeably less in the sesquicarbides than in the dicarbides. The compounds EuC2 and YbC2 differ in their lattice parameters and hydrolysis behaviour from the other LnC2 and this may be related to the relative stability of Eu and Yb (p. 1237). 

An interesting sesquicarbide, Mg2Q, can be prepared relatively pure by reaction of Mg dust with n-pentane at 680°C. In the solid state, it contains Q units these species are linear, isoelectronic with C02, and have C—C distances of 1.332(2) A.34 Other known sesquicarbides are SC3C4 (which contains a portion of Q groups next to Q and Q (units) and a relatively stable lithium derivative, Li4Q. The hydrolysis of this type of carbide proceeds according to ... 

KRU] Kmpka, M. C., High pressure synthesis of thorium sesquicarbide a... 

All rare earths easily form dicarbides (REEC2). Several of them (La-Sm and Gd-Ho) also form sesquicarbides (REE2C3). Solid solubility of carbon in rare earths also occurs easily. REE-carbides also form solid solutions with nitrogen and oxygen (Gupta and Krishnamurthy 2005). 

The existence of the sesquicarbide Th2C3 was found by Krupka (113) to be stable only at high temperatures and pressures (above 1300°C and 30 kbar)—which is a behavior sharply contrasting with U2C3 and PU2C3— and to be a stoichiometric compound. 

In these systems, two types of rare earth carbides have been confirmed to exist the rare earth dicarbides and the rare earth sesquicarbides. The existence of the monocarbides of cerium and praseodymium (Warf 1955, Brewer and Krikorian 1956, Dancy et al. 1962) has been discredited. It was found on the basis of an X-ray study (Spedding et al. 1958) that the earlier reported cerium monocarbide was most probably a solid solution of carbon in cerium the lattice parameter reported by Brewer and Krikorian (1956) for the cerium monocarbide was identical with that of cerium metal saturated with carbon. 

In addition to the hypocarbides and the dicarbides, there exist in the heavy-rare-earth-earbon systems the sesquicarbides and other carbides near 60%, as reported for the yttrium-carbon system, which has the same pseudocubic tetragonal cell as SC15C19 (Jedlicka et al. 1971). 

The existence of the europium sesquicarbide has been established by X-ray diffraction (Colquhoun et al. 1972), and a cubic Pu2C3-type structure was also determined with a lattice parameter, following the systematic variation of that of the neighboring sesquicarbides. 

The existence of the ytterbium sesquicarbide with the cubic Pu2C3-type structure was not established (Spedding et al. 1958) under common conditions. However, this type of ytterbium carbide can be prepared at high pressures, like the other high-pressure heavy-lanthanide sesquicarbides (Krupka and Krikorian 1970). 

Fig. 4. Decomposition temperature of the light lanthanide sesquicarbides (Gschneidner and Calderwood 1986).

The shortest R-R distances in the rare earth sesquicarbides, Rq-3Ri, are nearly 10% shorter than the shortest R R distances in the dicarbides, which are equal to their lattice parameters. Other R0-2R2 and R0-6R3 distances are also shorter than this value, indicating that there are stronger R-R interactions in the sesquicarbides than in the dicarbides. The interatomic distances in Ce2C3, particularly the Ce-Ce distances are markedly smaller than the expected values for CcjCj with the pure trivalent Ce atoms. This is in accordance with the result obtained from the paramagnetic scattering analysis (Atoji and Williams 1967). 

Yb -83 at.% ) when present in the tetragonal dicarbide, which was confirmed by magnetic susceptibility measurements (Flahaut 1969), and trivalent in the cubic hypocarbide. In the sesquicarbide, the behavior of europium and ytterbium is anomalous. Ytterbium forms a monoclinic Yb2C3 phase (Haschke and Eick 1970a) while europium forms a tetragonal phase with a Ca2C3-type structure (Pasto and Morgan 1976, Schwetz et al. 1979). 

As described above, in the dicarbides and the sesquicarbides of the rare earth elements there are pairs of carbon atoms, while in the hypocarbides, either the cubic structure or the trigonal one, the carbon atoms are isolated and no longer appear as pairs. In the intermediate carbides, R15C19, the tetragonal structure is made up of distorted metal octahedra, carbon pairs and also single carbon atoms. 

Superconductivity of the rare earth sesquicarbides and the Th-containing solid solutions... 

Incorporation of elements other than thorium into yttrium sesquicarbide also leads to a resultant enhancement of the superconducting temperature (Krupka et al. 1969b). These elements include the group IV and VI transition metals, as well as gold, germanium and silicon. But, in these cases, it is necessary to ensure the solubiUty of these elements in the parent sesquicarbide lattice. 

Specific heats and critical magnetic field curves of the rare earth and the rare-earth-thorium sesquicarbides... 

The critical magnetic field curve measurements of lanthanum and yttrium-thorium sesquicarbide (Francavilla et al. 1976, Francavilla and Carter 1976) show that the initial slopes of the critical magnetic field curve are (2.4 0.2) x lO OeK" and... 

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