Rare earth sesquicarbides

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. 

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). 

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

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). 

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). 

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. 

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

In this review, I briefly summarize important points about these compounds and especially focus on some new developments, such as the enhanced superconductivity recently discovered for yttrium sesquicarbide, and also new discoveries for the rare earth intercalated carbon compounds. 

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