Superconductivity in Layered Structures of Rare-Earth Carbide Halides

We report on the superconductivity of compounds of the composition $Y$₂C₂Br₂,Y₂C₂I₂ and $La$₂C₂Br₂,. As Figure 1 shows, the structures of the crystals, which cleave like graphite, are characterized by close-packed double sheets of metal atoms (M). The octahedral interstices are occupied by $C$₂ units and the metal double layers are sandwiched by layers of halogen atoms (X). Such $XMC$₂MX slabs are joined together by van der Waals forces. The crystals show good metallic conductivity within the layers. The conductivity is orders of magnitude smaller in the perpendicular direction.

Figure 1: Central projection of a slab of $M$₂C₂X₂ along $[010]$ of the monoclinic structure (X, M, and C atoms drawn in decreasing size; the diagram is based on the positional parameters of $Gd$₂C₂Br₂).

Figure 2: Dependence of the electrical resistance rho of $Y$₂C₂Br₂ on temperature.

Figure 2 shows the onset of superconductivity for $Y$₂C₂Br₂ as indicated by resistance measurements; the diamagnetic shielding displayed in Figure 3 confirms the superconducting state of all four phases. Superconductivity in these carbide halides is not only interesting because of its occurrence in a two-dimensionally delocalized electronic system. An analysis of the special electronic structure and bonding situation could also contribute to the understanding of chemical prerequisites for the phenomenon of superconductivity.

Extending an earlier idea about high-temperature superconductors, we attribute the attractive interaction between conducting electrons through electron-phonon coupling to a tendency toward localization of the electrons in pairs in special band states. These states may be bonding (covalent M-M bonds, for example in Nb, $Nb$₃Sn, $NbSe$₂), nonbonding (lone pairs in $Sn,Pb,(Ba,K)(Bi,Pb)O$₃) or antibonding in character (d states in oxucuprates). The carbide halides described here fall into the last category.

The C-C distance of 127 pm and 130 pm in $GdC$₂Br₂ determined by X-ray crystallography on single crystals - neutron diffraction on a powder sample of $Tb$₂C₂Br₂ gives a C-C distance of 129 pm - corresponds approximately to a double bond and leads to a formal bonding discription as $(M3+)$₂C₂^4-(X^-)₂ for the phases $M$₂C₂X₂. Semiconductor behaviour is therefore expected unless a strong overlap of M-d and C-p states closes the band gap. Metallic behaviour results, in terms of molecular chemistry, from the backbonding of occupied pi* states of the $C$₂ groups into empty d states on the M atom. This backbonding is already indicated by shortened C-C distances. Such a view of the bonding in $Gd$₂C₂Cl₂ was substantiated by calculations of the band structure using the Extended Hückel method. They show density of states at the Fermi edge, and their COOP (Crystal Orbital Overlap Population) confirmed the C-C antibonding (C pi*) and the Gd-C bonding character of these states.

Figure 3: Magnetization measurements on rare-earth carbide halides. All data points are normalized in such a way that at the lowest temperature the maximal diamagnetic shielding is achieved. For $Y$₂C₂Br₂ it is 40% of the ideal value.

Thus, a rather visual picture of the pairwise attractive interaction between the conduction electrons in the compounds $M$₂C₂X₂ results. The electrons are delocalized in covalently mixed p-d band states and have a tendency to pairwise occupation of the molecular pi* states of the $C$₂ units. In this sense, the stretching and wagging vibrations of the $C$₂ unit should promote an effective electron-phonon coupling through variations of the energy of the pi* level or variation of the p-d overlap. The parallels to other superconducting systems such as, for instance, type $BaBiO$₃ superconductors are obvious. Here, too, the electrons are delocalized in a band of covalently mixed O-Bi states and have a tendency to occupy in pairs the (in this case nonbonding and metal-centered) lone-pair states.

As anticipated, superconductivity in $M$₂C₂X₂ phases does not occur with magnetic lanthanoid ions; the corresponding metallic compounds of Gd and Tb undergo magnetic ordering at temperatures around 28 and 78 K, respectively, but show no superconductivity. The lowering of the valence electron concentration in $Y$₂C₂Br₂ by replacing 10% of the Y atoms by calcium atoms lowers $T$_c to 4 K. The particular halogen involved is critical: $T$_c is approximately 1 K higher for $Y$₂C₂I₂ than for $Y$₂C₂Br₂; however, $Y$₂C₂Cl₂ and $La$₂C₂I₂ are not superconducting.

For completeness one should mention that superconductivity also occurs in binary rare-earth metal carbides of the compositions $M$₂C₃, and $MC$₂. For example, a transition temperature of 17 K is observed for $C$_1.55Th_0.3Y_0.7. The structures also contain $C$₂ units as is required for this composition, and the observed C-C distances suggest strong p-d mixing.

(A. Simon, Hj. Mattausch, R.K. Kremer)

From the yearbook of the institute ("Wissenschaftlicher Tätigkeitsbericht") 1991

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