Superconductivity in Yttrium carbide halides

We report on the preparation of halogen-mixed phases Y$$₂C$$₂X$$₂(X=Cl, Br, I) and their structural as well as electrical characterization. As described earlier (Tätigkeitsbericht 1991) superconductivity is found in such phases in the case of non-magnetic rare-earth metals.

Figure 1: Central projection of a slab of Y$$₂C$$₂X₂ along [010] with X, M and C atoms drawn in decreasing sizes. The diagram is based on the positional parameters of Y$$₂C$$₂Br$$₂.

As Fig. 1 shows, the structures of these compounds are characterized by close-packed double layers of metal atoms (M). The octahedral interstices are occupied by C$$₂ units, and the metal atom double layers are sandwiched by layers of halogen atoms (X). These XMC$$₂MX slabs are joined to each other via van der Waals forces. Two monoclinic stacking types of the layer packages are known. Y$$₂C$$₂Br$$₂ crystallizes in the 3s-form, with three slabs necessary to reach identical atom positions. Only one slab is needed in the 1s-form, which is the structure of Y$$₂C$$₂I$$₂ and Y$$₂C$$₂Cl$$₂.

Figure 2: Resistance measurement on a Y$$₂C$$₂I$$₂ crystal

Figure 2 displays the in-plane resistance measured on a Y$$₂C$$₂I$$₂ crystal using a 4-point van der Pauw technique. The crystal is a thin platelet of approximately 0.5 mm diameter and a few microns thickness. A residual resistivity ratio about 2 is found. At T$$_c=9K the resistance drop indicates the transition into the superconducting state.

Figure 3: Lattice parameters a) and transition temperatures b) of the systems Y$$₂C$$₂Br$$_xCl$$_2-x (filled), Y$$₂C$$₂I$$_xBr$$_2-x (filled) and Y$$₂C$$₂I$$_xCl$$_2-x (open). The values for the ionic radii are: Cl$-$ 1.81 Å, Br$-$ 1.96 Å, I$-$ 2.20 Å. The 1s-form is represented by circles, the 3s-form by squares.

The metallic conductivity of these compounds is due to backbonding from occupied pi* orbitals of the C$$₂ unit into empty d states of the metal atoms. According to our hypothesis the pairwise attractive interaction between conduction electrons originates from a tendency towards (pairwise) localization in the pi* states at the Fermi level. In this sense the stretching and wagging vibrations of the C$$₂ unit offer an effective electron phonon coupling through dynamic variation of the energy of the pi* level and/or variation of the p-d overlap.

We report on measurements for compounds with different halogen compositions. The different sizes of the halogen atoms introduce variations of the lattice parameters and, as a consequence, p-d overlap. Fig. 3 shows a) the lattice parameters derived from Guinier films and b) the transition temperatures from SQUID measurements in the systems Y$$₂C$$₂Br$$_xCl$$_2-x, Y$$₂C$$₂I$$_xBr$$_2-x and Y$$₂C$$₂I$$_xCl$$_2-x.

A linear dependence of the lattice parameters a and b (parallel to the layers) on the averaged ion radius of the halogen atoms is observed. There is a maximum in T$$_c (11.1 K) at the composition Y$$₂C$$₂Br$$_0.5I$$_1.5. Obviously the p-d overlap is optimized with respect to T$$_c for this composition. In addition, it is of interest that changes in structure do not reflect in the values of T$$_c. As indicated by the discontinuities in the monoclinic angle beta (Fig. 3a) the stacking sequence of the slabs changes from 1s to 3s at an average radius near 1.87 Å and back to 1s near 2.06 Å. These changes are also seen for the c-axis lengths, however, the axes a and b change monotonically. These results shed light on the anisotropy of the interactions leading to superconductivity.

(M. Bäcker, R. Henn, Hj. Mattausch, R. K. Kremer and A. Simon)

From the yearbook of the institute ("Wissenschaftlicher Tätigkeitsbericht") 1994

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