CRYSTAL STRUCTURE, SUPERCONDUCTIVITY AND MAGNETISM OF THE QUASI-2D HEAVY FERMION MATERIALS CeTiN5 (T=Co, Rh, Ir)

The crystal structure of the recently discovered heavy-fermion (HF) superconductor CeCoIn{sub 5} (T{sub c} = 2.3 K) has been determined by high-resolution neutron powder diffraction. It is tetragonal (space group P4/mmm), with lattice parameters a = 4.61292(9) {angstrom} and c = 7.5513(2) {angstrom} at ambient conditions. Whereas CeCoIn{sub 5} is isostructural with the HF aniferromagnet CeRhIn{sub 5} and the HF superconductor CeIrIn{sub 5}, its cell constants and its only variable positional parameter, zIn2, differ significantly from the corresponding ones of CeRhIn{sub 5} and CeIrIn{sub 5}. As a result, the distortions of the cuboctahedron [CeIn{sub 3}], which is the key structural unit in all three materials, are different in CeCoIn{sub 5} from the ones in CeRhIn{sub 5} and CeIrIn{sub 5}. The compounds CeCoIn{sub 5} and CeIrIn{sub 5}, which contain the most distorted (in one or another way) [CeIn{sub 3}] cuboctahedra exhibit superconductivity at ambient pressure below 2.3 K and 0.4 K, respectively. On the other hand, CeRhIn{sub 5}, in which [CeIn{sub 3}] cuboctahedra are the less distorted, and the cubic HF CeIn{sub 3} are antiferromagnets at ambient pressure with T{sub N} = 3.8 K and 10 K respectively; they become superconductors under pressure of 16 kbar and 25 kbar with T{submore » c} = 2.1 and 0.2 K respectively.« less