Comparison of the Crystal Structure of the Heavy-Fermion Materials CeCoIn5, CeRhIn5 and CeIrIn5

The crystal structure of the recently discovered heavy-fermion (HF) superconductor CeCoIn5 (Tc = 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) A and c = 7.5513(2) A at ambient conditions. Whereas CeCoIn5 is isostructural with the HF aniferromagnet CeRhIn5 and the HF superconductor CeIrIn5, its cell constants and its only variable positional parameter, zIn2, differ significantly from the corresponding ones of CeRhIn5 and CeIrIn5. As a result, the distortions of the cuboctahedron [CeIn3], which is the key structural unit in all three materials, are different in CeCoIn5 from the ones in CeRhIn5 and CeIrIn5. The compounds CeCoIn5 and CeIrIn5, which contain the most distorted (in one or another way) [CeIn3] cuboctahedra exhibit superconductivity at ambient pressure below 2.3 K and 0.4 K respectively. On the other hand, CeRhIn5, in which [CeIn3] cuboctahedra are the less distorted, and the parent compound of the materials CeTIn5 i.e. the cubic HF CeIn3 are antiferromagnets at ambient pressure with TN = 3.8 K and 10 K correspondingly.