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 (T_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)Å and c = 7.5513(2) Å at ambient conditions. While CeCoIn₅ is isostructural with the HF antiferromagnet 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 way or another) cuboctahedra [CeIn₃] exhibit superconductivity at ambient pressure below 2.3 K and 0.4 K respectively. On the other hand, CeRhIn₅, in which the [CeIn₃] cuboctahedra are less distorted, and the parent compound of the materials CeTIn₅, i.e. the cubic HF CeIn₃, are antiferromagnets at ambient pressure with T_N ≅ 3.4 K and 10 K correspondingly.