  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.
