Distribution of Metal Ions and Magnetic Properties in Spinel System CdCr2-xGaxSe4

The structures of the new heavy fermion materials CeIrIn5 and CeRhIn5 have been determined by single-crystal neutron (R(F)= 0.051) and X-ray (R(F)=0.056) diffraction, respectively. Both compounds adopt tetragonal structure, space group P4/mmm (No. 123), Z=1, a=b=4.674(1) ?, and c=7.501(5) ? for CeIrIn5, and a=b=4.656(2) ?, and c=7.542(1) ? for CeRhIn5. The possible presence of antisite disorder, a long-standing question on this type of structure, was excluded by both electron andneutron or X-ray diffraction. The compounds are built by monolayers of face-sharing distorted cuboctahedra [CeIn3] and monolayers of edge-sharing rectangular parallelepipeds [RhIn2] or [IrIn2], stacked alternatively in the [001] direction. Therefore, they are new members of the inhomogeneous linear homologous series MmTnTʹ3m+2n. Because of their ordered intergrowth structure, the physical properties of the quasi-two-dimensional heavy electron systems CeIrIn5 and CeRhIn5 can be directly compared with the corresponding ones of their parent compound, the three-dimensional heavy fermion material CeIn3.