A crystallographic study of the Nd/Er site preferences in the Nd 5Ϫx Er x Tt 4 (Tt ϭ Si, Ge) series prepared by high-temperature methods is presented. For Nd 5Ϫx Er x Si 4 , phases with x Յ 1.0 adopt the tetragonal Zr 5 Si 4 -type structure. On the other hand, phases in the composition range of 2.0 Յ x Յ 5.0 exhibits the Gd 5 Si 4 -type structure. For all silicides, all silicon atoms belong to SiϪSi dimers. In the Nd 5Ϫx Er x Ge 4 system, phases with x < 3.2 and x ϭ 4.0 adopt the orthorhombic Sm 5 Ge 4 -type structure. For the composition range of 3.2 Յ x Յ 4.0, a monoclinic U 2 Mo 3 Si 4 -type structure (space group P2 1 /c) occurs as the majority phase. This monoclinic structure, until recently, was not reported for similar RE 5 T 4 systems, and differs from the known monoclinic Gd 5 Si 2 Ge 2 -type * Prof. G. J. Miller E-Mail: gmiller@iastate.edu [a] 889 (space group P2 1 /a) because all Ge···Ge contacts between slabs are equivalent. The structural relationships between the Zr 5 Si 4 -type, Gd 5 Si 4 -type, Sm 5 Ge 4 -type, and U 2 Mo 3 Si 4 -type structures are discussed. Single crystal refinements of the metal atom occupancies for the three different metal sites in the asymmetric unit of all structure types reveal a partially ordered nonstatistical arrangement of neodymium and erbium atoms. The magnetic properties of some Nd 5Ϫx Er x Ge 4 phases are also reported. Nd 4 ErGe 4 shows an antiferromagnetic-type transition similar to Gd 5 Ge 4 . As the erbium concentration increases (2 Յ x Յ 4), these phases exhibit, at least, ferrimagnetic-type ordering.

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