Polymorphism in binary rare-earth metal germanides. Synthesis, structure and properties of the new hexagonal forms of Tb3Ge5 and Dy3Ge5

Reported are the synthesis, crystal structure determination and magnetic properties of new polymorphic forms of the rare-earth metal germanides Tb3Ge5 and Dy3Ge5. Both compounds are isostructural and crystallize with the hexagonal space group P6¯2c (No. 190, Z = 2) with unit-cell parameters a = 6.861(2) Å; c = 8.339(6) Å for Tb3Ge5 and a = 6.8387(10) Å; c = 8.293(2) Å for Dy3Ge5, respectively. The structures are derivatives of the ubiquitous AlB2 type and can be regarded as its 6-fold superstructure (a′ = a × 31/2 and c′ = c × 2), arising from the long range ordering of Ge vacancies. They are therefore best described as flat Ge-layers, stacked in a hexagonal close-packed manner along the crystallographic c-axis, which are separated by layers of rare-earth metal atoms. Magnetic susceptibility measurements reveal that both Tb3Ge5 and Dy3Ge5 exhibit antiferromagnetic order at temperatures below 23 K and 9 K, respectively.