Many Metals Make the Cut: Quaternary Rare-Earth Germanides RE4M2InGe4 (M = Fe, Co, Ni, Ru, Rh, Ir) and RE4RhInGe4 Derived from Excision of Slabs in RE2InGe2

The formation of quaternary rare-earth (RE) germanides containing transition metals (M's) from groups 6 to 10 was investigated through arc-melting and annealing reactions at 800 degrees C; about 50 new compounds were obtained. These include several new series of quaternary germanides RE4M2InGe4 (M = Fe, Co, Ru, Rh, Ir), previously known only for M = Mn and Ni; additional members of RE4Ni2InGe4 extended to other RE substituents; and a different but closely related series RE4RhInGe4. Detailed crystal structures were determined by single-crystal X-ray diffraction studies for 20 compounds. Monoclinic structures in space group C2/m are adopted by RE4M2InGe4 (Ho4Ni2InGe4-type, a = 15.1-16.5 angstrom, b = 4.1-4.4 angstrom, c = 6.9-7.3 angstrom, beta = 106.2-108.6 degrees) and RE4RhInGe4 (own type, a = 20.0-20.2 angstrom, b = 4.2-4.3 angstrom, c = 10.1-10.2 angstrom, beta = 105.0-105.3 degrees). Both structures contain frameworks built from MGe4 tetrahedra, InGe4 square planes, and Ge-2 dimers, delimiting tunnels occupied by RE atoms. These structures can also be derived by cutting slabs along different directions from the more symmetrical RE2InGe2 structure. Although the Ge-2 dimers are relatively invariant, the InGe4 square planes can undergo distortion to form two sets of short versus long In-Ge distances. This distortion results from a competition between M-Ge bonding in the MGe4 tetrahedra and In-Ge bonding in the InGe4 square planes.