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 °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 Å, b = 4.1–4.4 Å, c = 6.9–7.3 Å, β = 106.2–108.6°) and RE₄RhInGe₄ (own type, a = 20.0–20.2 Å, b = 4.2–4.3 Å, c = 10.1–10.2 Å, β = 105.0–105.3°). Both structures contain frameworks built from MGe₄ tetrahedra, InGe₄ square planes, and Ge₂ 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 Ge₂ 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.