Tb₄FeGe₈ grown in liquid gallium: Trans-cis chains from the distortion of a planar Ge square net

The ternary germanide Tb₄FeGe₈ was obtained from Ga flux reactions. The crystal structure studied with single-crystal X-ray diffraction revealed the existence of an orthorhombic average substructure (Cmcm, Z = 1) with cell parameters a = 4.1118(14) Å, b = 15.844(5) Å, and c = 3.9885(15) A. The refinement [I > 2σ(I)] converged to final residuals R1/wR2 = 0.0363/0.0893. The average structure (CeNiSi ₂-type) consists of a 3D [Fe_1/4Ge₂] framework where Ge atoms form a square net and Fe atoms reside alternatively above and below it with only 1/4 occupation probability. X-ray and electron diffraction studies showed a modulation in the Ge net. The modulated structure was refined based on a 4-fold monoclinic supercell (P2₁/n) with parameters a = 5.7315(11) Å, b = 15.842(3) Å, c = 11.438(2) Å, and β = 91.724(4)° with R₁/WR₂ = 0.0643/0.1735 and uncovered a severe distortion of the Ge square net. The Ge atoms are displaced to form an array of cis-trans chains. The Ge-Ge distances within these chains are distinctively bonding, whereas those between the chains are nonbonding. Results of the electronic structure calculations and magnetic measurements are also reported. The structural distortions found in Tb₄FeGe₈ cast a doubt onto the correctness of many of the reported REM _1-xGe₂ disordered compounds and call for reinvestigation.