Structur–Composition Relations and Fractional Site Occupancy of New M5Ge4 Compounds in the System Ge–Ta–Zr

The new ternary phases Zr_4−xTa_1+xGe₄ (0.1<x<0.4) and Zr_2+xTa_3−xGe₄ (0.1<x<1.1) were prepared from the elements by arc melting and subsequent induction heating at 1400–1450°C. Single-crystal X-ray diffraction was used to determine their structures and to refine mixed site occupancies. Zr_4−xTa_1+xGe₄ was found to crystallize in the monoclinic space group P2₁/c (structure type: U₂Mo₃Si₄) and the compound Zr_2−xTa_3−xGe₄ shows orthorhombic symmetry (space group Pnma, structure type: Sm₅Ge₄). The close structural relationship between the two structures is discussed. Both phases exhibit pronounced differential fractional site occupancy of Ta and Zr on the metal sites and considerable composition ranges. Extended Hückel calculations were performed for various site occupancy models and Mulliken overlap populations for the different lattice sites of each structure were calculated for these models. The correlation of the cumulated Mulliken overlap populations and the atomic orbital populations with the actual site occupancies is discussed.