Al7+xFe23Ge14−x and Al7+xFe9Ge5−x: Two new ternary compounds related to Fe6Ge5

The title compounds were synthesised from the pure elements by arc melting and subsequent heat treatment at 800 °C. They were characterised by powder- and single-crystal XRD, their homogeneity ranges were investigated by scanning electron microscopy and EDX from a series of samples with distinct overall compositions. Al7+xFe23Ge14-x (−0.8 ≤ x ≤ 1.8, orthorhombic, space group Cmc21, Pearson symbol oC176; a = 7.912 (2), b = 19.786(6), c = 14.769(5) Å) adopts a new crystal structure type, while Al7+xFe9Ge5-x, (−0.7 ≤ x ≤ 3.5, monoclinic, space group C2/m, Pearson symbol mC42; a = 10.147(2), b = 7.5394(12), c = 7.8744(12) Å, β = 108.396(15)°) was found to be isotypic with Fe3Ga4. Within the accuracy of structure refinements the Fe atoms build an ordered and fully occupied sublattice, whereas extensive mixed occupation between aluminium and germanium atoms is evident. Both compounds exhibit noticeable homogeneity ranges based on extensive (Al,Ge) substitutions. Their crystal structures are related to the Fe6Ge5-structure type (monoclinic, space group C2/m, Pearson symbol mC44). Structural features and relations are discussed; homogeneity ranges as well as the corresponding lattice parameters are given.