Abstract
The new ternary intermetallic title compounds, namely trilanthanum undeca(zinc/magnesium), La(3)(Zn(0.874)Mg(0.126))(11), (I), and tricerium undeca(zinc/magnesium), Ce(3)(Zn(0.863)Mg(0.137))(11), (II), are isostructural and crystallize in the orthorhombic La(3)Al(11) structure type. These three phases belong to the same structural family, the representative members of which may be derived from the tetragonal BaAl(4) structure type by a combination of internal deformation and multiple substitution. Compared to the structure of La(3)Al(11), in (I), a significant decrease of 11.9% in the unit-cell b axis and an increase in the other two directions, of 3.6% along a and 5.2% along c, are observed. Such an atypical deformation is caused by the closer packing of atoms in the unit cell due to atom shifts that reflect strengthening of metallic-type bonding. This structural change is also manifested in a significant difference in the coordination around the smaller atoms at the 8l Wyckoff position (site symmetry m). The Al atom in La(3)Al(11) is in a tricapped trigonal prismatic environment (coordination number 9), while the Zn atoms in (I) and (II) are situated in a tetragonal antiprism with two added atoms (coordination number 10).