Location of Zn within the Mg-12(LaxCe1-x) lattice by X-ray incoherent channelling patterns

Systematic electron diffraction studies on intermetallic precipitates forme d within a lightweight Mg-RE-Zn alloy (RE = La or Ce) identify these to be of structural type Mn12Th (space group I4/mmm). Analytical electron microsc opy yields an overall composition of Mg-12(LaxCe1-x) with x similar to 0.43 , with 1 at.% Zn incorporated within the lattice. Variations in characteris tic X-ray emission rates, as an electron beam is rocked near zone-axis orie ntations, are used to form two-dimensional channelling patterns, termed X-r ay incoherent channelling patterns. This channelling contrast enables a spe cific sublattice site that is occupied by Zn to be unambiguously identified within the Mg12RE lattice. The particular sublattice site is denoted by th e Wyckoff letter f, and is one of the three different Mg sublattice sites f , i and j. Of these three sites, the Wigner-Seitz cell that is centred on t he f sublattice site has the largest Mg-RE interatomic distance, and theref ore the f site is expected to be favoured for accommodating the substitutio n of a larger Zn atom.