Crystal structure of epsilon-Ag7+xMg26-x - A binary alloy phase of the Mackay cluster type

dThe binary alloy phase epsilon -Ag7+xMg26-x with x approximate to 1 and sm all amounts of the beta ' -AgMg phase crystallize by annealing of Ag-Mg all oys with starting compositions between 24-28 At-% Ag at 390 to 420 degreesC . A model structure for the e-phase consisting of a fcc packing of Mackay c lusters was derived from the known structure of the epsilon ' -Ag17Mg54 pha se. Crystals of the c-phase were obtained by direct melting of the elements and annealing. The examination of a single crystal yielded a face-centered cubic unit cell , space group Fm3 with a = 1761.2(5) pm. The refinement was started with th e parameters of the model: wR2(all) = 0.0925 for 1093 symmetrically indepen dent reflections. A refinement of the occupancy parameters indicated a part ial replacement of silver for magnesium at two metal atom sites, resulting in the final composition epsilon -Ag7+xMg26-x with x = 0.96(2). There are 2 64 atoms in the unit cell and the calculated density is 3.568 gcm(-3). The topology of the model was confirmed. Mackay icosahedra are located at the l attice points of a face-centered cubic lattice. Differences between model a nd refined structure and their effects on the powder patterns are discussed . The new binary structure type of epsilon -Ag7+xMg26-x can be described in terms of the 13-cluster concept.