THE STRUCTURE OF A METASTABLE AU-SN PHASE DETERMINED BY CONVERGENT-BEAM ELECTRON-DIFFRACTION

The structure of a new metastable Au-Sn phase was determined by conver gent beam electron diffraction. Micrometer-sized crystals were grown b y heating a metal/oxide/metal trilayer and allowing the metals to reac t. The new phase was tetragonal with space group P4/nbm, lattice param eters a = 6.63(3) Angstrom and c = 5.92(3) Angstrom, and composition A uSn4. The phase was found to be isostructural with PtPb4 with Au atoms located at 2(a) sites and Sn atoms at 8(m) sites. Conditional Patters on transforms parallel to the c-axis were calculated from the quasi-ki nematic intensities of first-order Laue zone (FOLZ) reflections diffra cted from Bloch states localized on the Sn atom strings. The contour m aps showed clear peaks from Au-Sn and Sn-Sn interatomic vectors. A lea st-squares refinement using the FOLZ reflections gave the Sn atomic pa rameters as x = 0.167(1), z = 0.25(4). (C) 1996 Academic Press, Inc.