DIFFUSE-X-RAY SCATTERING OF AG-13.4 AT-PERCENT AL

A single crystal of Ag-13.4 at. % Al aged within the alpha phase at 67 3 K was investigated by diffuse x-ray scattering. The separated short- range-order scattering is only weakly modulated and shows two types of diffuse maxima simultaneously, at 2k(F)(110) (where k(F) is the Fermi wave number along [110]) and at 1/2,1/2,1/2 positions. Such a topolog y has previously been observed only in the Cu-Pt system. A detailed an alysis of nearest-neighbor configurations, however, exhibits large dif ferences between the two systems; the data suggest the A(5)B structure as a possible ground-state structure of the fee Ag-Al lattice, i.e., none of the structures observed in the Cu-Pt system. For this structur e an order-disorder transition temperature of similar to 135 K is esti mated on the basis of Monte Carlo simulations with effective pair-inte raction parameters of the first 18 shells.