INPLANE STRUCTURE OF THE LIQUID-VAPOR INTERFACES OF DILUTE BISMUTH-GALLIUM ALLOYS - X-RAY-SCATTERING STUDIES

The in-plane structure functions in the liquid-vapor interfaces of two bismuth-gallium alloys (0.18 at. % Bi and 0.07 at. % Bi) have been st udied by grazing incidence x-ray diffraction. The higher concentration alloy exhibits, at 25, 43, and 77 degrees C, segregation of Bi into n early complete monolayers atop the bulk alloy; at each temperature the Ri monolayer has the structure of a two-dimensional supercooled liqui d. The intensity distribution of the diffuse x-ray scattering at q(z)= 1.0 and 1.5 Angstrom(-1) is used to calculate the surface tension of t he alloy at 77 degrees C; the value obtained is 570+/-30 dyn/cm. This value, which is very different from the value obtained by extrapolatin g the surface tension of liquid Bi to 77 degrees C, is related to the structure of the Liquid-vapor interface. We have compared experimental grazing incidence x-ray diffraction studies on the liquid-vapor inter face of the 0.07% alloy with integral-equation theoretical calculation s on a two-dimensional hard-disk fluid mixture. The results suggest th at the segregated partial monolayer may consist of dimers and trimers in equilibrium with a small amount of two-dimensional Liquid bismuth. (C) 1996 American Institute of Physics.