STRUCTURE OF THE LIQUID-VAPOR INTERFACE OF A SN-GA ALLOY

The atomic distributions along the normal to, and in the plane of, the liquid-vapor interface of a dilute (9:91 at. %) Sn:Ga alloy have been studied by x-ray specular reflectivity and grazing incidence x-ray di ffraction at 57 degrees C, 41 degrees C, 26 degrees C, and 12 degrees C. Surface tensions, calculated from sample shape measurements, have a lso been determined at the same temperatures. The results of these exp eriments show that the atomic distribution in the liquid-vapor interfa ce of the Sn:Ga alloy studied has both similarities and differences wi th the atomic distribution in the liquid-vapor interface of a dilute B i:Ga alloy. In both alloys the atomic distribution along the normal to the liquid-vapor interface is stratified, and the composition of the outermost layer is, sensibly, 100% of the constituent with lower (pure liquid) surface tension, respectively, Sn and Bi. In both alloys the in-plane structure of the outermost layer is that of a liquid. In the homovalent Bi:Ga alloy the excess Bi segregates into a monolayer which forms the outermost layer of the stratified liquid-vapor interface, w ith no excess Bi in the second or deeper layers of the interface. In t he heterovalent Sn:Ga alloy, the excess Sn segregates into both the ou termost and the second layers of the stratified liquid-vapor interface . The outermost layer of the interface is found to be pure two-dimensi onal liquid Sn, in which the atomic diameter is smaller than that in n ormal liquid Sn by about 6%. In the second layer of the interface the Sn atomic concentration is found to be 22.3+/-1.6% at 57 degrees C and 22.7+/-1.7% at 41 degrees C. Within the limits of our experimental se nsitivity, the Sn concentration in deeper levels of the liquid-vapor i nterface cannot be distinguished from that in the bulk liquid alloy. T he surface tension of the (9:91 at. %) Sn:Ga alloy is found to be 671/-42 dyn/cm at 57 degrees C, 573+/-58 dyn/cm at 41 degrees C, 587+/-50 dyn/cm at 26 degrees C, and 527+/-40 dyn/cm at 12 degrees C. The diff erences between the structures of the Sn:Ga and Bi:Ga liquid-vapor int erfaces are interpreted, qualitatively, in terms of their electron den sity distributions. (C) 1997 American Institute of Physics.