Surface freezing in liquid Ga–Bi alloys: optical second harmonic and plasma generation study

The phenomenon of surface freezing has been studied by second harmonic and plasma generation measurements in the entire composition range of liquid Ga–Bi alloys. Surface freezing was observed for the first time for all alloys with compositions between the eutectic (xBi = 0.0022, Teut = 29.48 °C) and monotectic point (xBi = 0.085, Tmono = 222 °C). On cooling of such alloys a Bi-rich solid-like film forms on top of the bulk liquid phase at temperatures well above the liquidus line. On melting of these films a clear hysteresis behaviour is found, characterizing this type of transition as a first order phase transition. The maximum difference of 20 K between the liquidus temperature and the melting temperature of the surface freezing films was observed for the eutectic alloy. The line of surface freezing temperatures merges with the liquidus approaching the monotectic point. An estimate of the thickness of the Bi-rich surface freezing films from the interfacial free energies yields a value of the order of 10 nm. The correlation of the surface freezing and wetting transition occurring in the Ga–Bi system as well as the thermodynamics of surface freezing are qualitatively discussed.