Electron-Diffraction Study of Liquid-Solid Transition of Thin Metal Films

Structure changes of thin films of Pb, Sn and Bi at various temperatures has been studied by electron diffraction method. The films were prepared, in the electron diffraction camera by evaporating the metals, on cleavage surfaces of several kinds of crystals. The mean thickness of the films ranged from 10A to 1000A. The observed melting points are found to be lower than those of bulk metals. Anticipating this effect to be attributed to the small size of the crystal, the author has calculated the melting temperature as a function of crystal size. The observed values of the lowering of the melting point agree fairly well with the calculated values for the crystal sizes estimated from the mean thickness and the breadth of the Debye-Scherrer rings in diffraction patterns. On cooling, the films are supercooled down to considerablly lower temperatures which can be ascribed to the slowness of the rate of nucleation in a small crystal.