From the bulk to clusters: Solid-liquid phase transitions and precursor effects

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Abstract
A review is given of the experiments done in order to understand the initial stages of melting and solidification of a material. We emphasize on new results obtained for ultrafine metal particles by using high sensitivity reflectance and electron microscopy measurements. These experiments show that melting certainly begins through a continuous and reversible transition and proceeds through the usual first-order transition. For ultrafine particles, these two processes have a relative importance which depend on the size, the first process having the major importance for the smallest particles. Undercooled state and subsequent solidification depend also on the size of the particle. For the smallest particles, the maximum supercooling temperature is found to coincide with the final temperature of the continuous transition leading to disappearance of the hysteretic behaviour usually found in solid-liquid transitions for ultrafine particles.