Melting of Pb nanocrystals

The size-dependent melting and surface melting of Pb nanocrystals is demonstrated by x-ray powder diffraction in ultrahigh vacuum. Whereas some prior studies have measured the size-dependent melting temperature via the diffraction intensity, it is shown here that crystallite reorientation makes the diffraction intensity an unreliable indicator of melting. Instead of the diffraction intensity, the diffraction peak shape reveals the size-dependent melting via changes in the crystallite size distribution. Measurements showed that the melting temperature varies inversely with the crystallite size and quantitatively favors the liquid-skin melting model over the homogeneous melting model. Surface melting is demonstrated via the reversible growth of a 0.5 nm liquid skin on 50 nm crystallites just below the size-dependent melting temperature.