Channeling study of melting and solidification of lead nanocrystals in aluminium

Nanometer-sized Pb crystallites were obtained in Al single crystals by 423 K implantation of 150 keV Pb+ ions with a fluence of 2.3 X 10(16) cm(-2). The crystallites grow in perfect topotactical alignment with the matrix with a cube/cube orientation relationship, Lead depth profiles were obtained using the Rutherford backscattering (RES) technique. With the RES and channeling analysis two major distributions of nanocrystals were observed. The first one with the average size of crystallites of about 13 nm is located within depth region 30-65 nm, acid the second distribution (70-105 nm) has the average size of crystallites of 9.9 nm. Measurements of melting/solidification of Pb nanocrystals were performed with the channeling technique. A thermal hysteresis for crystallites as well as for channeling in Al matrix was observed. The nanocrystals show large superheating (similar to 75 K above the bulk melting point of Pb) as well as supercooling (similar to 35 K) during the heating cycle. The size dependence of melting of the crystallites is deduced from the measurements using Monte Carlo channeling simulations. These results are compared with those obtained by transmission electron microscopy (TEM). They are discussed in a phenomenological context considering the lack of free surfaces and a reduction of thermal vibrations for surface atoms in topotactical nanocrystals.