Melting of Pb clusters without free surfaces

Melting behaviors of Pb clusters without free surfaces have been studied using molecular dynamics and the Sutton-Chen (SC) potential. Two Pb clusters (PbN, with $N=201$ and 249) are selected and each of them has been embedded in the core of an Al⁴⁰³³ cubo-octahedron, but with different orientation relationship with Al coatings. For the Pb²⁰¹ cluster, where a parallel cube-cube relationship exists with the Al lattice, semicoherent interfaces can be formed and the core can be superheated up to 750 K, in comparing to the equilibrium melting point of 615±10K for bulk Pb predicted using the SC potential. On further elevating the temperature, a simultaneous melting of both the Pb core and the outer Al coating will occur. For the Pb²⁴⁹ cluster without a parallel orientation relationship with Al, no effective semicoherent interface can be formed, and the core premelts at about 500 K without breaking the crystalline structure of Al shells. The melting point of the Pb inclusions can be predicted using thermodynamic relations. For different Pb-Al interfaces, two melting mechanisms exist: one is the homogeneous melting which leads to a melting point elevation and the other is the heterogeneous melting which can induce a melting point depression.