INVERSE SURFACE MELTING IN CONFINED CLUSTERS - AR-13 IN ZEOLITE-L

Monte Carlo and molecular dynamics simulations on an Ar-13 cluster in zeolite L have been carried out at a series of temperatures to underst and the rigid-nonrigid transition corresponding to the solid-liquid tr ansition exhibited by the free Ar-13 cluster. The icosahedral geometry of the free cluster is no longer preferred when the cluster is confin ed in the zeolite. The root-mean-squared pair distance fluctuation, de lta, exhibits a sharp, well-defined rigid-nonrigid transition at 17 K as compared to 27 K for the free cluster. Multiple peaks in the distri bution of short-time averages of the guest-host interaction energy ind icate coexistence of two phases.; It is shown that this transition is associated with the inner atoms becoming mobile at 17 K even while the outer layer atoms, which are in close proximity to the zeolitic wall, continue to be comparatively immobile. This may be contrasted with th e melting of large free clusters of 40 or more atoms which exhibit sur face melting. Guest-host interactions seem to play a predominant role in determining the properties of confined clusters. We demonstrate tha t the volume of the cluster increases rather sharply at 17 and 27 K re spectively for the confined and the free cluster. Power spectra sugges t that the motion of the inner atoms is generally parallel to the atom s which form the cage wall.