Abstract
The vibrational dynamics and thermodynamics of atoms in the grain-boundaries are shown to present dual behavior. First, we find their average coordination, for the cases considered, to be less than that of the atoms in the bulk (10.6 instead of 12). This under-coordination leads to an enhancement of the vibrational density of states at low frequencies and consequently to a contribution to the lattice heat capacity, at low temperatures (T), which deviates from the bulk T-3 dependence. Second, substantial shortening of the bonds around the grain-boundary atoms yield a high frequency tail extending well above the top of the bulk band leading to substantial contribution to the excess vibrational entropy (relative to the bulk) from the atoms in the grain-boundaries.