  We have carried out extensive isokinetic {\it ab initio} molecular dynamic
simulations to investigate the finite temperature properties of the impurity
doped cluster Li$_6$Sn along with the host cluster Li$_7$. The data obtained
from about 20 temperatures and total simulation time of at least 3 ns is used
to extract thermodynamical quantities like canonical specific heat. We observe
a substantial charge transfer from all Li atoms to Sn which inturn weakens the
Li-Li bonds in Li$_6$Sn compared to the bonds in Li$_7$. This weakening of
bonds changes the finite temperature behavior of Li$_6$Sn significantly.
Firstly, Li$_6$Sn becomes liquid-like around 250 K, a much lower temperature
than that of Li$_7$ ($\approx$~425 K). Secondly, an additional quasirotational
motion of lithium atoms appears at lower temperatures giving rise to a shoulder
around 50 K in the specific heat curve of Li$_6$Sn. The peak in the specific
heat of Li$_7$ is very broad and the specific heat does not show any premelting
features.

Condensed Matter