  We report results of molecular dynamics simulations to understand the role of
solute and solute-solvent interaction on solute diffusivity in a solid solution
within a body centered cubic solid when the solute size is significantly
smaller than the size of the solvent atom. Results show that diffusivity is
maximum for two specific sizes of the solute atom. This is the first time that
twin maxima have been found. The solute with diffusivity maxima are larger in
case of rigid host as compared to flexible host. This suggests that the effect
of lattice vibrations is to decrease the size at which the maximum is seen. For
one of the $\epsilon_{uv}$ where two diffusivity maxima have been observed, we
have analyzed various properties to understand the anomalous diffusion
behavior. It is characterized by a lower activation energy, lower
backscattering in the velocity autocorrelation function, lower mean square
force, single exponential decay of the intermediate scattering function and
monotonic dependence on $k$ of the $\Delta \omega/2Dk^2$ where $\Delta \omega$
is the fwhm of the self part of the dynamic structure factor. Among the two
solute atoms at the anomalous maxima, the solute with higher diffusivity has
lower activation energy.
