  We report on the observation of unusually strong interactions in a thermal
mixture of ultracold atoms which cause a significant modification of the
spatial distribution. A mixture of $^{87}$Rb and $^{174}$Yb with a temperature
of a few $\mu$K is prepared in a hybrid trap consisting of a bichromatic
optical potential superimposed on a magnetic trap. For suitable trap parameters
and temperatures, a spatial separation of the two species is observed. We infer
that the separation is driven by a large interaction strength between
$^{174}$Yb and $^{87}$Rb accompanied by a large three-body recombination rate.
Based on this assumption we have developed a diffusion model which reproduces
our observations.
