  Neutron diffraction has been used to study the lattice and magnetic
structures of the insulating and superconducting Rb$_y$Fe$_{1.6+x}$Se$_2$. For
the insulating Rb$_{y}$Fe$_{1.6+x}$Se$_2$, neutron polarization analysis and
single crystal neutron diffraction unambiguously confirm the earlier proposed
$\sqrt{5}\times\sqrt{5}$ block antiferromagnetic structure. For superconducting
samples ($T_c=30$ K), we find that in addition to the tetragonal
$\sqrt{5}\times\sqrt{5}$ superlattice structure transition at 513 K, the
material develops a separate $\sqrt{2}\times \sqrt{2}$ superlattice structure
at a lower temperature of 480 K. These results suggest that superconducting
Rb$_{y}$Fe$_{1.6+x}$Se$_2$ is phase separated with coexisting $\sqrt{2}\times
\sqrt{2}$ and $\sqrt{5}\times\sqrt{5}$ superlattice structures.
