  The effect of rare-earth ions (R = Er^{3+}, Tb^{3+}) with strong spin-orbit
coupling on the dielectric properties and the electric polarization induced by
local polar phase separation domains in RMn_2O_5 multiferroics has been
studied. These parameters were found to be qualitatively distinguished from
those studied before in GdMn_2O_5, in which Gd^{3+} ion in the ground ^8S_{7/2}
state is weakly bounded with the lattice. It is shown that the properties of
the polar phase separation domains, which form in the subsystem of Mn^{3+} and
Mn^{4+} ions, are substantially dependent on the values of crystal fields, in
which these domains exist.
