  Relativistic and QED corrections are calculated for the hyperfine splitting
(hfs) in the $2S_{1/2}$ ground state of $^{9}$Be$^+$ ions with an exact account
for electronic correlations. The achieved accuracy is sufficient to determine
the finite nuclear size effects from the comparison to the experimental hfs
value. The obtained results establish the ground to determine the neutron halo
in $^{11}$Be.
