  Low-temperature powder neutron diffraction measurements are performed in the
ordered magnetic state of the pyrochlore antiferromagnet Gd2Sn2O7. Symmetry
analysis of the diffraction data indicates that this compound has the ground
state predicted theoretically for a Heisenberg pyrochlore antiferromagnet with
dipolar interactions. The difference in magnetic structures of Gd2Sn2O7 and of
nominally analogous Gd2Ti2O7 is found to be determined by a specific type of
third-neighbor superexchange interaction on the pyrochlore lattice between
spins across empty hexagons.
