  We report on the magnetic structures of Tb3RuO7 and Nd3RuO7 determined from
powder neutron-diffraction experiments. In Tb3RuO7, alternating-bond spin-3/2
Ru1-Ru2 chains are formed. The Ru1 and Ru2 moments are ordered and disordered
(paramagnetic), respectively, below 15 K. This result indicates the appearance
of a partially disordered (PD) state, although the two Ru sites are very
similar to each other. In Nd3RuO7, the Ru1 and Ru2 ions form independent
uniform chains. Both the Ru1 and Ru2 moments are ordered below 17.5 K.
Probably, the main source of the PD state in Tb3RuO7 is the magnetic
frustration in the exchange interactions. The internal magnetic field generated
by the Tb ordered moments at the Ru1 sites is different from that at the Ru2
sites. We speculate that the different (nonuniform) internal magnetic field
enhances the difference in the properties between the Ru1 and Ru2 moments.
