  We have performed magnetic susceptibility, heat capacity, muon spin
relaxation, and neutron scattering measurements on three members of the family
Ba3MRu2O9, where M = In, Y and Lu. These systems consist of mixed-valence Ru
dimers on a triangular lattice with antiferromagnetic interdimer exchange.
Although previous work has argued that charge order within the dimers or
intradimer double exchange plays an important role in determining the magnetic
properties, our results suggest that the dimers are better described as
molecular units due to significant orbital hybridization, resulting in one
spin-1/2 moment distributed equally over the two Ru sites. These molecular
building blocks form a frustrated, quasi-two-dimensional triangular lattice.
Our zero and longitudinal field muSR results indicate that the molecular
moments develop a collective, static magnetic ground state, with oscillations
of the zero field muon spin polarization indicative of long-range magnetic
order in the Lu sample. The static magnetism is much more disordered in the Y
and In samples, but they do not appear to be conventional spin glasses.
