  We report detailed magnetic, transport, heat-capacity, and neutron
diffraction measurements of Ba5Ru3O12, a compound consisting of isolated Ru3O12
trimers. We show that this system develops long-range antiferromagnetic
ordering at 60 K (TN) without structural distortion and metal-insulator-type
transition, which is in sharp contrast to other Barium Ruthenate trimer systems
such as 9R-BaRuO3 and Ba4Ru3O10. A complex magnetic structure is revealed which
is attributable to the magnetic frustration due to competing exchange
interactions between Ru ions on different crystallographic sites within the
Ru3O12 trimer.
