The crystal structure of Ba4NbRu3O12 is based on triangular planes of elongated Ru3O12 trimers oriented perpendicular to the plane. We report that it is semiconducting, that its Weiss temperature and effective magnetic moment are -155 K and 2.59 {mu}B/f.u. respectively, and that magnetic susceptibility and specific heat data indicate that it exhibits magnetic ordering near 4 K. The presence of a high density of low energy states is evidenced by a substantial Sommerfeld-like T-linear term (gamma = 31(2) mJ/mole-K^2) in the specific heat. Electronic structure calculations reveal that the electronic states at the Fermi Energy reside on the Ru3O12 trimers and that the calculated density of electronic states is high and continuous around the Fermi Energy - in other words density functional theory calculates the material to be a metal. Our results imply that Ba4NbRu3O12 is a geometrically frustrated trimer-based Mott insulator.
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