Geometrically frustrated trimer-based Mott insulator

The crystal structure of $\mathrm{B}{\mathrm{a}}₄\mathrm{NbR}{\mathrm{u}}₃{\mathrm{O}}₁₂$ is based on triangular planes of elongated $\mathrm{R}{\mathrm{u}}₃{\mathrm{O}}₁₂$ trimers oriented perpendicular to the plane. We report that it is semiconducting, that its Weiss temperature and effective magnetic moment are \ensuremath{-}155 K and $2.59\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$, respectively, and that the 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 $\mathrm{[}\ensuremath{\gamma}=31(2)\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}]$ in the specific heat. Electronic-structure calculations reveal that the electronic states at the Fermi energy reside on the $\mathrm{R}{\mathrm{u}}₃{\mathrm{O}}₁₂$ 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. The results imply that $\mathrm{B}{\mathrm{a}}₄\mathrm{NbR}{\mathrm{u}}₃{\mathrm{O}}₁₂$ is a geometrically frustrated trimer-based Mott insulator.